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Prior to the Web's development, CERN had been a pioneer in the
introduction of Internet technology, beginning in the early 1980s. A
short history of this period can be found at CERN.ch [11]
More recently, CERN has become a centre for the development of Grid
computing, hosting among others the Enabling Grids for E-sciencE
(EGEE) and LHC Computing Grid projects. It also hosts the CERNInternet Exchange Point (CIXP), one of the two main Internet
Exchange Points in Switzerland. CERN's computer network is
connected to JANET (formerly UKERNA), the research and education
network, JANET aids CERN to disperse large data over a network grid
for closer analysis.
Particle accelerators
Current complex
Map of the Large Hadron Collider together with
the Super Proton Synchrotron at CERN
CERN operates a network of six accelerators and a decelerator. Each
machine in the chain increases the energy of particle beams before
delivering them to experiments or to the next more powerful
accelerator. Currently active machines are:
• Two linear accelerators generate low energy particles. Linac2
accelerates protons to 50 MeV for injection into the Proton
Synchrotron Booster (PSB), and Linac3 provides heavy ions at
4.2 MeV/u for injection into the Low Energy Ion Ring (LEIR).[12]
• The Proton Synchrotron Booster increases the energy of particles
generated by the proton linear accelerator before they are
transferred to the other accelerators.
• The Low Energy Ion Ring (LEIR) accelerates the ions from the ion
linear accelerator, before transferring them to the Proton
Synchrotron (PS). This accelerator was commissioned in 2005, after
having been reconfigured from the previous Low Energy Antiproton Ring (LEAR).
• The 28 GeV Proton Synchrotron (PS), built in 1959 and still operating as a feeder to the more powerful SPS.
• The Super Proton Synchrotron (SPS), a circular accelerator with a diameter of 2 kilometres built in a tunnel,which started operation in 1976. It was designed to deliver an energy of 300 GeV and was gradually upgraded to
450 GeV. As well as having its own beamlines for fixed-target experiments, it has been operated as a
proton –antiproton collider (the SppS collider), and for accelerating high energy electrons and positrons which
were injected into the Large Electron –Positron Collider (LEP). From 2008 onwards, it will inject protons and
heavy ions into the Large Hadron Collider (LHC).
• The On-Line Isotope Mass Separator (ISOLDE), which is used to study unstable nuclei. The radioactive ions are
produced by the impact of protons at an energy of 1.0 –1.4 GeV from the Proton Synchrotron Booster. It was first
commissioned in 1967 and was rebuilt with major upgrades in 1974 and 1992.
• REX-ISOLDE increases the charge states of ions coming from the ISOLDE targets, and accelerates them to a
and ALICE) will run on the collider; each of them will study particle collisions
from a different point of view, and with different technologies. Construction for
these experiments required an extraordinary engineering effort. Just as an
example, a special crane had to be rented from Belgium in order to lower pieces
of the CMS detector into its underground cavern, since each piece weighed
nearly 2,000 tons. The first of the approximately 5,000 magnets necessary for construction was lowered down a
special shaft at 13:00 GMT on 7 March 2005.
This accelerator will generate vast quantities of computer data, which CERN will stream to laboratories around the
world for distributed processing (making use of a specialised grid infrastructure, the LHC Computing Grid). In April
2005, a trial successfully streamed 600 MB/s to seven different sites across the world. If all the data generated by the
LHC is to be analysed, then scientists must achieve 1,800 MB/s before 2008.
The initial particle beams were injected into the LHC August 2008.[14] The first attempt to circulate a beam throughthe entire LHC was at 8:28 GMT on 10 September 2008,[15] but the system went wrong because of a faulty magnet
connection, and it was stopped for repairs on 19 September 2008.
The LHC resumed its operation on Friday the 20th of November 2009 by successfully circulating two beams, each
with a power of 3.5 trillion electron volts. The challenge that the engineers then faced was to try and line up the two
beams so that they smashed into each other. This is like "firing two needles across the Atlantic and getting them to
hit each other" according to the LHC's main engineer Steve Myers, director for accelerators and technology at the
Swiss laboratory.
At 1200 BST on Tuesday the 30th of March 2010 the LHC successfully smashed two proton particle beams
travelling at 3.5 TeV (trillion electron volts), with a resultant force of 7 TeV. However this is just the start of a longroad toward the expected discovery of the Higgs boson. This is mainly because the amount of data produced is so
huge it could take up to 24 months to completely analyse it all. At the end of the 7 TeV experimental period, the
LHC will be shut down for maintenance for up to a year, with the main purpose of this shut down being to strengthen
the huge magnets inside the accelerator. When it re-opens, it will attempt to create 14 TeV events.
• The 600 MeV Synchrocyclotron (SC) which started operation in 1957 and was shut down in 1991.
• The Intersecting Storage Rings (ISR), an early collider built from 1966 to 1971 and operated until 1984.
• The Large Electron –Positron Collider (LEP), which operated from 1989 to 2000 and was the largest machine of
its kind, housed in a 27 km-long circular tunnel which now houses the Large Hadron Collider.• The Low Energy Antiproton Ring (LEAR), commissioned in 1982, which assembled the first pieces of true
antimatter, in 1995, consisting of nine atoms of antihydrogen. It was closed in 1996, and superseded by the
Antiproton Decelerator.
Sites
CERN's main site, as seen from Switzerland looking towards France.
Interior of office building 40 at the
Meyrin site. Building 40 hosts many
offices for scientists working for CMS
and Atlas.
The smaller accelerators are located on the main
Meyrin site (also known as the West Area), which was
originally built in Switzerland alongside the French
border, but has been extended to span the border since1965. The French side is under Swiss jurisdiction and
so there is no obvious border within the site, apart from
a line of marker stones. There are six entrances to the
Meyrin site:
• A, in Switzerland. Open for all CERN personnel at
specific times.
• B, in Switzerland. Open for all CERN personnel at
all times. Often referred to as the main entrance.
• C , in Switzerland. Open for all CERN personnel at
specific times.
• D, in Switzerland. Open for goods reception at
specific times.
• E , in France. Open for French-resident CERN
personnel at specific times. Controlled by customs
personnel. Named "Porte Charles de Gaulle" in
recognition of his role in the creation of the
CERN.[16]
• Tunnel entrance, in France. Open for equipment
transfer to and from CERN sites in France bypersonnel with a specific permit. This is the only
permitted route for such transfers. Under the CERN
treaty, no taxes are payable when such transfers are
made. Controlled by customs personnel.
The SPS and LEP/LHC tunnels are located
underground almost entirely outside the main site, and are mostly buried under French farmland and invisible from
the surface. However they have surface sites at various points around them, either as the location of buildings
associated with experiments or other facilities needed to operate the colliders such as cryogenic plants and access
shafts. The experiments themselves are located at the same underground level as the tunnels at these sites.
Three of these experimental sites are in France, with ATLAS in Switzerland, although some of the ancillarycryogenic and access sites are in Switzerland. The largest of the experimental sites is the Prévessin site, also known
as the North Area, which is the target station for non-collider experiments on the SPS accelerator. Other sites are the
ones which were used for the UA1, UA2 and the LEP experiments (the latter which will be used for LHC
experiments).
Outside of the LEP and LHC experiments, most are officially named and numbered after the site where they were
located. For example, NA32 was an experiment looking at the production of charmed particles and located at the
Prévessin (North Area) site while WA22 used the Big European Bubble Chamber (BEBC) at the Meyrin (WestArea) site to examine neutrino interactions. The UA1 and UA2 experiments were considered to be in the
Underground Area, i.e. situated underground at sites on the SPS accelerator.
Relativistic Heavy Ion Collider BNL, 2000 –present
Superconducting Super Collider Cancelled in 1993
Large Hadron Collider CERN, 2009 –present
Super Large Hadron Collider Proposed, CERN, 2019 –
Very Large Hadron Collider Theoretical
The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It is expected that
it will address the most fundamental questions of physics, advancing our understanding of the deepest laws of nature.
The LHC lies in a tunnel 27 kilometres (17 mi) in circumference, as much as 175 metres (574 ft) beneath the
Franco-Swiss border near Geneva, Switzerland. This synchrotron is designed to collide opposing particle beams of
either protons at an energy of 7 teraelectronvolts (1.12 microjoules) per particle, or lead nuclei at an energy of
574 TeV (92.0 µJ) per nucleus.[1] [2] The term hadron refers to particles composed of quarks.
The Large Hadron Collider was built by the European Organization for Nuclear Research (CERN) with the intention
of testing various predictions of high-energy physics, including the existence of the hypothesized Higgs boson[3] and
of the large family of new particles predicted by supersymmetry.[4] It is funded by and built in collaboration with
over 10,000 scientists and engineers from over 100 countries as well as hundreds of universities and laboratories.[5]
On 10 September 2008, the proton beams were successfully circulated in the main ring of the LHC for the firsttime,[6] but 9 days later operations were halted due to a serious fault.[7] On 20 November 2009 they were
successfully circulated again,[8] with the first proton –proton collisions being recorded 3 days later at the injection
energy of 450 GeV per beam.[9] After the 2009 winter shutdown, the LHC was restarted and the beam was ramped
up to 3.5 TeV per beam,[10] half its designed energy,[11] which is planned for after its 2012 shutdown. On 30 March
2010, the first planned collisions took place between two 3.5 TeV beams, which set a new world record for the
The LHC physics program is mainly based on proton –proton collisions.
However, shorter running periods, typically one month per year, with heavy-ion
collisions are included in the program. While lighter ions are considered as well,
the baseline scheme deals with lead ions[29] (see A Large Ion Collider
Experiment). The lead ions will be first accelerated by the linear accelerator
LINAC 3, and the Low-Energy Ion Ring (LEIR) will be used as an ion storageand cooler unit. The ions will then be further accelerated by the PS and SPS
before being injected into LHC ring, where they will reach an energy of 2.76
TeV per nucleon (or 575 TeV per ion), higher than the energies reached by the
Relativistic Heavy Ion Collider. The aim of the heavy-ion program is to
investigate quark –gluon plasma, which existed in the early universe.
DetectorsSix detectors have been constructed at the LHC, located underground in large caverns excavated at the LHC's
intersection points. Two of them, the ATLAS experiment and the Compact Muon Solenoid (CMS), are large, general
purpose particle detectors.[23] A Large Ion Collider Experiment (ALICE) and LHCb, have more specific roles and
the last two, TOTEM and LHCf, are very much smaller and are for very specialized research. The BBC's summary
of the main detectors is:[30]
Detector Description
ATLAS one of two general purpose detectors. ATLAS will be used to look for signs of new physics, including the origins of mass and extra
dimensions.
CMS the other general purpose detector will, like ATLAS, hunt for the Higgs boson and look for clues to the nature of dark matter.
ALICE will study a "liquid" form of matter called quark –gluon plasma that existed shortly after the Big Bang.
LHCb equal amounts of matter and antimatter were created in the Big Bang. LHCb will try to investigate what happened to the "missing"
antimatter.
Operational timeline
The first beam was circulated through the collider on the morning of 10 September 2008.[30] CERN successfully
fired the protons around the tunnel in stages, three kilometres at a time. The particles were fired in a clockwise
direction into the accelerator and successfully steered around it at 10:28 local time.[31]
The LHC successfullycompleted its first major test: after a series of trial runs, two white dots flashed on a computer screen showing the
protons travelled the full length of the collider. It took less than one hour to guide the stream of particles around its
inaugural circuit.[32] CERN next successfully sent a beam of protons in a counterclockwise direction, taking slightly
longer at one and a half hours due to a problem with the cryogenics, with the full circuit being completed at 14:59. In
the original timeline of the LHC commissioning, the first "modest" high-energy collisions at a center-of-mass energy
of 900 GeV were expected to take place before the end of September 2008, and the LHC was expected to be
operating at 10 TeV by the time of the official inauguration on 21 October 2008.[33] However, due to the delay
caused by the above-mentioned incident, the collider was not operational until November 2009.[34] Despite the
delay, LHC was officially inaugurated on 21 October 2008, in the presence of political leaders, science ministers
from CERN's 20 Member States, CERN officials, and members of the worldwide scientific community.[35] CERNscientists estimate that if the Standard Model is correct, a single Higgs boson may be produced every few hours. At
this rate, it may take about two to three years to collect enough data to discover the Higgs boson unambiguously.
Similarly, it may take one year or more before sufficient results concerning supersymmetric particles have been
gathered to draw meaningful conclusions.[1]
On 19 September 2008, a quench occurred in about 100 bending magnets in sectors 3 and 4, causing a loss of
approximately six tonnes of liquid helium, which was vented into the tunnel, and a temperature rise of about 100
kelvin in some of the affected magnets. Vacuum conditions in the beam pipe were also lost.[36]
Shortly after theincident CERN reported that the most likely cause of the problem was a faulty electrical connection between two
magnets, and that – due to the time needed to warm up the affected sectors and then cool them back down to
operating temperature – it would take at least two months to fix it.[37] Subsequently, CERN released a preliminary
analysis of the incident on 16 October 2008,[38] and a more detailed one on 5 December 2008.[39] Both analyses
confirmed that the incident was indeed initiated by a faulty electrical connection. A total of 53 magnets were
damaged in the incident and were repaired or replaced during the winter shutdown.[40]
Most of 2009 was spent on repairs and reviews from the damage caused by the quench incident. On November 20,
the first low-energy beams circulated in the tunnel for the first time since the incident. On December 15, 2009, the
first physics results from the LHC were reported, involving 284 collisions that took place in the ALICE detector.[41]
The early part of 2010 saw the continue ramp-up of beam in energies and early physic experiments. The results of the first proton –proton collisions at energies higher than Fermilab's Tevatron proton –antiproton collisions have been
published, yielding greater-than-predicted charged hadron production.[42] The CMS paper reports that the increase in
the production rate of charged hadrons when the center-of-mass energy goes from 0.9 TeV to 2.36 TeV exceeds the
predictions of the theoretical models used in the analysis, with the excess ranging from 10% to 14%, depending upon
which model is used. The charged hadrons were primarily mesons (kaons and pions).[43] On 30 March 2010, LHC
set a record for high-energy collisions, by colliding proton beams at a combined energy level of 7 TeV. The attempt
was the third that day, after two unsuccessful attempts in which the protons had to be "dumped" from the collider
and new beams had to be injected.[44] CERN has declared a schedule to operate the LHC through the rest of 2010
and most of 2011 before the next scheduled shutdown.[45]
Timeline
Date Event
10 Sep 2008 CERN successfully fired the first protons around the entire tunnel circuit in stages.
19 Sep 2008 Magnetic quench occurred in about 100 bending magnets in sectors 3 and 4, causing a loss of approximately 6 tonnes of liquid
helium.
30 Sep 2008 First "modest" high-energy collisions planned but postponed due to accident.
16 Oct 2008 CERN released a preliminary analysis of the incident.
21 Oct 2008 Official inauguration.5 Dec 2008 CERN released detailed analysis.
20 Nov 2009 Low-energy beams circulated in the tunnel for the first time since the incident.[46]
23 Nov 2009 First particle collisions in all four detectors at 450 GeV.[9]
30 Nov 2009 LHC becomes the world's highest-energy particle accelerator achieving 1.18 TeV per beam, beating the Tevatron's previous record of
0.98 TeV per beam held for eight years.[47]
28 Feb 2010 The LHC continues operations ramping energies to run at 3.5 TeV for 18 months to two years, after which it will be shut down to
prepare for the 14 TeV collisions (7 TeV per beam).[48]
30 Mar 2010 The two beams collided at 7 TeV (3.5 TeV per beam) in the LHC at 13:06 CEST, marking the start of the LHC research program.
After some years of running, any particle physics experiment typically begins to suffer from diminishing returns:
each additional year of operation discovers less than the year before. The way around the diminishing returns is to
upgrade the experiment, either in energy or in luminosity. A luminosity upgrade of the LHC, called the Super LHC,
has been proposed,[49] to be made after ten years of LHC operation.
The optimal path for the LHC luminosity upgrade includes an increase in the beam current (i.e., the number of protons in the beams) and the modification of the two high-luminosity interaction regions, ATLAS and CMS. To
achieve these increases, the energy of the beams at the point that they are injected into the (Super) LHC should also
be increased to 1 TeV. This will require an upgrade of the full pre-injector system, the needed changes in the Super
Proton Synchrotron being the most expensive.
Cost
With a budget of 9 billion US dollars (approx. €7.5bn or £6.19bn as of Jun 2010), the LHC is one of the most
expensive scientific instruments[50] ever built.[51] The total cost of the project is expected to be of the order of 4.6bn
Swiss francs (approx. $4.4bn, €3.1bn, or £2.8bn as of Jan 2010) for the accelerator and SFr 1.16bn (approx. $1.1bn,€0.8bn, or £0.7bn as of Jan 2010) for the CERN contribution to the experiments.[52]
The construction of LHC was approved in 1995 with a budget of SFr 2.6bn, with another SFr 210M towards the
experiments. However, cost overruns, estimated in a major review in 2001 at around SFr 480M for the accelerator,
and SFr 50M for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005
to April 2007.[53] The superconducting magnets were responsible for SFr 180M of the cost increase. There were also
further costs and delays due to engineering difficulties encountered while building the underground cavern for the
Compact Muon Solenoid,[54] and also due to faulty parts provided by Fermilab.[55] Due to lower electricity costs
during the summer, it is expected that the LHC will normally not operate over the winter months, [56] although an
exception was made to make up for the 2008 start-up delays over the 2009/10 winter.
Computing resources
Data produced by LHC as well as LHC-related simulation will produce a total data output of 15 petabytes per year
(max throughput while running not stated).[57]
The LHC Computing Grid is being constructed to handle the massive amounts of data produced. It incorporates both
private fiber optic cable links and existing high-speed portions of the public Internet, enabling data transfer from
CERN to academic institutions around the world.
The Open Science Grid is used as the primary infrastructure in the United States, and also as part of an interoperable
federation with the LHC Computing Grid.
The distributed computing project LHC@home was started to support the construction and calibration of the LHC.
The project uses the BOINC platform, enabling anybody with an internet connection to use their computer idle time
to simulate how particles will travel in the tunnel. With this information, the scientists will be able to determine how
the magnets should be calibrated to gain the most stable "orbit" of the beams in the ring.
Safety of particle collisions
The experiments at the Large Hadron Collider sparked fears among the public that the particle collisions might
produce doomsday phenomena, involving the production of stable microscopic black holes or the creation of
hypothetical particles called strangelets.[58] Two CERN-commissioned safety reviews examined these concerns and
concluded that the experiments at the LHC present no danger and that there is no reason for concern,[59] [60] [61] aconclusion expressly endorsed by the American Physical Society.[62]
The size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account of
the amount of energy stored in the magnets and the beams.[28] [63] While operating, the total energy stored in the
magnets is 10 GJ (equivalent to 2.4 tons of TNT) and the total energy carried by the two beams reaches 724 MJ (173
kilograms of TNT).[64]
Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet, while thebeam dump must absorb 362 MJ (87 kilograms of TNT) for each of the two beams. These energies are carried by
very little matter: under nominal operating conditions (2,808 bunches per beam, 1.15×10 11 protons per bunch), the
beam pipes contain 1.0×10−9 gram of hydrogen, which, in standard conditions for temperature and pressure, would
fill the volume of one grain of fine sand.
Construction accidents and delays
• On 25 October 2005, José Pereira Lages, a technician, was killed in the LHC when a switchgear that was being
transported fell on him.[65]
• On 27 March 2007 a cryogenic magnet support broke during a pressure test involving one of the LHC's innertriplet (focusing quadrupole) magnet assemblies, provided by Fermilab and KEK. No one was injured. Fermilab
director Pier Oddone stated "In this case we are dumbfounded that we missed some very simple balance of
forces". This fault had been present in the original design, and remained during four engineering reviews over the
following years.[66] Analysis revealed that its design, made as thin as possible for better insulation, was not strong
enough to withstand the forces generated during pressure testing. Details are available in a statement from
Fermilab, with which CERN is in agreement.[67] [68] Repairing the broken magnet and reinforcing the eight
identical assemblies used by LHC delayed the startup date, then planned for November 2007.
• Problems occurred on 19 September 2008 during powering tests of the main dipole circuit, when an electrical
fault in the bus between magnets caused a rupture and a leak of six tonnes of liquid helium. The operation was
delayed for several months.[69]
It is currently believed that a faulty electrical connection between two magnetscaused an arc, which compromised the liquid-helium containment. Once the cooling layer was broken, the helium
flooded the surrounding vacuum layer with sufficient force to break 10-ton magnets from their mountings. The
explosion also contaminated the proton tubes with soot.[39] [70] This accident was more recently thoroughly
discussed in a 22 February 2010 Superconductor Science and Technology article by CERN physicist Lucio
Rossi.[71]
• Two vacuum leaks were identified in July 2009, and the start of operations was further postponed to
mid-November 2009.[72]
Popular culture
The Large Hadron Collider has gained considerable attention from outside the scientific community and its progress
is followed by most popular science media. The LHC has also sparked the imaginations of authors of works of
fiction, such as novels, TV series, and video games, although descriptions of what it is, how it works, and projected
outcomes of the experiments are often only vaguely accurate, occasionally causing concern among the general
public.
The novel Angels & Demons, by Dan Brown, involves antimatter created at the LHC to be used in a weapon against
the Vatican. In response CERN published a "Fact or Fiction?" page discussing the accuracy of the book's portrayal of
the LHC, CERN, and particle physics in general.[73] The movie version of the book has footage filmed on-site at one
of the experiments at the LHC; the director, Ron Howard, met with CERN experts in an effort to make the science in
the story more accurate.[74]
In The Big Bang Theory, the episode The Large Hadron Collision features the Large Hadron Collider prominently.
[12] BBC News (30 March 2010). "CERN LHC sees high-energy success" (http://news. bbc. co. uk/2/hi/science/nature/8593780. stm). Press
release. . Retrieved 2010-03-30.
[13] Brian Greene (11 September 2008). "The Origins of the Universe: A Crash Course" (http://www. nytimes. com/2008/09/12/opinion/
12greene. html?_r=1&oref=slogin). The New York Times. . Retrieved 2009-04-17.
[14] "... in the public presentations of the aspiration of particle physics we hear too often that the goal of the LHC or a linear collider is to check
off the last missing particle of the Standard Model, this year's Holy Grail of particle physics, the Higgs boson. The truth is much less boring
than that! What we're trying to accomplish is much more exciting, and asking what the world would have been like without the Higgsmechanism is a way of getting at that excitement." – Chris Quigg (2005). "Nature's Greatest Puzzles". arΧiv:hep-ph/0502070 [hep-ph].
[16] "Zeroing in on the elusive Higgs boson" (http://www.science. doe. gov/Accomplishments_Awards/Decades_Discovery/35. html). US
Department of Energy. March 2001. . Retrieved 2008-12-11.
[17] "Accordingly, in common with many of my colleagues, I think it highly likely that both the Higgs boson and other new phenomena will be
found with the LHC."..."This mass threshold means, among other things, that something new —either a Higgs boson or other novel
phenomena —is to be found when the LHC turns the thought experiment into a real one." Chris Quigg (February 2008). "The coming
revolutions in particle physics" (http://www.scientificamerican.com/article.cfm?id=the-coming-revolutions-in-particle-physics&page=3).
Scientific American. pp. 38 –45. . Retrieved 2009-09-28.
[18] Shaaban Khalil (2003). "Search for supersymmetry at LHC". Contemporary Physics 44 (3): 193 –201. doi:10.1080/0010751031000077378.
[19] Alexander Belyaev (2009). "Supersymmetry status and phenomenology at the Large Hadron Collider". Pramana 72 (1): 143 –160.
doi:10.1007/s12043-009-0012-0.[20] Anil Ananthaswamy (11 November 2009). "In SUSY we trust: What the LHC is really looking for" (http://www. newscientist. com/
article/mg20427341. 200-in-susy-we-trust-what-the-lhc-is-really-looking-for. html). New Scientist. .
[32] Mark Henderson (10 September 2008). "Scientists cheer as protons complete first circuit of Large Hadron Collider" (http://www.timesonline. co. uk/tol/news/uk/science/article4722261. ece). Times Online (London). . Retrieved 2008-10-06.
[33] Mark Henderson (18 September 2008). "'Big bang machine' is back on collision course after its glitches are fixed" (http://www.
timesonline. co. uk/tol/news/uk/science/article4774817. ece). Times Online (London). . Retrieved 2009-09-28.
[34] CERN Press Office (9 February 2009). "CERN management confirms new LHC restart schedule" (http://press. web. cern. ch/press/
[58] Alan Boyle (2 September 2008). "Courts weigh doomsday claims" (http://cosmiclog. msnbc. msn. com/archive/2008/09/02/1326534.
aspx). Cosmic Log. MSNBC. . Retrieved 2009-09-28.
[59] J.-P. Blaizot, J. Iliopoulos, J. Madsen, G.G. Ross, P. Sonderegger, H.-J. Specht (2003). "Study of Potentially Dangerous Events During
Heavy-Ion Collisions at the LHC" (http://doc.cern.ch/yellowrep/2003/2003-001/p1. pdf). CERN. . Retrieved 2009-09-28.
[60] J. Ellis J, G. Giudice, M.L. Mangano, T. Tkachev, U. Wiedemann (LHC Safety Assessment Group) (5 September 2008). "Review of the
Safety of LHC Collisions". Journal of Physics G 35: 115004. doi:10.1088/0954-3899/35/11/115004. arXiv:0806.3414.
[61] "The safety of the LHC" (http://public. web. cern.ch/public/en/LHC/Safety-en. html). CERN. 2008. . Retrieved 2009-09-28.
[62] Division of Particles & Fields (http://www.aps.org/units/dpf/). "Statement by the Executive Committee of the DPF on the Safety of Collisions at the Large Hadron Collider" (http://www.aps.org/units/dpf/governance/reports/upload/lhc_saftey_statement. pdf). American
Physical Society. . Retrieved 2009-09-28.
[63] "Challenges in accelerator physics" (http://lhc.web. cern.ch/lhc/general/acphys.htm). CERN. 14 January 1999. . Retrieved 2009-09-28.
[64] John Poole (2004). "Beam Parameters and Definitions" (https://edms.cern.ch/file/445830/5/Vol_1_Chapter_2. pdf). .
[65] CERN Press Office (26 October 2005). "Message from the Director-General" (http://user. web. cern. ch/user/QuickLinks/
The ALICE Time Projection Chamber (TPC) is the main particle tracking device in ALICE. Charged particles
crossing the gas of the TPC ionize the gas atoms along their path, liberating electrons that drift towards the end
plates of the detector. An avalanche effect in the vicinity of the anode wires strung in the readout, will give the
necessary signal amplification. The positive ions created in the avalanche will induce a positive current signal on the
pad plane. The readout is done by the 557 568 pads that form the cathode plane of the multi-wire proportionalchambers (MWPC) located at the end plates. This gives the r and coordinates. The last coordinate, z, is given by
the drift time.
Transition Radiation Detector
The completed ALICE detector showing the
eighteen TRD modules (trapezoidal prisms in a
radial arrangement).
Electrons and positrons can be discriminated from other charged
particles using the emission of transition radiation, X-rays emitted
when the particles cross many layers of thin materials. To develop such
a Transition Radiation Detector (TRD) for ALICE many detector
prototypes were tested in mixed beams of pions and electrons.
Time of Flight
Charged particles are identified in ALICE by Time-Of-Flight (TOF);
heavier particles are slower and so take longer to reach the outer layers
of the detector. For its TOF system ALICE uses detectors called
Multigap Resistive Plate Chambers (MRPC). There are approximately
160 000 MRPC pads with time resolution of about 100 ps distributed
over the large surface of 150 square meters. Using the tracking information from other detectors every track firing a
sensor is identified.
Photon Spectrometer
The Photon Spectrometer (PHOS) is designed to measure the temperature of collisions by detecting photons
emerging from them. It will be made of lead tungstate crystals. When high energy photons strike lead tungstate, they
make it glow, or scintillate, and this glow can be measured. Lead tungstate is extremely dense (denser than iron),
stopping most photons that reach it.
High Momentum Particle Identification Detector
The High Momentum Particle Identification Detector (HMPID) is a RICH detector to determine the speed of particles beyond the momentum range available through energy loss (in ITS and TPC, p = 600 MeV) and through
time-of-flight measurements (in TOF, p = 1.2 –1.4 GeV). Its momentum range is up to 3 GeV for pion/kaon
discrimination and up to 5 GeV for kaon/proton discrimination. It is the world's largest caesium iodide RICH
detector, with an active area of 11 m². A prototype was successfully tested at CERN in 1997 and currently takes data
at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory in the US.
TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf LHC-forward
MoEDAL Monopole and Exotics Detector At the LHC
LHC preaccelerators
p and Pb Linear accelerators for protons (Linac 2) and Lead (Linac 3)
(not marked) Proton Synchrotron Booster
PS Proton Synchrotron
SPS Super Proton Synchrotron
Geographical coordinates: 46°14′8″N 6°3′19″E
ATLAS logo.
ATLAS (A Toroidal LHC ApparatuS) is one of the six particle detector
experiments (ALICE, ATLAS, CMS, TOTEM, LHCb, and LHCf) constructed at
the Large Hadron Collider (LHC), a new particle accelerator at the European
Organization for Nuclear Research (CERN) in Switzerland. ATLAS is 44 metres
long and 25 metres in diameter, weighing about 7,000 tonnes. The project is led by
Fabiola Gianotti and involves roughly 2,000 scientists and engineers at 165
institutions in 35 countries.[1] The construction was originally scheduled to be
completed in June 2007, but was ready and detected its first beam events on 10September 2008.[2] The experiment is designed to observe phenomena that involve
highly massive particles which were not observable using earlier lower-energy
accelerators and might shed light on new theories of particle physics beyond the
Standard Model.
The ATLAS collaboration, the group of physicists building the detector, was
formed in 1992 when the proposed EAGLE (Experiment for Accurate Gamma,
Lepton and Energy Measurements) and ASCOT (Apparatus with Super
COnducting Toroids) collaborations merged their efforts into building a single, general-purpose particle detector for
the Large Hadron Collider.[3] The design was a combination of those two previous designs, as well as the detectorresearch and development that had been done for the Superconducting Supercollider. The ATLAS experiment was
proposed in its current form in 1994, and officially funded by the CERN member countries beginning in 1995.
Additional countries, universities, and laboratories joined in subsequent years, and further institutions and physicists
continue to join the collaboration even today. The work of construction began at individual institutions, with detector
components shipped to CERN and assembled in the ATLAS experimental pit beginning in 2003.
ATLAS is designed as a general-purpose detector. When the proton beams produced by the Large Hadron Collider
interact in the center of the detector, a variety of different particles with a broad range of energies may be produced.Rather than focusing on a particular physical process, ATLAS is designed to measure the broadest possible range of
signals. This is intended to ensure that, whatever form any new physical processes or particles might take, ATLAS
will be able to detect them and measure their properties. Experiments at earlier colliders, such as the Tevatron and
Large Electron-Positron Collider, were designed based on a similar philosophy. However, the unique challenges of
the Large Hadron Collider —its unprecedented energy and extremely high rate of collisions —require ATLAS to be
larger and more complex than any detector ever built.
Background
ATLAS experiment detector under construction
in October 2004 in its experimental pit; the
current status of construction can be seen on theCERN website.
[4]Note the people in the
background, for comparison.
The first cyclotron, an early type of particle accelerator, was built by
Ernest O. Lawrence in 1931, with a radius of just a few centimetres
and a particle energy of 1 megaelectronvolt (MeV). Since then,
accelerators have grown enormously in the quest to produce new
particles of greater and greater mass. As accelerators have grown, so
too has the list of known particles that they might be used to
investigate. The most comprehensive model of particle interactions
available today is known as the Standard Model of Particle Physics.
With the important exception of the Higgs boson, all of the particles
predicted by the model have been observed. While the standard model
predicts that quarks, electrons, and neutrinos should exist, it does notexplain why the masses of the particles are so very different. Due to
this violation of "naturalness" most particle physicists believe it is
possible that the Standard Model will break down at energies beyond
the current energy frontier of about one teraelectronvolt (TeV) (set at the Tevatron). If such
beyond-the-Standard-Model physics is observed it is hoped that a new model, which is identical to the Standard
Model at energies thus far probed, can be developed to describe particle physics at higher energies. Most of the
currently proposed theories predict new higher-mass particles, some of which are hoped to be light enough to be
observed by ATLAS. At 27 kilometres in circumference, the Large Hadron Collider (LHC) will collide two beams of
protons together, each proton carrying about 7 TeV of energy — enough energy to produce particles with masses up
to roughly ten times more massive than any particles currently known — assuming of course that such particlesexist. With an energy seven million times that of the first accelerator the LHC represents a "new generation" of
particle accelerators.
Particles that are produced in accelerators must also be observed, and this is the task of particle detectors. While
interesting phenomena may occur when protons collide it is not enough to just produce them. Particle detectors must
be built to detect particles, their masses, momentum, energies, charges, and nuclear spins. In order to identify all
particles produced at the interaction point where the particle beams collide, particle detectors are usually designed
with a similarity to an onion. The layers are made up of detectors of different types, each of which is adept at
observing specific types of particles. The different features that particles leave in each layer of the detector allow for
effective particle identification and accurate measurements of energy and momentum. (The role of each layer in thedetector is discussed below.) As the energy of the particles produced by the accelerator increases, the detectors
attached to it must grow to effectively measure and stop higher-energy particles. ATLAS is the largest detector ever
A schematic, called a Feynman diagram, of twovirtual gluons from colliding LHC protons
interacting to produce a hypothetical Higgs
boson, a top quark, and an antitop quark. These in
turn decay into a specific combination of quarks
and leptons that is very unlikely to be duplicated
by other processes. Collecting sufficient evidence
of signals like this one may eventually allow
ATLAS collaboration members to discover the
Higgs boson.
ATLAS is intended to investigate many different types of physics that
might become detectable in the energetic collisions of the LHC. Some
of these are confirmations or improved measurements of the StandardModel, while many others are searches for new physical theories.
One of the most important goals of ATLAS is to investigate a missing
piece of the Standard Model, the Higgs boson.[5] The Higgs
mechanism, which includes the Higgs boson, is invoked to give masses
to elementary particles, giving rise to the differences between the weak
force and electromagnetism by giving the W and Z bosons masses
while leaving the photon massless. If the Higgs boson is not discovered
by ATLAS, it is expected that another mechanism of electroweak
symmetry breaking that explains the same phenomena, such astechnicolour, will be discovered. The Standard Model is simply not
mathematically consistent at the energies of the LHC without such a
mechanism. The Higgs boson would be detected by the particles it
decays into; the easiest to observe are two photons, two bottom quarks,
or four leptons. Sometimes these decays can only be definitively
identified as originating with the Higgs boson when they are associated
with additional particles; for an example of this, see the diagram at
right.
The asymmetry between the behavior of matter and antimatter, known as CP violation, will also be investigated.[5]
Current CP-violation experiments, such as BaBar and Belle, have not yet detected sufficient CP violation in the
Standard Model to explain the lack of detectable antimatter in the universe. It is possible that new models of physics
will introduce additional CP violation, shedding light on this problem; these models might either be detected directly
by the production of new particles, or indirectly by measurements made of the properties of B-mesons. (LHCb, an
LHC experiment dedicated to B-mesons, is likely to be better suited to the latter).[6]
The top quark, discovered at Fermilab in 1995, has thus far had its properties measured only approximately. With
much greater energy and greater collision rates, LHC will produce a tremendous number of top quarks, allowing
ATLAS to make much more precise measurements of its mass and interactions with other particles. [7] These
measurements will provide indirect information on the details of the Standard Model, perhaps revealing
inconsistencies that point to new physics. Similar precision measurements will be made of other known particles; forexample, ATLAS may eventually measure the mass of the W boson twice as accurately as has previously been
achieved.
Perhaps the most exciting lines of investigation are those searching directly for new models of physics. One theory
that is the subject of much current research is broken supersymmetry. The theory is popular because it could
potentially solve a number of problems in theoretical physics and is present in almost all models of string theory.
Models of supersymmetry involve new, highly massive particles; in many cases these decay into high-energy quarks
and stable heavy particles that are very unlikely to interact with ordinary matter. The stable particles would escape
the detector, leaving as a signal one or more high-energy quark jets and a large amount of "missing" momentum.
Other hypothetical massive particles, like those in Kaluza-Klein theory, might leave a similar signature, but its
discovery would certainly indicate that there was some kind of physics beyond the Standard Model.
One remote possibility (if the universe contains large extra dimensions) is that microscopic black holes might be
produced by the LHC.[8] These would decay immediately by means of Hawking radiation, producing all particles in
the Standard Model in equal numbers and leaving an unequivocal signature in the ATLAS detector.[9] In fact, if this
occurs, the primary studies of Higgs bosons and top quarks would be conducted on those produced by the black
holes.
Components
The ATLAS detector consists of a series of ever-larger concentric cylinders around the interaction point where the
proton beams from the LHC collide. It can be divided into four major parts: the Inner Detector, the calorimeters, the
muon spectrometer and the magnet systems.[10] Each of these is in turn made of multiple layers. The detectors are
complementary: the Inner Detector tracks particles precisely, the calorimeters measure the energy of easily stopped
particles, and the muon system makes additional measurements of highly penetrating muons. The two magnet
systems bend charged particles in the Inner Detector and the muon spectrometer, allowing their momenta to be
measured.
The only established stable particles that cannot be detected directly are neutrinos; their presence is inferred by
noticing a momentum imbalance among detected particles. For this to work, the detector must be "hermetic", and
detect all non-neutrinos produced, with no blind spots. Maintaining detector performance in the high radiation areas
immediately surrounding the proton beams is a significant engineering challenge.
Inner detector
The ATLAS TRT central section, the outermost
part of the Inner Detector, as of September 2005,
assembled on the surface and taking data from
cosmic rays.
The Inner Detector begins a few centimetres from the proton beam
axis, extends to a radius of 1.2 metres, and is seven metres in length
along the beam pipe. Its basic function is to track charged particles by
detecting their interaction with material at discrete points, revealing
detailed information about the type of particle and its momentum.[11]
The magnetic field surrounding the entire inner detector causes
charged particles to curve; the direction of the curve reveals a particle's
charge and the degree of curvature reveals its momentum. The starting
points of the tracks yield useful information for identifying particles;
for example, if a group of tracks seem to originate from a point other
than the original proton –proton collision, this may be a sign that the
particles came from the decay of a bottom quark (see B-tagging). The
Inner Detector has three parts, which are explained below.
The Pixel Detector, the innermost part of the detector, contains three layers and three disks on each end-cap, with a
total of 1744 modules, each measuring two centimetres by six centimetres. The detecting material is 250 µm thick
silicon. Each module contains 16 readout chips and other electronic components. The smallest unit that can be read
out is a pixel (each 50 by 400 micrometres); there are roughly 47,000 pixels per module. The minute pixel size is
designed for extremely precise tracking very close to the interaction point. In total, the Pixel Detector will have over
80 million readout channels, which is about 50% of the total readout channels; such a large count created a design
and engineering challenge. Another challenge was the radiation the Pixel Detector will be exposed to because of its
proximity to the interaction point, requiring that all components be radiation hardened in order to continue operating
after significant exposures.
The Semi-Conductor Tracker (SCT) is the middle component of the inner detector. It is similar in concept and
function to the Pixel Detector but with long, narrow strips rather than small pixels, making coverage of a larger areapractical. Each strip measures 80 micrometres by 12.6 centimetres. The SCT is the most critical part of the inner
detector for basic tracking in the plane perpendicular to the beam, since it measures particles over a much larger area
than the Pixel Detector, with more sampled points and roughly equal (albeit one dimensional) accuracy. It is
composed of four double layers of silicon strips, and has 6.2 million readout channels and a total area of 61 square
meters.
The Transition radiation tracker (TRT), the outermost component of the inner detector, is a combination of a straw
tracker and a transition radiation detector. The detecting elements are drift tubes (straws), each four millimetres indiameter and up to 144 centimetres long. The uncertainty of track position measurements (position resolution) is
about 200 micrometres, not as precise as those for the other two detectors, a necessary sacrifice for reducing the cost
of covering a larger volume and having transition radiation detection capability. Each straw is fil led with gas that
becomes ionized when a charged particle passes through. The straws are held at about -1500V, driving the negative
ions to a fine wire down the centre of each straw, producing a current pulse (signal) in the wire. The wires with
signals create a pattern of 'hit' straws that allow the path of the particle to be determined. Between the straws,
materials with widely varying indices of refraction cause ultra-relativistic charged particles to produce transition
radiation and leave much stronger signals in some straws. Xenon gas is used to increase the number of straws with
strong signals. Since the amount of transition radiation is greatest for highly relativistic particles (those with a speed
very near the speed of light), and particles of a particular energy have a higher speed the lighter they are, particlepaths with many very strong signals can be identified as the lightest charged particles, electrons. The TRT has about
298,000 straws in total.
Calorimeters
September 2005: the main barrel section of the
ATLAS hadronic calorimeter, waiting to be
moved inside the toroid magnets.
One of the sections of the extensions of the
hadronic calorimeter, waiting to be inserted in
late February 2006
The calorimeters are situated outside the solenoidal magnet that
surrounds the inner detector. Their purpose is to measure the energy
from particles by absorbing it. There are two basic calorimeter
systems: an inner electromagnetic calorimeter and an outer hadronic
calorimeter.[12] Both are sampling calorimeters; that is, they absorb
energy in high-density metal and periodically sample the shape of the
resulting particle shower, inferring the energy of the original particle
from this measurement.
The electromagnetic (EM) calorimeter absorbs energy from particles
that interact electromagnetically, which include charged particles and
photons. It has high precision, both in the amount of energy absorbed
and in the precise location of the energy deposited. The angle between
the particle's trajectory and the detector's beam axis (or more precisely
the pseudorapidity) and its angle within the perpendicular plane are
both measured to within roughly 0.025 radians. The energy-absorbingmaterials are lead and stainless steel, with liquid argon as the sampling
material, and a cryostat is required around the EM calorimeter to keep
it sufficiently cool.
The hadron calorimeter absorbs energy from particles that pass through
the EM calorimeter, but do interact via the strong force; these particles
are primarily hadrons. It is less precise, both in energy magnitude and
in the localization (within about 0.1 radians only).[6] The
energy-absorbing material is steel, with scintillating tiles that sample
the energy deposited. Many of the features of the calorimeter are chosen for their cost-effectiveness; the instrument
is large and comprises a huge amount of construction material: the main part of the calorimeter —the tile
calorimeter —is eight metres in diameter and covers 12 metres along the beam axis. The far-forward sections of the
hadronic calorimeter are contained within the EM calorimeter's cryostat, and use liquid argon as it does.
Muon spectrometer
The muon spectrometer is an extremely large tracking system, extending from a radius of 4.25 m around thecalorimeters out to the full radius of the detector (11 m).[10] Its tremendous size is required to accurately measure the
momentum of muons, which penetrate other elements of the detector; the effort is vital because one or more muons
are a key element of a number of interesting physical processes, and because the total energy of particles in an event
could not be measured accurately if they were ignored. It functions similarly to the inner detector, with muons
curving so that their momentum can be measured, albeit with a different magnetic field configuration, lower spatial
precision, and a much larger volume. It also serves the function of simply identifying muons —very few particles of
other types are expected to pass through the calorimeters and subsequently leave signals in the muon spectrometer. It
has roughly one million readout channels, and its layers of detectors have a total area of 12,000 square meters.
Magnet system
The ends of four of eight ATLAS toroid magnets,seen from the surface, about 90 metres above, in
September 2005.
The ATLAS detector uses two large superconducting magnet systems
to bend charged particles so that their momenta can be measured. This
bending is due to the Lorentz force, which is proportional to velocity.
Since all particles produced in the LHC's proton collisions will be
traveling at very close to the speed of light, the force on particles of
different momenta is equal. (In the theory of relativity, momentum is
not proportional to velocity at such speeds.) Thus high-momentum
particles will curve very little, while low-momentum particles will
curve significantly; the amount of curvature can be quantified and the
particle momentum can be determined from this value.
The inner solenoid produces a two tesla magnetic field surrounding the
Inner Detector.[13] This high magnetic field allows even very energetic
particles to curve enough for their momentum to be determined, and its nearly uniform direction and strength allow
measurements to be made very precisely. Particles with momenta below roughly 400 MeV will be curved so strongly
that they will loop repeatedly in the field and most likely not be measured; however, this energy is very small
compared to the several TeV of energy released in each proton collision.
The outer toroidal magnetic field is produced by eight very large air-core superconducting barrel loops and two
end-caps, all situated outside the calorimeters and within the muon system.[13] This magnetic field is 26 metres long
and 20 metres in diameter, and it stores 1.6 gigajoules of energy. Its magnetic field is not uniform, because a
solenoid magnet of sufficient size would be prohibitively expensive to build. Fortunately, measurements need to be
much less precise to measure momentum accurately in the large volume of the muon system.
The ATLAS detector will be complemented with a set of detectors in
the very forward region. These detectors will be located in the LHC
tunnel far away from the interaction point. The basic idea is to measure
elastic scattering at very small angles in order to get a handle on theabsolute luminosity at the interaction point of ATLAS.
Data systems and analysis
The detector generates unmanageably large amounts of raw data, about 25 megabytes per event (raw; zero
suppression reduces this to 1.6 MB) times 40 million beam crossings per second in the center of the detector, for a
total of 1 petabyte/second of raw data.[14] The trigger system uses simple information to identify, in real time, the
most interesting events to retain for detailed analysis. There are three trigger levels, the first based in electronics on
the detector and the other two primarily run on a large computer cluster near the detector. After the first-level trigger,
about 100,000 events per second have been selected. After the third-level trigger, a few hundred events remain to be
stored for further analysis. This amount of data will require over 100 megabytes of disk space per second — at least
a petabyte each year.[15]
Offline event reconstruction will be performed on all permanently stored events, turning the pattern of signals fromthe detector into physics objects, such as jets, photons, and leptons. Grid computing will be extensively used for
event reconstruction, allowing the parallel use of university and laboratory computer networks throughout the world
for the CPU-intensive task of reducing large quantities of raw data into a form suitable for physics analysis. The
software for these tasks has been under development for many years, and will continue to be refined once the
experiment is running.
Individuals and groups within the collaboration will write their own code to perform further analysis of these objects,
searching in the pattern of detected particles for particular physical models or hypothetical particles. These studies
are already being developed and tested on detailed simulations of particles and their interactions with the detector.
Such simulations give physicists a good sense of which new particles can be detected and how long it will take to
confirm them with sufficient statistical certainty.
[4] "UX15 Installation; WEB cameras" (http://atlaseye-webpub.web. cern. ch/atlaseye-webpub/web-sites/pages/UX15_webcams. htm).
ATLAS Control Room. cern.ch. . Retrieved September 15, 2010.[5] "Introduction and Overview" (http://atlas. web. cern.ch/Atlas/TP/NEW/HTML/tp9new/node4.
The purpose of the Hadronic Calorimeter (HCAL) is both to
measure the energy of individual hadrons produced in each event,
and to be as near to hermetic around the interaction region as
possible to allow events with missing energy to be identified.
The HCAL consists of layers of dense material (brass or steel)
interleaved with tiles of plastic scintillators, read out via
wavelength-shifting fibres by hybrid photodiodes. This
combination was determined to allow the maximum amount of
absorbing material inside of the magnet coil.
The high pseudorapidity region is instrumented by the Hadronic Forward detector. Located
11 m either side of the interaction point, this uses a slightly different technology of steel absorbers and quartz fibresfor readout, designed to allow better separation of particles in the congested forward region.
The brass used in the endcaps of the HCAL used to be Russian artillery shells.[4]
Layer 4 – The magnet
Like most particle physics detectors, CMS has a large solenoid magnet. This allows the charge/mass ratio of particles
to be determined from the curved track that they follow in the magnetic field. It is 13 m long and 6 m in diameter,
and its refrigerated superconducting niobium-titanium coils were originally intended to produce a 4 T magnetic field.
It was recently announced that the magnet will run at 3.8 T instead of the full design strength in order to maximize
longevity.[5]
The inductance of the magnet is 14 Η and the nominal current for 4 T is 19,500 A, giving a total stored energy of 2.66 GJ, equivalent to about half-a-tonne of TNT. There are dump circuits to safely dissipate this energy should the
magnet quench. The circuit resistance (essentially just the cables from the power converter to the cryostat) has a
value of 0.1 mΩ which leads to a circuit time constant of nearly 39 hours. This is the longest time constant of any
circuit at CERN. The operating current for 3.8 T is 18,160 A, giving a stored energy of 2.3 GJ.
while a small amount of key information is used to perform a fast, approximate calculation to identify features of
interest such as high energy jets, muons or missing energy. This "Level 1" calculation is completed in around 1 µs,
and event rate is reduced by a factor of about thousand down to 50 kHz. All these calculations are done on fast,
custom hardware using reprogrammable FPGAs.
If an event is passed by the Level 1 trigger all the data still buffered in the detector is sent over fibre-optic links to
the "High Level" trigger, which is software (mainly written in C++) running on ordinary computer servers. Thelower event rate in the High Level trigger allows time for much more detailed analysis of the event to be done than
in the Level 1 trigger. The High Level trigger reduces the event rate by a further factor of about a thousand down to
around 100 events per second. These are then stored on tape for future analysis.
Data analysis
Data that has passed the triggering stages and been stored on tape is duplicated using the Grid to additional sites
around the world for easier access and redundancy. Physicists are then able to use the Grid to access and run their
analyses on the data.
Some possible analyses might be:
• Looking at events with large amounts of apparently missing energy, which implies the presence of particles that
have passed through the detector without leaving a signature, such as neutrinos.
• Looking at the kinematics of pairs of particles produced by the decay of a parent, such as the Z boson decaying to
a pair of electrons or the Higgs boson decaying to a pair of tau leptons or photons, to determine the properties and
mass of the parent.
• Looking at jets of particles to study the way the quarks in the collided protons have interacted.
Milestones
1998 Construction of surface buildings for CMS begins.2000 LEP shut down, construction of cavern begins.
2004 Cavern completed.
10 September 2008 First beam in CMS.
23 November 2009 First collisions in CMS.
30 March 2010 First 7 TeV collisions in CMS.
The insertion of the vacuum tank,
June 2002
YE+2 descent into the cavern YE+1, a component of CMS
TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf LHC-forward
MoEDAL Monopole and Exotics Detector At the LHC
LHC preaccelerators
p and Pb Linear accelerators for protons (Linac 2) and Lead (Linac 3)
(not marked) Proton Synchrotron Booster
PS Proton SynchrotronSPS Super Proton Synchrotron
The LHCb (standing for "Large Hadron Collider beauty" where "beauty" refers to the bottom quark) experiment is
one of six particle physics detector experiments built on the Large Hadron Collider accelerator at CERN. LHCb is a
specialized b-physics experiment, particularly aimed at measuring the parameters of CP violation in the interactions
of b-hadrons (heavy particles containing a bottom quark).
The LHCb detector
The fact that both B hadrons are predominantly produced in the same forward cone as B meson production is
exploited in the layout of the LHCb detector. The LHCb detector is a single arm forward spectrometer with a polar
angular coverage from 10 to 300 milliradians (mrad) in the horizontal and 250 mrad in the vertical plane. The
asymmetry between the horizontal and vertical plane is determined by a large dipole magnet with the main
component in the vertical direction.
The vertex detector (known as the vertex locator or VELO) is built around the proton interaction region. It is used to
measure the particle trajectories close to the interaction point in order to precisely separate primary and secondary
vertices, e.g. for B-tagging.
The RICH-1 detector (Ring imaging Cherenkov detector) is located directly after the vertex detector. It is used for
particle identification of low-momentum tracks.
The main tracking system is placed before and after the dipole magnet. It is used to reconstruct the trajectories of charged particles and to measure their momenta.
Following the tracking system is RICH-2. It allows the identification of the particle type of high-momentum tracks.
The electromagnetic and hadronic calorimeters provide measurement of the energy of electrons, photons, and
hadrons. These measurements are used at trigger level to identify the particles with high transversal moment (high-Pt
particles).
The muon system is used to identify and trigger on muons in the events.
LHCb Physics analyses
After the LHC starts colliding protons at a useful rate for LHCb, in early 2010, LHCb aims to make several
measurements on physics phenomena involving B mesons as an early priority. These include:
• Measuring an upper limit on the branching ratio of the rare decay.• Measuring the forward-backward asymmetry of the muon pair in the flavour changing neutral current
decay. Such a flavour changing neutral current cannot occur at tree-level in the Standard
Model of Particle Physics, and only occurs through box and loop Feynman diagrams; properties of the decay canbe strongly modified by new Physics.• Measuring the CP violating phase in the decay , caused by interference between the decays with
and without oscillations. This phase is one of the CP observables with the smallest theoretical
uncertainty in the Standard Model, and can be significantly modified by new Physics.• Measuring properties of radiative B decays, i.e. B meson decays with photons in the final states. Specifically,
these are again flavour changing neutral current decays.
LHC Computing GridThe LHC Computing Grid, launched on October 3, 2008,[1] is a distribution network designed by CERN to handle
the massive amounts of data produced by the Large Hadron Collider (LHC). It incorporates both private fiber optic
cable links and existing high-speed portions of the public Internet.
The data stream from the detectors provides approximately 300 GB/s, which is filtered for "interesting events",
resulting in a "raw data" stream of about 300 MB/s. The CERN computer center, considered "Tier 0" of the LHC
Computing Grid, has a dedicated 10 Gb/s connection to the counting room.[2]
The project is expected to generate 27 TB of raw data per day, plus 10 TB of “event summary data”, which represents
the output of calculations done by the CPU farm at the CERN data center. [2] This data is sent out from CERN to
eleven Tier 1 academic institutions in Europe, Asia, and North America, via dedicated 10 Gbit/s links. More than150 Tier 2 institutions are connected to the Tier 1 institutions by general-purpose national research and education
networks.[2] The data produced by the LHC on all of its distributed computing grid is expected to add up to 10 –15
PB of data each year.[3]
The Tier 1 institutions receive specific subsets of the raw data, for which they serve as a backup repository for
CERN. They also perform reprocessing when recalibration is necessary.[2] The primary configuration for the
computers used in the grid is based on Scientific Linux.
Distributed computing resources for analysis by end-user physicists are provided by the Open Science Grid,
Enabling Grids for E-sciencE,[2] and LHC@home projects.
LHC@home is a distributed computing project using the BOINC framework, run by volunteers for the CERN in
Switzerland. Its goal is to help maintain and improve the Large Hadron Collider (LHC), a CERN project to create a
large particle accelerator which became active in September 2008. Data from the project is used by engineers to
improve the operation and efficiency of the accelerator, and to predict problems that could arise from adjustment ormodification of the LHC's equipment. The project is administered by volunteers, and receives no funding from
CERN.
BOINC users who are considering joining this project should know that it only occasionally has work; the project is
used for design and repair considerations related to the LHC. There are currently no plans to use the project to do
computation on the data that will be collected by the LHC.
A CERN collider project
The project was first introduced as a beta on September 1, 2004 and a record 1000 users signed up within 24 hours.
The project went public, with a 5000 user limit, on September 29 to commemorate CERN's 50th anniversary.Currently there is no user limit and qualification.
Project software
The project software involves a program called "SixTrack", created by Frank Schmidt, downloaded via BOINC onto
participant computers running Windows or Linux. SixTrack simulates particles accelerating through the 27 km (17
mi)-long LHC to find their orbit stability.
• In one workunit, 60 particles are simulated travelling 100,000 or 1,000,000 loops, which would take about 10
seconds in an actual run. This is sixtrack.
• The orbit stability data is used to detect if a particle in orbit goes off-course and runs into the tube wall —if this
happened too often in actual running, this would cause damage to the accelerator which would need repairs.
• A new experimental version called SixTrackbnl started to be sent to computers in early November.
• Garfield is a newer application, although not many workunits have been seen lately.
See also
• List of distributed computing projects
• LHC Computing Grid
External links
• LHC@home Project Page [1]
• Berkeley Open Infrastructure for Network Computing (BOINC) [2]
The Standard Model of elementary particles, with the gauge bosons in the
rightmost column.
The Standard Model of particle physics is
a theory concerning the electromagnetic,
weak, and strong nuclear interactions, which
mediate the dynamics of the known
subatomic particles. Developed throughout
the early and middle 20th century, the
current formulation was finalized in the mid
1970s upon experimental confirmation of
the existence of quarks. Since then,discoveries of the bottom quark (1977), the
top quark (1995) and the tau neutrino (2000)
have given credence to the standard model.
Because of its success in explaining a wide
variety of experimental results, the standard
model is sometimes regarded as a theory of
almost everything.
Still, the standard model falls short of being
a complete theory of fundamentalinteractions because it does not incorporate
the physics of general relativity, such as
gravitation and dark energy. The theory
does not contain any viable dark matter
particle that possesses all of the required properties deduced from observational cosmology. It also does not correctly
account for neutrino oscillations (and their non-zero masses). Although the standard model is theoretically
self-consistent, it has several unnatural properties giving rise to puzzles like the strong CP problem and the hierarchy
problem.
Nevertheless, the standard model is important to theoretical and experimental particle physicists alike. For
theoreticians, the standard model is a paradigm example of a quantum field theory, which exhibits a wide range of physics including spontaneous symmetry breaking, anomalies, non-perturbative behavior, etc. It is used as a basis for
building more exotic models which incorporate hypothetical particles, extra dimensions and elaborate symmetries
(such as supersymmetry) in an attempt to explain experimental results at variance with the Standard Model such as
the existence of dark matter and neutrino oscillations. In turn, the experimenters have incorporated the standard
model into simulators to help search for new physics beyond the standard model from relatively uninteresting
background.
Recently, the standard model has found applications in other fields besides particle physics such as astrophysics and
The first step towards the Standard Model was Sheldon Glashow's discovery, in 1960, of a way to combine the
electromagnetic and weak interactions.[1] In 1967, Steven Weinberg[2] and Abdus Salam[3] incorporated the Higgs
mechanism[4] [5] [6] into Glashow's electroweak theory, giving it its modern form.
The Higgs mechanism is believed to give rise to the masses of all the elementary particles in the Standard Model.
This includes the masses of the W and Z bosons, and the fermions. The Higgs mechanism is also believed to giverise to the masses of quarks and leptons.
After the neutral weak currents caused by Z boson exchange were discovered at CERN in 1973, [7] [8] [9] [10] the
electroweak theory became widely accepted and Glashow, Salam, and Weinberg shared the 1979 Nobel Prize in
Physics for discovering it. The W and Z bosons were discovered experimentally in 1981, and their masses were
found to be as the Standard Model predicted.
The theory of the strong interaction, to which many contributed, acquired its modern form around 1973 –74, when
experiments confirmed that the hadrons were composed of fractionally charged quarks.
OverviewAt present, matter and energy are best understood in terms of the kinematics and interactions of elementary particles.
To date, physics has reduced the laws governing the behavior and interaction of all known forms of matter and
energy to a small set of fundamental laws and theories. A major goal of physics is to find the "common ground" that
would unite all of these theories into one integrated theory of everything, of which all the other known laws would
be special cases, and from which the behavior of all matter and energy could be derived (at least in principle).[11]
The Standard Model groups two major extant theories — quantum electroweak and quantum chromodynamics —
into an internally consistent theory that describes the interactions between all known particles in terms of quantum
field theory. For a technical description of the fields and their interactions, see Standard Model (mathematical
formulation).
Particle content
Elementary particles: fermions
Organization of Fermions
Charge First generation Second generation Third generation
Quarks +2 ⁄ 3
Up u Charm c Top t
−1 ⁄ 3
Down d Strange s Bottom b
Leptons −1 Electron e− Muon μ− Tau τ−
0 Electron neutrino νe
Muon neutrino νμ
Tau neutrino ντ
The Standard Model includes 12 elementary particles of spin-1 ⁄ 2
known as fermions. According to the spin-statistics
theorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle.
The fermions of the Standard Model are classified according to how they interact (or equivalently, by what charges
they carry). There are six quarks (up, down, charm, strange, top, bottom), and six leptons (electron, electron
neutrino, muon, muon neutrino, tau, tau neutrino). Pairs from each classification are grouped together to form a
generation, with corresponding particles exhibiting similar physical behavior (see table).
The defining property of the quarks is that they carry color charge, and hence, interact via the strong interaction. A
phenomenon called color confinement results in quarks being perpetually (or at least since very soon after the start of
the Big Bang) bound to one another, forming color-neutral composite particles (hadrons) containing either a quark
and an antiquark (mesons) or three quarks (baryons). The familiar proton and the neutron are the two baryons having
the smallest mass. Quarks also carry electric charge and weak isospin. Hence they interact with other fermions both
electromagnetically and via the weak nuclear interaction.
The remaining six fermions do not carry color charge and are called leptons. The three neutrinos do not carry electric
charge either, so their motion is directly influenced only by the weak nuclear force, which makes them notoriously
difficult to detect. However, by virtue of carrying an electric charge, the electron, muon, and tau all interact
electromagnetically.
Each member of a generation has greater mass than the corresponding particles of lower generations. The first
generation charged particles do not decay; hence all ordinary (baryonic) matter is made of such particles.
Specifically, all atoms consist of electrons orbiting atomic nuclei ultimately constituted of up and down quarks.
Second and third generations charged particles, on the other hand, decay with very short half lives, and are observed
only in very high-energy environments. Neutrinos of all generations also do not decay, and pervade the universe, but
rarely interact with baryonic matter.
Force mediating particles
Summary of interactions between particles described by the Standard Model.
Interactions in physics are the ways
that particles influence other particles.
At a macroscopic level,
electromagnetism allows particles to
interact with one another via electric
and magnetic fields, and gravitation
allows particles with mass to attractone another in accordance with
Einstein's general relativity. The
standard model explains such forces as
resulting from matter particles
exchanging other particles, known as
force mediating particles (Strictly
speaking, this is only so if interpreting
literally what is actually an
approximation method known as
perturbation theory, as opposed to theexact theory). When a force mediating particle is exchanged, at a macroscopic level the effect is equivalent to a force
influencing both of them, and the particle is therefore said to have mediated (i.e., been the agent of) that force. The
Feynman diagram calculations, which are a graphical form of the perturbation theory approximation, invoke "force
mediating particles" and when applied to analyze high-energy scattering experiments are in reasonable agreement
with the data. Perturbation theory (and with it the concept of "force mediating particle") in other situations fails.
These include low-energy QCD, bound states, and solitons.
The known force mediating particles described by the Standard Model also all have spin (as do matter particles), but
in their case, the value of the spin is 1, meaning that all force mediating particles are bosons. As a result, they do not
follow the Pauli exclusion principle. The different types of force mediating particles are described below.
• Photons mediate the electromagnetic force between electrically charged particles. The photon is massless and is
well-described by the theory of quantum electrodynamics.
• The W+, W−, and Z gauge bosons mediate the weak interactions between particles of different flavors (all quarks
and leptons). They are massive, with the Z being more massive than the W±. The weak interactions involving the
W± act on exclusively left-handed particles and right-handed antiparticles. Furthermore, the W± carry an electric
charge of +1 and −1 and couple to the electromagnetic interactions. The electrically neutral Z boson interacts withboth left-handed particles and antiparticles. These three gauge bosons along with the photons are grouped together
which collectively mediate the electroweak interactions.
• The eight gluons mediate the strong interactions between color charged particles (the quarks). Gluons are
massless. The eightfold multiplicity of gluons is labeled by a combination of color and an anticolor charge (e.g.,
red –antigreen).[12] Because the gluon has an effective color charge, they can interact among themselves. The
gluons and their interactions are described by the theory of quantum chromodynamics.
The interactions between all the particles described by the Standard Model are summarized by the diagram at the top
of this section.
The Higgs boson
The Higgs particle is a hypothetical massive scalar elementary particle theorized by Robert Brout, François Englert,
Peter Higgs, Gerald Guralnik, C. R. Hagen, and Tom Kibble in 1964 (see 1964 PRL symmetry breaking papers) and
is a key building block in the Standard Model.[13] [14] [15] [16] It has no intrinsic spin, and for that reason is classified
as a boson (like the force mediating particles, which have integer spin). Because an exceptionally large amount of
energy and beam luminosity are theoretically required to observe a Higgs boson in high energy colliders, it is the
only fundamental particle predicted by the Standard Model that has yet to be observed.
The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, the
photon and gluon excepted, are massive. In particular, the Higgs boson would explain why the photon has no mass,
while the W and Z bosons are very heavy. Elementary particle masses, and the differences betweenelectromagnetism (mediated by the photon) and the weak force (mediated by the W and Z bosons), are critical to
many aspects of the structure of microscopic (and hence macroscopic) matter. In electroweak theory, the Higgs
boson generates the masses of the leptons (electron, muon, and tau) and quarks.
As yet, no experiment has directly detected the existence of the Higgs boson. It is hoped that the Large Hadron
Collider at CERN will confirm the existence of this particle. It is also possible that the Higgs boson may already
have been produced but overlooked.[17]
Field content
The standard model has the following fields:
Spin 1
1. A U(1) gauge field B μν
with coupling g′ (weak U(1), or weak hypercharge)
2. An SU(2) gauge field W μν
with coupling g (weak SU(2), or weak isospin)
3. An SU(3) gauge field G μν
with coupling gs
(strong SU(3), or color charge)
Spin1 ⁄
2
The spin 1 ⁄ 2
particles are in representations of the gauge groups. For the U(1) group, we list the value of the weak
hypercharge instead. The left-handed fermionic fields are:
1. An SU(3) triplet, SU(2) doublet, with U(1) weak hypercharge 1 ⁄ 3
(left-handed quarks)
2. An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge 2 ⁄ 3
3. An SU(3) singlet, SU(2) doublet with U(1) weak hypercharge −1 (left-handed lepton)
4. An SU(3) triplet, SU(2) singlet, with U(1) weak hypercharge −4 ⁄ 3
(left-handed up-type antiquark)
5. An SU(3) singlet, SU(2) singlet with U(1) weak hypercharge 2 (left-handed antilepton)
By CPT symmetry, there is a set of right-handed fermions with the opposite quantum numbers.
This describes one generation of leptons and quarks, and there are three generations, so there are three copies of each
field. Note that there are twice as many left-handed lepton field components as left-handed antilepton fieldcomponents in each generation, but an equal number of left-handed quark and antiquark fields.
Spin 0
1. An SU(2) doublet H with U(1) hyper-charge −1 (Higgs field)
Note that | H |2, summed over the two SU(2) components, is invariant under both SU(2) and under U(1), and so it can
appear as a renormalizable term in the Lagrangian, as can its square.
This field acquires a vacuum expectation value, leaving a combination of the weak isospin, I3, and weak hypercharge
unbroken. This is the electromagnetic gauge group, and the photon remains massless. The standard formula for the
electric charge (which defines the normalization of the weak hypercharge, Y , which would otherwise be somewhatarbitrary) is:[18]
Lagrangian
The Lagrangian for the spin 1 and spin 1 ⁄ 2
fields is the most general renormalizable gauge field Lagrangian with no
fine tunings:
• Spin 1:
where the traces are over the SU(2) and SU(3) indices hidden in W and G respectively. The two-index objects are the
field strengths derived from W and G the vector fields. There are also two extra hidden parameters: the theta angles
for SU(2) and SU(3).
The spin-1 ⁄ 2
particles can have no mass terms because there is no right/left helicity pair with the same SU(2) and
SU(3) representation and the same weak hypercharge. This means that if the gauge charges were conserved in the
vacuum, none of the spin 1 ⁄ 2
particles could ever swap helicity, and they would all be massless.
For a neutral fermion, for example a hypothetical right-handed lepton N (or N α in relativistic two-spinor notation),
with no SU(3), SU(2) representation and zero charge, it is possible to add the term:
This term gives the neutral fermion a Majorana mass. Since the generic value for M will be of order 1, such a particle
would generically be unacceptably heavy. The interactions are completely determined by the theory – the leptons
The Lagrangian for the Higgs includes the most general renormalizable self interaction:
The parameter v2 has dimensions of mass squared, and it gives the location where the classical Lagrangian is at a
minimum. In order for the Higgs mechanism to work, v2 must be a positive number. v has units of mass, and it is theonly parameter in the standard model which is not dimensionless. It is also much smaller than the Planck scale; it is
approximately equal to the Higgs mass, and sets the scale for the mass of everything else. This is the only real
fine-tuning to a small nonzero value in the standard model, and it is called the Hierarchy problem.
It is traditional to choose the SU(2) gauge so that the Higgs doublet in the vacuum has expectation value (v,0).
Masses and CKM matrix
The rest of the interactions are the most general spin-0 spin-1 ⁄ 2
Yukawa interactions, and there are many of these.
These constitute most of the free parameters in the model. The Yukawa couplings generate the masses and mixings
once the Higgs gets its vacuum expectation value.The terms L
* HR generate a mass term for each of the three generations of leptons. There are 9 of these terms, but by
relabeling L and R, the matrix can be diagonalized. Since only the upper component of H is nonzero, the upper
SU(2) component of L mixes with R to make the electron, the muon, and the tau, leaving over a lower massless
component, the neutrino. {Neutrino oscillation show neutrinos have mass. http:/ / operaweb. lngs. infn. it/ spip.
php?rubrique14 31May2010 Press Release.}
The terms QHU generate up masses, while QHD generate down masses. But since there is more than one
right-handed singlet in each generation, it is not possible to diagonalize both with a good basis for the fields, and
Technically, quantum field theory provides the mathematical framework for the standard model, in which a
Lagrangian controls the dynamics and kinematics of the theory. Each kind of particle is described in terms of a
dynamical field that pervades space-time. The construction of the standard model proceeds following the modern
method of constructing most field theories: by first postulating a set of symmetries of the system, and then by writing
down the most general renormalizable Lagrangian from its particle (field) content that observes these symmetries.
The global Poincaré symmetry is postulated for all relativistic quantum field theories. It consists of the familiar
translational symmetry, rotational symmetry and the inertial reference frame invariance central to the theory of special relativity. The local SU(3)×SU(2)×U(1) gauge symmetry is an internal symmetry that essentially defines the
standard model. Roughly, the three factors of the gauge symmetry give rise to the three fundamental interactions.
The fields fall into different representations of the various symmetry groups of the Standard Model (see table). Upon
writing the most general Lagrangian, one finds that the dynamics depend on 19 parameters, whose numerical values
are established by experiment. The parameters are summarized in the table at right.
The QCD sector
The QCD sector defines the interactions between quarks and gluons, with SU(3) symmetry, generated by T a. Since
leptons do not interact with gluons, they are not affected by this sector.
is the gluon field strength, are the Dirac matrices, D stands for the isospin doublet section, U stands for a
unitary matrix, and gs
is the strong coupling constant.
The electroweak sector
The electroweak sector is a Yang –Mills gauge theory with the symmetry group U(1)×SU(2)L,
where B μ
is the U(1) gauge field; Y W
is the weak hypercharge — the generator of the U(1) group; is the
three-component SU(2) gauge field; are the Pauli matrices — infinitesimal generators of the SU(2) group. The
subscript L indicates that they only act on left fermions; g′ and g are coupling constants.
The SM also makes several predictions about the decay of Z bosons, which have been experimentally confirmed by
the Large Electron-Positron Collider at CERN.
Challenges to the standard model
There is some experimental evidence consistent with neutrinos having mass, which the Standard Model does not
allow.[21] To accommodate such findings, the Standard Model can be modified by adding a non-renormalizable
interaction of lepton fields with the square of the Higgs field. This is natural in certain grand unified theories, and if
new physics appears at about 1016 GeV, the neutrino masses are of the right order of magnitude.
Currently, there is one elementary particle predicted by the Standard Model that has yet to be observed: the Higgs
boson. A major reason for building the Large Hadron Collider is that the high energies of which it is capable are
expected to make the Higgs observable. However, as of August 2008, there is only indirect empirical evidence forthe existence of the Higgs boson, so that its discovery cannot be claimed. Moreover, there are serious theoretical
reasons for supposing that elementary scalar Higgs particles cannot exist (see Quantum triviality).
A fair amount of theoretical and experimental research has attempted to extend the Standard Model into a Unified
Field Theory or a Theory of everything, a complete theory explaining all physical phenomena including constants.
Inadequacies of the Standard Model that motivate such research include:
• Does not attempt to explain gravitation, and there is no known way of adapting the quantum field theory of the
sort the Standard Model employs freely, with general relativity, the canonical theory of gravitation. This means,
among other things, that we have no good theory for the very early universe;
• Seems rather ad-hoc and inelegant, requiring 19 numerical constants whose values are unrelated and arbitrary.Although the Standard Model, as it now stands, cannot explain why neutrinos have masses (and the specifics of
neutrino mass are still unclear), it is believed that explaining neutrino mass will require an additional 7 or 8
constants;
• Gives rise to the hierarchy problem, namely why the weak scale and Planck scale are so disparate;
• Should be modified so as to be consistent with the emerging "standard model of cosmology." Specifically, a truly
satisfactory theory of the elementary particles and of the fundamental interactions must explain the initial
conditions of the universe that gave rise to certain observed properties of the present-day universe, properties such
as the predominance of matter over antimatter (matter/antimatter asymmetry), and its isotropy and homogeneity
over large distances.
It should be remarked that neither Unified Field Theory nor the Theory of everything are presently able to addressand solve these problems in conclusive ways.
[9] F.J. Hasert et al. (1974). "Observation of neutrino-like interactions without muon or electron in the Gargamelle neutrino experiment". Nuclear
Physics B 73: 1. doi:10.1016/0550-3213(74)90038-8.
[10] D. Haidt (4 October 2004). "The discovery of the weak neutral currents" (http://cerncourier. com/cws/article/cern/29168). CERN
Courier . . Retrieved 2008-05-08.
[11] "Details can be worked out if the situation is simple enough for us to make an approximation, which is almost never, but often we can
understand more or less what is happening." from The Feynman Lectures on Physics, Vol 1. pp. 2 –7
[12] Technically, there are nine such color –anticolor combinations. However there is one color symmetric combination that can be constructed
out of a linear superposition of the nine combinations, reducing the count to eight.[13] F. Englert, R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters 13: 321 –323.
doi:10.1103/PhysRevLett.13.321.
[14] P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters 13: 508 –509.
• S.F. Novaes (2000). "Standard Model: An Introduction". arΧiv:hep-ph/0001283 [hep-ph].
• D.P. Roy (1999). "Basic Constituents of Matter and their Interactions — A Progress Report.".
arΧiv:hep-ph/9912523 [hep-ph].
• F. Wilczek (2004). "The Universe Is A Strange Place". arΧiv:astro-ph/0401347 [astro-ph].
External links
• " Standard Model - explanation for beginners (http://cms. web.cern. ch/cms/Physics/StandardPackage/index.
html)" LHC
• " Standard Model may be found incomplete, (http://www. newscientist. com/news/news. jsp?id=ns9999404)"
New Scientist .
• " Observation of the Top Quark (http://www-cdf. fnal.gov/top_status/top. html)" at Fermilab.
• " The Standard Model Lagrangian. (http://cosmicvariance. com/2006/11/23/thanksgiving)" After electroweak
symmetry breaking, with no explicit Higgs boson.
• " Standard Model Lagrangian (http://nuclear.ucdavis. edu/~tgutierr/files/stmL1. html)" with explicit Higgsterms. PDF, PostScript, and LaTeX versions.
• " The particle adventure. (http://particleadventure. org/)" Web tutorial.
• Nobes, Matthew (2002) "Introduction to the Standard Model of Particle Physics" on Kuro5hin: Part 1, (http://
www. kuro5hin. org/story/2002/5/1/3712/31700) Part 2, (http://www. kuro5hin. org/story/2002/5/14/
19363/8142) Part 3a, (http://www. kuro5hin. org/story/2002/7/15/173318/784) Part 3b. (http://www.
kuro5hin. org/story/2002/8/21/195035/576)
Particle physics
Collision of 2 beams of gold atoms recorded by RHIC
Particle physics is a branch of physics that studies the
elementary subatomic constituents of matter and
radiation, and the interactive relationship between
them. It is also called high energy physics, because
many elementary particles do not occur under normal
circumstances in nature due to energetic instability, but
can be created and detected during high energy
collisions with other particles, as is done in particle
accelerators.
Scientific research in this area has produced a long list
An image showing 6 quarks, 6 leptons and the interacting particles, according to
the Standard Model
Modern particle physics research is focused
on subatomic particles, including atomic
constituents such as electrons, protons, and
neutrons (protons and neutrons are actually
composite particles, made up of quarks),particles produced by radioactive and
scattering processes, such as photons,
neutrinos, and muons, as well as a wide
range of exotic particles.
Strictly speaking, the term particle is a
misnomer because the dynamics of particle
physics are governed by quantum
mechanics. As such, they exhibit
wave-particle duality, displayingparticle-like behavior under certain
experimental conditions and wave-like
behavior in others (more technically they are
described by state vectors in a Hilbert space;
see quantum field theory). Following the
convention of particle physicists,
"elementary particles" refer to objects such
as electrons and photons, it is well known
that these "particles" display wave-like properties as well.
All the particles and their interactions observed to date can almost be described entirely by a quantum field theory
called the Standard Model. The Standard Model has 17 species of elementary particles: 12 fermions (24 if you count
antiparticles separately), 4 vector bosons (5 if you count antiparticles separately), and 1 scalar boson. These
elementary particles can combine to form composite particles, accounting for the hundreds of other species of
particles discovered since the 1960s. The Standard Model has been found to agree with almost all the experimental
tests conducted to date. However, most particle physicists believe that it is an incomplete description of nature, and
that a more fundamental theory awaits discovery. In recent years, measurements of neutrino mass have provided the
first experimental deviations from the Standard Model.
Particle physics has had a large impact on the philosophy of science. Some particle physicists adhere to
reductionism, a point of view that has been criticized and defended by philosophers and scientists. Part of the debateis described below.[1] [2] [3] [4]
History
The idea that all matter is composed of elementary particles dates to at least the 6th century BC. The philosophical
doctrine of atomism and the nature of elementary particles were studied by ancient Greek philosophers such as
Leucippus, Democritus and Epicurus; ancient Indian philosophers such as Kanada, Dignāga and Dharmakirti;
medieval scientists such as Alhazen, Avicenna and Algazel; and early modern European physicists such as Pierre
Gassendi, Robert Boyle and Isaac Newton. The particle theory of light was also proposed by Alhazen, Avicenna,
Gassendi and Newton. These early ideas were founded in abstract, philosophical reasoning rather than
In the 19th century, John Dalton, through his work on stoichiometry, concluded that each element of nature was
composed of a single, unique type of particle. Dalton and his contemporaries believed these were the fundamental
particles of nature and thus named them atoms, after the Greek word atomos, meaning "indivisible". However, near
the end of the century, physicists discovered that atoms were not, in fact, the fundamental particles of nature, but
conglomerates of even smaller particles. The early 20th century explorations of nuclear physics and quantum physics
culminated in proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclear
fusion by Hans Bethe in the same year. These discoveries gave rise to an active industry of generating one atom from
another, even rendering possible (although not profitable) the transmutation of lead into gold. They also led to the
development of nuclear weapons. Throughout the 1950s and 1960s, a bewildering variety of particles were found in
scattering experiments. This was referred to as the "particle zoo". This term was deprecated after the formulation of
the Standard Model during the 1970s in which the large number of particles was explained as combinations of a
(relatively) small number of fundamental particles.
The Standard Model
The current state of the classification of elementary particles is the Standard Model. It describes the strong, weak,
and electromagnetic fundamental forces, using mediating gauge bosons. The species of gauge bosons are the gluons,W− and W+ and Z bosons, and the photons. The model also contains 24 fundamental particles, which are the
constituents of matter. Finally, it predicts the existence of a type of boson known as the Higgs boson, which is yet to
be discovered.
Experimental laboratories
In particle physics, the major international laboratories are:
• Brookhaven National Laboratory (Long Island, United States). Its main facility is the Relativistic Heavy Ion
Collider (RHIC) which collides heavy ions such as gold ions and polarized protons. It is the world's first heavy
ion collider, and the world's only polarized proton collider.• Budker Institute of Nuclear Physics (Novosibirsk, Russia)
• CERN, (Franco-Swiss border, near Geneva). Its main project is now the Large Hadron Collider (LHC), which had
its first beam circulation on 10 September 2008, and is now the world's most energetic collider of protons. It will
also be the most energetic collider of heavy ions when it begins colliding lead ions in 2010. Earlier facilities
include the Large Electron –Positron Collider (LEP), which was stopped in 2001 and then dismantled to give way
for LHC; and the Super Proton Synchrotron, which is being reused as a pre-accelerator for LHC.
• DESY (Hamburg, Germany). Its main facility is the Hadron Elektron Ring Anlage (HERA), which collides
electrons and positrons with protons.
• Fermilab, (Batavia, United States). Its main facility is the Tevatron, which collides protons and antiprotons and
was the highest energy particle collider in the world until the Large Hadron Collider surpassed it on 29 November2009.
• KEK, (Tsukuba, Japan). It is the home of a number of experiments such as K2K, a neutrino oscillation experiment
and Belle, an experiment measuring the CP violation of B mesons.
• SLAC National Accelerator Laboratory (Menlo Park, United States). Its main facility is PEP-II, which collides
electrons and positrons.
Many other particle accelerators exist.
The techniques required to do modern experimental particle physics are quite varied and complex, constituting a
sub-specialty nearly completely distinct from the theoretical side of the field. See Category:Experimental particle
physics for a partial list of the ideas required for such experiments.
Theoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools to
understand current experiments and make predictions for future experiments. See also theoretical physics. There are
several major interrelated efforts in theoretical particle physics today. One important branch attempts to better
understand the standard model and its tests. By extracting the parameters of the Standard Model from experiments
with less uncertainty, this work probes the limits of the Standard Model and therefore expands our understanding of nature's building blocks. These efforts are made challenging by the difficulty of calculating quantities in quantum
chromodynamics. Some theorists working in this area refer to themselves as phenomenologists and may use the
tools of quantum field theory and effective field theory. Others make use of lattice field theory and call themselves
lattice theorists.
Another major effort is in model building where model builders develop ideas for what physics may lie beyond the
Standard Model (at higher energies or smaller distances). This work is often motivated by the hierarchy problem and
is constrained by existing experimental data. It may involve work on supersymmetry, alternatives to the Higgs
mechanism, extra spatial dimensions (such as the Randall-Sundrum models), Preon theory, combinations of these, or
other ideas.
A third major effort in theoretical particle physics is string theory. String theorists attempt to construct a unified
description of quantum mechanics and general relativity by building a theory based on small strings, and branes
rather than particles. If the theory is successful, it may be considered a "Theory of Everything".
There are also other areas of work in theoretical particle physics ranging from particle cosmology to loop quantum
gravity.
This division of efforts in particle physics is reflected in the names of categories on the preprint archive [5]: hep-th
Particle physicists internationally agree on the most important goals of particle physics research in the near and
intermediate future. The overarching goal, which is pursued in several distinct ways, is to find and understand what
physics may lie beyond the standard model. There are several powerful experimental reasons to expect new physics,
including dark matter and neutrino mass. There are also theoretical hints that this new physics should be found at
accessible energy scales. Most importantly, though, there may be unexpected and unpredicted surprises which will
give us the most opportunity to learn about nature.
Much of the efforts to find this new physics are focused on new collider experiments. A (relatively) near term goal is
the completion of the Large Hadron Collider (LHC) in 2008 which will continue the search for the Higgs boson,
supersymmetric particles, and other new physics. An intermediate goal is the construction of the International Linear
Collider (ILC) which will complement the LHC by allowing more precise measurements of the properties of newlyfound particles. A decision for the technology of the ILC has been taken in August 2004, but the site has still to be
agreed upon.
Additionally, there are important non-collider experiments which also attempt to find and understand physics beyond
the Standard Model. One important non-collider effort is the determination of the neutrino masses since these masses
may arise from neutrinos mixing with very heavy particles. In addition, cosmological observations provide many
useful constraints on the dark matter, although it may be impossible to determine the exact nature of the dark matter
without the colliders. Finally, lower bounds on the very long lifetime of the proton put constraints on Grand
Unification Theories at energy scales much higher than collider experiments will be able to probe any time soon.
In particle physics, a superpartner (also sparticle) is a hypothetical elementary particle. Supersymmetry is one of
the synergistic bleeding-edge theories in current high-energy physics which predicts the existence of these "shadow"
particles.
The word superpartner is a portmanteau of the words supersymmetry and partner (sparticle is a portmanteau of
supersymmetry and particle).
Theoretical predictions
According to the supersymmetry theory, each fermion should have a partner boson, the fermion's superpartner and
each boson should have a partner fermion. When the more familiar leptons, photons, and quarks were produced in
the Big Bang, each one was accompanied by a matching sparticle: sleptons, photinos and squarks. This state of
affairs occurred at a time when the universe was undergoing a rapid phase change, and theorists believe this state of
affairs lasted only some ten trillionth of a ten trillionth of a nanosecond (10 −35 seconds) before the particles we see
now "condensed" out and froze into space-time. Sparticles have not existed naturally since that time.Exact unbroken supersymmetry would predict that a particle and its superpartners would have the same mass. No
superpartners of the Standard Model particles have yet been found. This may indicate that supersymmetry is
incorrect, or it may also be the result of the fact that supersymmetry is not an exact, unbroken symmetry of nature. If
a superpartner is found, its mass would determine the scale at which supersymmetry is broken.
For particles that are real scalars (such as an axion), there is a fermion superpartner as well as a second, real scalar
field. For axions, these particles are often referred to as axinos and saxions.
In extended supersymmetry there may be more than one superparticle for a given particle. For instance, with two
copies of supersymmetry in four dimensions, a photon would have two fermion superpartners and a scalar
superpartner.In zero dimensions (often known as matrix mechanics), it is possible to have supersymmetry, but no superpartners.
However, this is the only situation where supersymmetry does not imply the existence of superpartners.
Recreating superpartners
If the supersymmetry theory is correct, it should be possible to recreate these particles in high-energy particle
accelerators. Doing so will not be an easy task; these particles may have masses up to a thousand times greater than
their corresponding "real" particles.
Until recently, colliders did not have the power to create these supermassive particles, but the newly built Large
Hadron Collider at CERN in Switzerland and France will be able to achieve collisions in the 14 TeV(tera-electron-volt) range, which is more than adequate to determine if these superpartner particles exist.
In particle physics, supersymmetry (often abbreviated SUSY) is a symmetry that relates elementary particles of one
spin to other particles that differ by half a unit of spin and are known as superpartners. In a theory with unbroken
supersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internal
quantum numbers, and vice-versa.
So far, there is only indirect evidence for the existence of supersymmetry.[1]
Since the superpartners of the StandardModel particles have not been observed, supersymmetry, if it exists, must be a broken symmetry, allowing the
superparticles to be heavier than the corresponding Standard Model particles.
If supersymmetry exists close to the TeV energy scale, it allows for a solution of the hierarchy problem of the
Standard Model, i.e., the fact that the Higgs boson mass is subject to quantum corrections which — barring
extremely fine-tuned cancellations among independent contributions — would make it so large as to undermine the
internal consistency of the theory. In supersymmetric theories, on the other hand, the contributions to the quantum
corrections coming from Standard Model particles are naturally canceled by the contributions of the corresponding
superpartners. Other attractive features of TeV-scale supersymmetry are the fact that it allows for the high-energy
unification of the weak interactions, the strong interactions and electromagnetism, and the fact that it provides a
candidate for Dark Matter and a natural mechanism for electroweak symmetry breaking.Another advantage of supersymmetry is that supersymmetric quantum field theory can sometimes be solved.
Supersymmetry is also a feature of most versions of string theory, though it can exist in nature even if string theory
is incorrect.
The Minimal Supersymmetric Standard Model is one of the best studied candidates for physics beyond the Standard
Model. Theories of gravity that are also invariant under supersymmetry are known as supergravity theories.
A supersymmetry relating mesons and baryons was first proposed, in the context of hadronic physics, by Hironari
Miyazawa in 1966, but his work was ignored at the time.[2] [3] [4] [5] In the early 1970s, J. L. Gervais and B. Sakita
(in 1971), Yu. A. Golfand and E.P. Likhtman (also in 1971), D.V. Volkov and V.P. Akulov (in 1972) and J. Wess
and B. Zumino (in 1974) independently rediscovered supersymmetry, a radically new type of symmetry of spacetime
and fundamental fields, which establishes a relationship between elementary particles of different quantum nature,bosons and fermions, and unifies spacetime and internal symmetries of the microscopic world. Supersymmetry first
arose in the context of an early version of string theory by Pierre Ramond, John H. Schwarz and Andre Neveu, but
the mathematical structure of supersymmetry has subsequently been applied successfully to other areas of physics;
firstly by Wess, Zumino, and Abdus Salam and their fellow researchers to particle physics, and later to a variety of
fields, ranging from quantum mechanics to statistical physics. It remains a vital part of many proposed theories of
physics.
The first realistic supersymmetric version of the Standard Model was proposed in 1981 by Howard Georgi and Savas
Dimopoulos and is called the Minimal Supersymmetric Standard Model or MSSM for short. It was proposed to solve
the hierarchy problem and predicts superpartners with masses between 100 GeV and 1 TeV. As of 2009 there is no
irrefutable experimental evidence that supersymmetry is a symmetry of nature. In 2010 the Large Hadron Collider at
CERN is scheduled to produce the world's highest energy collisions and offers the best chance at discovering
superparticles for the foreseeable future. Recently prediction markets like intrade offered scientific contracts that
give estimates for that probability.
Applications
Extension of possible symmetry groups
One reason that physicists explored supersymmetry is because it offers an extension to the more familiar symmetries
of quantum field theory. These symmetries are grouped into the Poincaré group and internal symmetries and theColeman –Mandula theorem showed that under certain assumptions, the symmetries of the S-matrix must be a direct
product of the Poincaré group with a compact internal symmetry group or if there is no mass gap, the conformal
group with a compact internal symmetry group. In 1971 Golfand and Likhtman were the first to show that the
Poincaré algebra can be extended through introduction of four anticommuting spinor generators (in four
dimensions), which later became known as supercharges. In 1975 the Haag-Lopuszanski-Sohnius theorem analyzed
all possible superalgebras in the general form, including those with an extended number of the supergenerators and
central charges. This extended super-Poincaré algebra paved the way for obtaining a very large and important class
of supersymmetric field theories.
The supersymmetry algebraTraditional symmetries in physics are generated by objects that transform under the tensor representations of the
Poincaré group and internal symmetries. Supersymmetries, on the other hand, are generated by objects that transform
under the spinor representations. According to the spin-statistics theorem, bosonic fields commute while fermionic
fields anticommute. Combining the two kinds of fields into a single algebra requires the introduction of a Z2-grading
under which the bosons are the even elements and the fermions are the odd elements. Such an algebra is called a Lie
superalgebra.
The simplest supersymmetric extension of the Poincaré algebra, expressed in terms of two Weyl spinors, has the
One piece of evidence for supersymmetry existing is gauge coupling unification. The renormalization group
evolution of the three gauge coupling constants of the Standard Model is somewhat sensitive to the present particle
content of the theory. These coupling constants do not quite meet together at a common energy scale if we run the
renormalization group using the Standard Model.[1] With the addition of minimal SUSY joint convergence of the
coupling constants is projected at approximately 1016
GeV.[1]
Supersymmetric quantum mechanics
Supersymmetric quantum mechanics adds the SUSY superalgebra to quantum mechanics as opposed to quantum
field theory. Supersymmetric quantum mechanics often comes up when studying the dynamics of supersymmetric
solitons and due to the simplified nature of having fields only functions of time (rather than space-time), a great deal
of progress has been made in this subject and is now studied in its own right.
SUSY quantum mechanics involves pairs of Hamiltonians which share a particular mathematical relationship, which
are called partner Hamiltonians. (The potential energy terms which occur in the Hamiltonians are then called
partner potentials.) An introductory theorem shows that for every eigenstate of one Hamiltonian, its partner
Hamiltonian has a corresponding eigenstate with the same energy. This fact can be exploited to deduce many
properties of the eigenstate spectrum. It is analogous to the original description of SUSY, which referred to bosons
and fermions. We can imagine a "bosonic Hamiltonian", whose eigenstates are the various bosons of our theory. The
SUSY partner of this Hamiltonian would be "fermionic", and its eigenstates would be the theory's fermions. Each
boson would have a fermionic partner of equal energy.
SUSY concepts have provided useful extensions to the WKB approximation. In addition, SUSY has been applied to
non-quantum statistical mechanics through the Fokker-Planck equation.
Mathematics
SUSY is also sometimes studied mathematically for its intrinsic properties. This is because it describes complexfields satisfying a property known as holomorphy, which allows holomorphic quantities to be exactly computed.
This makes supersymmetric models useful toy models of more realistic theories. A prime example of this has been
the demonstration of S-duality in four-dimensional gauge theories that interchanges particles and monopoles.
General supersymmetry
Supersymmetry appears in many different contexts in theoretical physics that are closely related. It is possible to
have multiple supersymmetries and also have supersymmetric extra dimensions.
Extended supersymmetry
It is possible to have more than one kind of supersymmetry transformation. Theories with more than one
supersymmetry transformation are known as extended supersymmetric theories. The more supersymmetry a theory
has, the more constrained the field content and interactions are. Typically the number of copies of a supersymmetry
is a power of 2, i.e. 1, 2, 4, 8. In four dimensions, a spinor has four degrees of freedom and thus the minimal number
of supersymmetry generators is four in four dimensions and having eight copies of supersymmetry means that there
are 32 supersymmetry generators.
The maximal number of supersymmetry generators possible is 32. Theories with more than 32 supersymmetry
generators automatically have massless fields with spin greater than 2. It is not known how to make massless fields
with spin greater than two interact, so the maximal number of supersymmetry generators considered is 32. This
corresponds to an N = 8 supersymmetry theory. Theories with 32 supersymmetries automatically have a graviton.
It is possible to have supersymmetry in dimensions other than four. Because the properties of spinors change
drastically between different dimensions, each dimension has its characteristic. In d dimensions, the size of spinors is
roughly 2d /2 or 2(d − 1)/2. Since the maximum number of supersymmetries is 32, the greatest number of dimensions in
which a supersymmetric theory can exist is eleven.
Supersymmetry as a quantum group
Supersymmetry can be reinterpreted in the language of noncommutative geometry and quantum groups. In
particular, it involves a mild form of noncommutativity, namely supercommutativity. See the main article for more
details.
Supersymmetry in quantum gravity
Supersymmetry is part of a larger enterprise of theoretical physics to unify everything we know about the physical
world into a single fundamental framework of physical laws, known as the quest for a Theory of Everything (TOE).
A significant part of this larger enterprise is the quest for a theory of quantum gravity, which would unify the
classical theory of general relativity and the Standard Model, which explains the other three basic forces in physics
(electromagnetism, the strong interaction, and the weak interaction), and provides a palette of fundamental particles
upon which all four forces act. Two of the most active approaches to forming a theory of quantum gravity are string
theory and loop quantum gravity (LQG), although in theory, supersymmetry could be a component of other
theoretical approaches as well.
For string theory to be consistent, supersymmetry appears to be required at some level (although it may be a strongly
broken symmetry). In particle theory, supersymmetry is recognized as a way to stabilize the hierarchy between theunification scale and the electroweak scale (or the Higgs boson mass), and can also provide a natural dark matter
candidate. String theory also requires extra spatial dimensions which have to be compactified as in Kaluza-Klein
theory.
Loop quantum gravity (LQG), in its current formulation, predicts no additional spatial dimensions, nor anything else
about particle physics. These theories can be formulated in three spatial dimensions and one dimension of time,
although in some LQG theories dimensionality is an emergent property of the theory, rather than a fundamental
assumption of the theory. Also, LQG is a theory of quantum gravity which does not require supersymmetry. Lee
Smolin, one of the originators of LQG, has proposed that a loop quantum gravity theory incorporating either
supersymmetry or extra dimensions, or both, be called "loop quantum gravity II".
If experimental evidence confirms supersymmetry in the form of supersymmetric particles such as the neutralino that
is often believed to be the lightest superpartner, some people believe this would be a major boost to string theory.
A simulated event, featuring the appearance of the Higgs boson
Composition: Elementary particle
Particle statistics: Bosonic
Status: Hypothetical
Theorized: F. Englert, R. Brout, P. Higgs, G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble 1964
Mass: between 115 and 185 GeV/c2 (predicted)
Spin: 0
The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of
particle physics. At present there are no known elementary scalar bosons (spin-0 particles) in nature, although many
composite spin-0 particles are known. The existence of the particle is postulated as a means of resolving
inconsistencies in current theoretical physics, and attempts are being made to confirm the existence of the particle byexperimentation, using the Large Hadron Collider (LHC) at CERN and the Tevatron at Fermilab. Other theories
exist that do not anticipate the Higgs boson, described elsewhere as the Higgsless model.
The Higgs boson is the only Standard Model particle that has not been observed and is thought to be the mediator of
mass. Experimental detection of the Higgs boson would help explain the origin of mass in the universe. The Higgs
boson would explain the difference between the massless photon, which mediates electromagnetism, and the massive
W and Z bosons, which mediate the weak force. If the Higgs boson exists, it is an integral and pervasive component
of the material world.
Arguments based on the Standard Model suggest the mass of the Higgs is below 1.4 TeV. Therefore the Large
Hadron Collider
[1]
is expected to provide experimental evidence of the existence or non-existence of the Higgsboson. Experiments at Fermilab also continue previous attempts at detection, albeit hindered by the lower energy of
the Tevatron accelerator, although it theoretically has the necessary energy to produce the Higgs boson. It has been
reported that Fermilab physicists suggest that the odds of the Tevatron detecting the Higgs boson, if indeed it exists,
are between 50% and 96%, depending on its mass.[2]
"Englert-Brout-Higgs-Guralnik-Hagen-Kibble" [3] ) which gives mass
to vector bosons, was theorized in 1964 by François Englert and
Robert Brout ("boson scalaire");[4] in October of the same year by
Peter Higgs,[5] working from the ideas of Philip Anderson; andindependently by Gerald Guralnik, C. R. Hagen, and Tom Kibble,[6]
who worked out the results by the spring of 1963.[7]
The three papers written on this discovery by Guralnik, Hagen, Kibble,
Higgs, Brout, and Englert were each recognized as milestone papers
during Physical Review Letters 50th anniversary celebration.[8] While
each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL
Symmetry Breaking papers is noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for
Theoretical Particle Physics for this work.[9]
Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetrybreaking. The electroweak theory predicts a neutral particle whose mass is not far from that of the W and Z bosons.
Theoretical overview
A one-loop Feynman diagram of the first-order
correction to the Higgs mass. The Higgs boson couples
strongly to the top quark so it may decay into top
anti-top quark pairs.
The Higgs boson particle is one quantum component of the
theoretical Higgs field. In empty space, the Higgs field has an
amplitude different from zero; i.e., a non-zero vacuum expectation
value. The existence of this non-zero vacuum expectation plays a
fundamental role: it gives mass to every elementary particle that
couples to the Higgs field, including the Higgs boson itself. Inparticular, the acquisition of a non-zero vacuum expectation value
spontaneously breaks electroweak gauge symmetry, which
scientists often refer to as the Higgs mechanism. This is the
simplest mechanism capable of giving mass to the gauge bosons
while remaining compatible with gauge theories. In essence, this
field is analogous to a pool of molasses that "sticks" to the otherwise massless fundamental particles that travel
through the field, converting them into particles with mass that form, for example, the components of atoms. Prof.
David J. Miller of University College London provided a simple explanation of the Higgs Boson, for which he won
an award.[10]
In the Standard Model, the Higgs field consists of two neutral and two charged component fields. Both of the
charged components and one of the neutral fields are Goldstone bosons, which act as the longitudinal
third-polarization components of the massive W+, W –, and Z bosons. The quantum of the remaining neutral
component corresponds to the massive Higgs boson. Since the Higgs field is a scalar field, the Higgs boson has no
spin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even.
The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, then
the Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV). Many theorists
expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the
Standard Model. The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry
breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such amechanism, because unitarity is violated in certain scattering processes. Many models of supersymmetry predict that
the lightest Higgs boson (of several) will have a mass only slightly above the current experimental limits, at around
120 GeV or less.
Supersymmetric extensions of the Standard Model (so called SUSY) predict the existence of whole families of Higgs
bosons, as opposed to a single Higgs particle of the Standard Model. Among the SUSY models, in the Minimal
Supersymmetric extension (MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two
Higgs doublets, leading to the existence of a quintet of scalar particles: two CP-even neutral Higgs bosons h and H, aCP-odd neutral Higgs boson A, and two charged Higgs particles H±.
There are over a hundred theoretical Higgs-mass predictions.[11]
Experimental search
Status as of August 2010, to 95% confidence interval
A Feynman diagram of one way the Higgs boson may
be produced at the LHC. Here, two gluons decay into a
top/anti-top pair, which then combine to make a neutral
Higgs.
As of August 2010, the Higgs boson
has yet to be confirmed
experimentally,[12] despite large efforts
invested in accelerator experiments at
CERN and Fermilab.Prior to the year 2000, the data
gathered at the LEP collider at CERN
allowed an experimental lower bound
to be set for the mass of the Standard
Model Higgs boson of 114.4 GeV/ c2 at
95% confidence level. The same
experiment has produced a small
number of events that could be
interpreted as resulting from Higgs
bosons with mass just above saidcutoff —around 115 GeV —but the
number of events was insufficient to
draw definite conclusions.[13] The LEP
was shut down in 2000 due to
construction of its successor, the Large
Hadron Collider which is expected to
be able to confirm or reject the
existence of the Higgs boson. Full
operational mode was delayed until mid-November 2009, because of a serious fault discovered with a number of
magnets during the calibration and startup phase.[14] [15]
At the Fermilab Tevatron, there are ongoing experiments searching for the Higgs boson. As of July 2010, combined
data from CDF and DØ experiments at the Tevatron were
emit a W or Z boson, which combine to make a neutral
Higgs.
sufficient to exclude the Higgs boson in the range between
158 GeV/ c2 and 175 GeV/ c2 at the 95% confidence level.[16] [17]
Data collection and analysis in search of Higgs are intensifying
since March 30, 2010 when the LHC began operating at 3.5 Tev
and is rapidly approaching in its design range of 7 Tev, well above
that at which detection should occur.[18]
It may be possible to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has a
number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons.Precision measurements of electroweak parameters, such as the Fermi constant and masses of W/Z bosons, can be
used to constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusion
of a Standard Model Higgs boson having a mass greater than 285 GeV/ c2 at 95% CL, and estimated its mass to be
129 GeV/ c2 (the central value corresponds to approximately 138 proton masses). [19] As of August 2009, the
Standard Model Higgs boson is excluded by electroweak measurements above 186 GeV at 95% CL. However, it
should be noted that these indirect constraints make the assumption that the Standard Model is correct. One may still
discover a Higgs boson above 186 GeV if it is accompanied by other particles between Standard Model and GUT
scales.
Some have argued that there already exists potential evidence,[20] [21] [22] but to date no such evidence has convinced
the physics community.In a 2009 preprint,[23] it has been suggested (and reported under headlines such as Higgs could reveal itself in
Dark-Matter collisions[24] ) that the Higgs Boson might not only interact with the above-mentioned particles of the
Standard model of particle physics, but also with the mysterious WIMPs ("weakly interacting massive particles") of
the Dark matter, playing a most-important role in recent astrophysics. In this case, it is natural to augment the above
Feynman diagrams by terms representing such an interaction.
In principle, a relation between the Higgs particle and the Dark matter would be "not unexpected", since, (i), the
Higgs field does not directly couple to the quanta of light (i.e. the photons), while at the same time, (ii), it generates
mass. However, "dark matter" is a metonymy for the discrepancy between the apparent observed mass of the
universe and that given by the standard model and is not a component of any known theory of physics so the
usefulness of this conjecture is limited.
Barring discovery during current intensive efforts, it will be sometime after the end of the current physics fill at the
LHC in 2011 and some further months or years of analysis of the collected data before scientists can confidently
html). CERN Press Office. 9 February 2009. . Retrieved 2009-02-10.
[15] "CERN reports on progress towards LHC restart" (http://press.web.cern. ch/press/PressReleases/Releases2009/PR09. 09E. html).
CERN Press Office. 19 June 2009. . Retrieved 2009-07-21.[16] T. Aaltonen et al. (CDF and DØ Collaborations) (2010). "Combination of Tevatron searches for the standard model Higgs boson in the
W+W− decay mode". arΧiv:1001.4162 [hep-ex].
[17] "Fermilab experiments narrow allowed mass range for Higgs boson" (http://www. fnal. gov/pub/presspass/press_releases/
Higgs-mass-constraints-20100726-images.html). Fermilab. 26 July 2010. . Retrieved 2010-07-26.
Safety of particle collisions at the Large Hadron Collider 92
Particle accelerator
The LHC's CMS detector.
The Large Hadron Collider (LHC) is the world's largest and highest-energy
particle accelerator complex, intended to collide opposing beams of either
protons or lead nuclei with very high kinetic energy.[10] [11] It was built by the
European Organization for Nuclear Research (CERN) near Geneva, in
Switzerland. The LHC's main purpose is to explore the validity and limitations of the Standard Model, the current theoretical picture for particle physics. The first
particle collisions at the LHC took place shortly after startup in November 2009,
at energies up to 1.2 TeV per beam.[12] .
On 30 March 2010, the first planned collisions took place between two 3.5 TeV
beams, which set another new world record for the highest energy man-made
particle collisions.[13]
Due to problematic connections between the superconducting magnets that guide
the beams, the LHC will not run at its designed 7 TeV per beam (14 TeV
center-of-mass) until after a long shutdown that is scheduled to begin at the end of 2011.[14]
Safety concerns
In the run up to the commissioning of the LHC, Walter L. Wagner (an original opponent of the RHIC), Lu is Sancho
(a Spanish science writer) and Otto Rössler (a German biochemist) have expressed concerns over the safety of the
LHC, and have attempted to halt the beginning of the experiments through petitions to the US and European
Courts.[1] [15] [16] [17] [18] These opponents assert that the LHC experiments have the potential to create low velocity
micro black holes that could grow in mass or release dangerous radiation leading to doomsday scenarios, such as the
destruction of the Earth.[3] [19] Other claimed potential risks include the creation of theoretical particles called
strangelets, magnetic monopoles and vacuum bubbles.
[3]
[19]
Based on such safety concerns, US federal judge Richard Posner[20] , Future of Humanity Institute research associate
Toby Ord[21] and others[22] [23] [24] [25] have argued that the LHC experiments are too risky to undertake. In the book
Our Final Century: Will the Human Race Survive the Twenty-first Century? , English cosmologist and astrophysicist
Martin Rees calculated an upper limit of 1 in 50 million for the probability that the Large Hadron Collider will
produce a global catastrophe or black hole.[15] However, Rees has also reported not to be "losing sleep over the
collider," and trusts the scientists who have built it.[26] He has stated: "My book has been misquoted in one or two
places. I would refer you to the up-to-date safety study."[27]
These risk assessments of catastrophic scenarios at the LHC have sparked fears among the public, [1] and scientists
associated with the project have received protests. The Large Hadron Collider team revealed that they had received
death threats and threatening emails and phone calls demanding the experiment be halted.[27] On 9 September 2008,Romania's Conservative Party held a protest before the European Commission mission to Bucharest, demanding that
the experiment be halted because it feared that the LHC could create dangerous black holes.[28] [29]
Media coverage of safety concerns
The safety concerns regarding the LHC collisions have attracted widespread media attention. [1] [30] Various widely
circulated newspapers have reported doomsday fears in connection with the collider, including The Times,[31] The
Guardian,[32] The Independent ,[33] The Sydney Morning Herald ,[34] and Time.[35] Among other media sources, CNN
mentioned that "Some have expressed fears that the project could lead to the Earth's demise," [36] but it assured its
readers with comments from scientists like John Huth, who said that it was "baloney".[36]
MSNBC said that, "thereare more serious things to worry about"[37] and allayed fears that "the atom-smasher might set off earthquakes or
Safety of particle collisions at the Large Hadron Collider 93
other dangerous rumblings".[37] The results of an online survey it conducted "indicate that a lot of [the public] know
enough not to panic".[37] The BBC stated, "the scientific consensus appears to be on the side of CERN's theorists"[38]
who say the LHC has "no conceivable danger".[38] Brian Greene in the New York Times reassured readers by
saying,"If a black hole is produced under Geneva, might it swallow Switzerland and continue on a ravenous rampage
until the earth is devoured? It’s a reasonable question with a definite answer: no."[39]
The tabloids also covered the safety concerns. The Daily Mail produced headlines such as "Are we all going to dienext Wednesday?"[40] and "End of the world postponed as broken Hadron Collider out of commission until the
spring"[41] . The Sun quoted Otto Rössler saying, "The weather will change completely, wiping out life. There will
be a Biblical Armageddon."[42] After the launch of the collider, it had a story entitled, "Success! The world hasn't
ended".[43]
On 10 September 2008, a 16-year-old girl from Sarangpur, Madhya Pradesh, India committed suicide, having
become distressed about predictions of an impending "doomsday" made on an Indian news channel (Aaj Tak)
covering the LHC.[44]
After the dismissal of the federal lawsuit, The Daily Show's correspondent John Oliver interviewed Walter L.
Wagner, who declared that he believed the chance of the LHC destroying the earth to be 50%, since it will either
happen or it won't.[45] [46]
Safety reviews
Concerns similar to those for the LHC were raised in connection with the RHIC particle accelerator.[47] [48] [49] [50]
After detailed studies, scientists reached such conclusions as "beyond reasonable doubt, heavy-ion experiments at
RHIC will not endanger our planet"[51] and that there is "powerful empirical evidence against the possibility of
dangerous strangelet production."[52]
CERN-commissioned reports
Drawing from research performed to assess the safety of the RHIC collisions, the LHC Safety Study Group, a groupof independent scientists, performed a safety analysis of the LHC, and released their findings in the 2003 report
Study of Potent ially Dangerous Events During Heavy-Ion Collisions at the LHC . The report concluded that there is
"no basis for any conceivable threat".[4] Several of its arguments were based on the predicted evaporation of
hypothetical micro black holes by Hawking radiation and on the theoretical predictions of the Standard Model with
regard to the outcome of events to be studied in the LHC. One argument raised against doomsday fears was that
collisions at energies equivalent to and higher than those of the LHC have been happening in nature for billions of
years apparently without hazardous effects, as ultra-high-energy cosmic rays impact Earth's atmosphere and other
bodies in the universe.[4]
In 2007, CERN mandated a group of five particle physicists not involved in the LHC experiments — the LHC Safety
Assessment Group (LSAG), consisting of John Ellis, Gian Giudice, Michelangelo Mangano and Urs Wiedemann, of
CERN, and Igor Tkachev, of the Institute for Nuclear Research in Moscow — to monitor the latest concerns about
the LHC collisions.[6] On 20 June 2008, in light of new experimental data and theoretical understanding, the LSAG
issued a report updating the 2003 safety review, in which they reaffirmed and extended its conclusions that "LHC
collisions present no danger and that there are no reasons for concern".[5] [6] The LSAG report was then reviewed by
CERN’s Scientific Policy Committee (SPC), a group of external scientists that advises CERN ’s governing body, its
Council.[7] [17] [53] The report was reviewed and endorsed by a panel of five independent scientists, Peter
Braun-Munzinger, Matteo Cavalli-Sforza, Gerard 't Hooft, Bryan Webber and Fabio Zwirner, and their conclusions
were unanimously approved by the full 20 members of the SPC.[53] On 5 September 2008, the LSAG's "Review of
the safety of LHC collisions" was published in the Journal of Physics G: Nuclear and Particle Physics by the UK
Institute of Physics, which endorsed its conclusions in a press release that announced the publication.[5] [9]
Safety of particle collisions at the Large Hadron Collider 94
Following the July 2008 release of the LSAG safety report,[5] the Executive Committee of the Division of Particles
and Fields (DPF) of the American Physical Society, the world's second largest organization of physicists, issued a
statement approving the LSAG's conclusions and noting that "this report explains why there is nothing to fear from
particles created at the LHC".[8] On 1 August 2008, a group of German quantum physicists, the Committee for
Elementary Particle Physics (KET),[54] published an open letter further dismissing concerns about the LHC
experiments and carrying assurances that they are safe based on the LSAG safety review.[2] [55]
Other publications
On 20 June 2008, Steven Giddings and Michelangelo Mangano issued a research paper titled the "Astrophysical
implications of hypothetical stable TeV-scale black holes", where they develop arguments to exclude any risk of
dangerous black hole production at the LHC.[56] On 18 August 2008, this safety review was published in the
Physical Review D,[57] and a commentary article which appeared the same day in the journal Physics endorsed
Giddings' and Mangano's conclusions.[58] The LSAG report draws heavily on this research.[17]
On 9 February 2009, a paper titled "Exclusion of black hole disaster scenarios at the LHC" was published in the
journal Physics Letters B.[59] The article, which summarizes proofs aimed at ruling out any possible black hole
disaster at the LHC, relies on a number of new safety arguments as well as certain arguments already present inGiddings' and Mangano's paper "Astrophysical implications of hypothetical stable TeV-scale black holes".[56]
Safety arguments
Micro black holes
Although the Standard Model of particle physics predicts that LHC energies are far too low to create black holes,
some extensions of the Standard Model posit the existence of extra spatial dimensions, in which it would be possible
to create micro black holes at the LHC at a rate of the order of one per second.[60] [61] [62] [63] [64] According to the
standard calculations these are harmless because they would quickly decay by Hawking radiation.
[62]
Hawkingradiation is a thermal radiation predicted to be emitted by black holes due to quantum effects. Because Hawking
radiation allows black holes to lose mass, black holes that lose more matter than they gain through other means are
expected to dissipate, shrink, and ultimately vanish. Smaller micro black holes (MBHs), which could be produced at
the LHC, are currently predicted by theory to be larger net emitters of radiation than larger black holes, and to shrink
and dissipate instantly.[65] The LHC Safety Assessment Group (LSAG) indicates that "there is broad consensus
among physicists on the reality of Hawking radiation, but so far no experiment has had the sensitivity required to
find direct evidence for it."[5]
According to the LSAG, even if micro black holes were produced by the LHC and were stable, they would be unable
to accrete matter in a manner dangerous for the Earth. They would also have been produced by cosmic rays and have
stopped in neutron stars and white dwarfs, and the stability of these astronomical bodies means that they cannot bedangerous:[5] [66]
Stable black holes could be either electrically charged or neutral. [...] If stable microscopic black holes had no
electric charge, their interactions with the Earth would be very weak. Those produced by cosmic rays would
pass harmlessly through the Earth into space, whereas those produced by the LHC could remain on Earth.
However, there are much larger and denser astronomical bodies than the Earth in the Universe. Black holes
produced in cosmic-ray collisions with bodies such as neutron stars and white dwarf stars would be brought to
rest. The continued existence of such dense bodies, as well as the Earth, rules out the possibility of the LHC
Safety of particle collisions at the Large Hadron Collider 95
Strangelets
Strangelets are small fragments of strange matter —a hypothetical form of quark matter —that contain roughly equal
numbers of up, down, and strange quarks and that are more stable than ordinary nuclei (strangelets would range in
size from a few femtometers to a few meters across).[5] If strangelets can actually exist, and if they were produced at
the LHC, they could conceivably initiate a runaway fusion process in which all the nuclei in the planet would be
converted to strange matter, similar to a strange star.[5]
The probability of the creation of strangelets decreases at higher energies.[5] As the LHC operates at higher energies
than the RHIC or the heavy ion programs of the 1980s and 1990s, the LHC is less likely to produce strangelets than
its predecessors.[5] Furthermore, models indicate that strangelets are only stable or long-lived at low temperatures.
Strangelets are bound at low energies (in the range of 1 –10 MeV), while the collisions in the LHC release energies in
the range of 14 TeV. The second law of thermodynamics precludes the formation of a cold condensate that is an
order of magnitude cooler than the surrounding medium. This can be illustrated by the example of trying to form an
ice cube in boiling water.[5]
Specific concerns and responses
Otto Rössler, a German chemistry professor at the University of Tübingen, argues that micro black holes created in
the LHC could grow exponentially.[67] [68] [69] [70] [71] On 4 July 2008, Rössler met with a CERN physicist, Rolf
Landua, with whom he discussed his safety concerns.[72] Following the meeting, Landua asked another expert,
Hermann Nicolai, Director of the Albert Einstein Institute, in Germany, to examine Rössler's arguments.[72] Nicolai
reviewed Otto Rössler's research paper on the safety of the LHC[68] and issued a statement highlighting logical
inconsistencies and physical misunderstandings in Rössler's arguments.[73] Nicolai concluded that "this text would
not pass the referee process in a serious journal."[71] [73] Domenico Giulini also commented with Hermann Nicolai
on Otto Rössler's thesis, concluding that "his argument concerns only the General Theory of Relativity (GRT), and
makes no logical connection to LHC physics; the argument is not valid; the argument is not self-consistent."[74] On 1
August 2008, a group of German physicists, the Committee for Elementary Particle Physics (KET),[54]
published anopen letter further dismissing Rössler's concerns and carrying assurances that the LHC is safe.[2] [55] Otto Rössler
was due to meet Swiss president Pascal Couchepin in August 2008 to discuss this concern,[75] but it was later
reported that the meeting had been canceled as it was believed Rössler and his fellow opponents would have used the
meeting for their own publicity.[76]
On 10 August 2008, Rainer Plaga, a German astrophysicist, posted a research paper on the arXiv Web archive
concluding that LHC safety studies have not definitely ruled out the potential catastrophic threat from microscopic
black holes, including the possible danger from Hawking radiation emitted by black holes. [3] [77] [78] [79] In a
follow-up paper posted on the arXiv on 29 August 2008, Steven Giddings and Michelangelo Mangano, the authors
of the research paper "Astrophysical implications of hypothetical stable TeV-scale black holes", [56] responded to
Plaga's concerns.[80] They pointed out what they see as a basic inconsistency in Plaga's calculation, and argued thattheir own conclusions on the safety of the collider, as referred to in the LHC safety assessment (LSAG) report, [5]
remain robust.[80] Giddings and Mangano also referred to the research paper "Exclusion of black hole disaster
scenarios at the LHC", which relies on a number of new arguments to conclude that there is no risk due to mini black
holes at the LHC.[3] [59] . On 19 January 2009 Roberto Casadio, Sergio Fabi and Benjamin Harms posted on the
arXiv a paper, later published on Physical Review D, ruling out the catastrophic growth of black holes in the scenario
considered by Plaga.[81] In reaction to the criticisms, Plaga updated his paper on the arXiv on 26 September 2008
and again on 9 August 2009.[77] So far, Plaga's paper has not been published in a peer-reviewed journal.
Safety of particle collisions at the Large Hadron Collider 96
Legal challenges
On 21 March 2008, a complaint requesting an injunction to halt the LHC's startup was filed by Walter L. Wagner
and Luis Sancho against CERN and its American collaborators, the US Department of Energy, the National Science
Foundation and the Fermi National Accelerator Laboratory, before the United States District Court for the District of
Hawaii.[19] [82] [83] The plaintiffs demanded an injunction against the LHC's activation for 4 months after issuance of
the LHC Safety Assessment Group's (LSAG) most recent safety documentation, and a permanent injunction until theLHC can be demonstrated to be reasonably safe within industry standards.[84] The US Federal Court scheduled trial
to begin 16 June 2009.[85]
The LSAG review, issued on 20 June 2008 after outside review, found "no basis for any concerns about the
consequences of new particles or forms of matter that could possibly be produced by the LHC".[5] The US
Government, in response, called for summary dismissal of the suit against the government defendants as untimely
due to the expiration of a six-year statute of limitations (since funding began by 1999 and has essentially been
completed already), and also called the hazards claimed by the plaintiffs "overly speculative and not credible".[86]
The Hawaii District Court heard the government's motion to dismiss on 2 September 2008,[1] and on 26 September
the Court issued an order granting the motion to dismiss on the grounds that it had no jurisdiction over the LHC
project.[87] A subsequent appeal by the plaintiffs was dismissed by the Court on 24 August 2010.[88]
On 26 August 2008, a group of European citizens, led by a German biochemist Otto Rössler, filed a suit against
CERN in the European Court of Human Rights in Strasbourg.[69] The suit, which was summarily rejected on the
same day, alleged that the Large Hadron Collider posed grave risks for the safety of the 27 member states of the
European Union and their citizens.[31] [35] [69]
Late in 2009 a review of the legal situation by Eric Johnson, a lawyer, was published in the Tennessee Law
Review.[89] [90] [91] In February 2010 a summary of Johnson's article appeared as an opinion piece in New
Scientist.[92]
In February 2010, the German Constitutional Court (Bundesverfassungsgericht) rejected an injunction petition to
halt the LHC's operation as unfounded, without hearing the case, stating that the opponents had failed to produceplausible evidence for their theories.[93]
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[20] Catastrophe: "Risk and Response"; http://www.bsos.umd. edu/gvpt/lpbr/subpages/reviews/posner505. htm
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[45] Andersen, Kurt The Genesis 2.0 Project (http://www.vanityfair. com/culture/features/2010/01/hadron-collider-201001) published inVanity Fair Jan. 2010. p.96
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MBH redirects here. It can also refer to the Hayist Bases Movement, or a unit of power - a thousand BTUs
per hour.
Micro black holes are tiny black holes, also called quantum mechanical black holes or mini black holes, for
which quantum mechanical effects play an important role.[1]
It is possible that such quantum primordial black holes were created in the high-density environment of the early
Universe (or big bang), or possibly through subsequent phase transitions. They might be observed by astrophysicists
in the near future, through the particles they are expected to emit by Hawking radiation.
Some theories involving additional space dimensions predict that micro black holes could be formed at an energy as
low as the TeV range, which will be available in particle accelerators such as the LHC (Large Hadron Collider).
Popular concerns have then been raised over end-of-the-world scenarios (see Safety of particle collisions at the Large
Hadron Collider). However, such quantum black holes would instantly evaporate, either totally or leaving only a
very weakly interacting residue. Beside the theoretical arguments, we can notice that the cosmic rays bombarding the
Earth do not produce any damage, although they reach center of mass energies in the range of hundreds of TeV.
Minimum mass of a black hole
In principle, a black hole can have any mass above the Planck mass. To make a black hole, one must concentrate
mass or energy sufficiently that the escape velocity from the region in which it is concentrated exceeds the speed of
light. This condition gives the Schwarzschild radius, , where G is Newton's constant and c is the
speed of light, as the size of a black hole of mass M. On the other hand, the Compton wavelength, ,
where h is Planck's constant, represents a limit on the minimum size of the region in which a mass M at rest can be
localized. For sufficiently small M, the reduced Compton wavelength ( , where ħ is Dirac's constant)
exceeds half the Schwarzschild radius, and no black hole description exists. This smallest mass for a black hole isthus approximately the Planck mass.Some extensions of present physics posit the existence of extra dimensions of space. In higher-dimensional
spacetime, the strength of gravity increases more rapidly with decreasing distance than in three dimensions. With
certain special configurations of the extra dimensions, this effect can lower the Planck scale to the TeV range.
Examples of such extensions include large extra dimensions, special cases of the Randall-Sundrum model, and
String theory configurations like the GKP solutions. In such scenarios, black hole production could possibly be an
important and observable effect at the LHC.[1] [2] [3] [4] [5] It would also be a common natural phenomenon induced
by the cosmic rays.
Stability of a micro black hole
Hawking radiation
In 1974 Stephen Hawking argued that due to quantum effects, black holes "evaporate" by a process now referred to
as Hawking Radiation in which elementary particles (photons, electrons, quarks, gluons, etc.) are emitted.[6] His
calculations show that the smaller the size of the black hole, the faster the evaporation rate, resulting in a sudden
burst of particles as the micro black hole suddenly explodes.
Any primordial black hole of sufficiently low mass will Hawking evaporate to near the Planck mass within the
lifetime of the Universe. In this process, these small black holes radiate away matter. A rough picture of this is that
pairs of virtual particles emerge from the vacuum near the event horizon, with one member of a pair being captured,
and the other escaping the vicinity of the black hole. The net result is the black hole loses mass (due to conservation
of energy). According to the formulae of black hole thermodynamics, the more the black hole loses mass the hotter it
becomes, and the faster it evaporates, until it approaches the Planck mass. At this stage a black hole would have a
Hawking temperature of TP
/ 8π (5.6×1032 K), which means an emitted Hawking particle would have an energy
comparable to the mass of the black hole. Thus a thermodynamic description breaks down. Such a mini-black hole
would also have an entropy of only 4π nats, approximately the minimum possible value. At this point then, the
object can no longer be described as a classical black hole, and Hawking's calculations also break down.
While Hawking radiation is sometimes questioned,[7]
Leonard Susskind summarizes an expert perspective in hisrecent book:[8] "Every so often, a physics paper will appear claiming that black holes don't evaporate. Such papers
quickly disappear into the infinite junk heap of fringe ideas."
Conjectures for the final state
Conjectures for the final fate of the black hole include total evaporation and production of a Planck mass-sized black
hole remnant. It is possible that such Planck-mass black holes, no longer able either to absorb energy gravitationally
like a classical black hole because of the quantised gaps between their allowed energy levels, nor to emit Hawking
particles for the same reason, may in effect be stable objects. In such case, they would be WIMPs (weakly
interacting massive particles); this could explain dark matter.
Primordial black holes
Formation in the early Universe
Production of a black hole requires concentration of mass or energy within the corresponding Schwarzschild radius.
It is hypothesized that shortly after the big bang the Universe was dense enough to fit within its own Schwarzschild
radius. Even so, at that time the Universe was not able to collapse into a singularity due to its uniform mass
distribution and rapid growth. This, however, does not fully exclude the possibility that black holes of various sizes
may have emerged locally. A black hole formed in this way is called a primordial black hole and is the most widely
accepted theory for the possible creation of micro black holes.
Expected observable effects
Primordial black holes of initial masses around 1015 grams would be completing their evaporation today; lighter
primordial black holes would have already evaporated.[1] In optimistic circumstances, the Fermi Gamma-ray Space
Telescope satellite, launched in June 2008, might detect experimental evidence for evaporation of nearby black holes
by observing gamma ray bursts.[9] [10] [11] It is unlikely that a collision between a microscopic black hole and an
object such as a star or a planet would be noticeable. This is due to the fact that the small radius and high density of
the black hole would allow it to pass straight through any object consisting of normal atoms, interacting with only
few of its atoms while doing so. It has, however, been suggested that a small black hole (of sufficient mass) passing
through the Earth would produce a detectable acoustic or seismic signal.[12]
[13]
[14]
[15]
Manmade micro black holes
Can we produce micro black holes?
In familiar three-dimensional gravity, the minimum energy of a microscopic black hole is 1019 GeV, which would
have to be condensed into a region on the order of the Planck length. This is far beyond the limits of any current
technology. It is estimated that to collide two particles to within a distance of a Planck length with currently
achievable magnetic field strengths would require a ring accelerator about 1000 light years in diameter to keep the
particles on track. Stephen Hawking also said in chapter 6 of his Brief History of Time that physicist John Archibald
Wheeler once calculated that a very powerful hydrogen bomb using all the deuterium in all the water on Earth could
also generate such a black hole, but Hawking does not provide this calculation or any reference to it to support this
However, in some scenarios involving extra dimensions of space, the Planck mass can be as low as the TeV range.
The Large hadron collider (LHC) has a design energy of 14 TeV for proton-proton collisions and 1150 TeV for
Pb-Pb collisions. In these circumstances, it was argued in 2001 that black hole production could be an important and
observable effect at the LHC [2] [3] [4] [5] [16] or future higher-energy colliders. Such quantum black holes should
decay emitting sprays of particles that could be seen by detectors at these facilities.[2]
[3]
A recent paper by Choptuikand Pretorius, published on March 17, 2010 in Physical Review Letters presents a computer-generated proof that
micro black holes must form from two colliding particles with sufficient energy, which might be allowable at the
energies of the LHC if additional dimensions are present other than the customary four (three space, one time).[17]
[18]
Safety arguments
Hawking's calculation[6] and more general quantum mechanical arguments predict that micro black holes evaporate
almost instantaneously. Additional safety arguments beyond those based on Hawking radiation were given in the
paper [19] [20] , which showed that in hypothetical scenarios with stable black holes that could damage Earth, such
black holes would have been produced by cosmic rays and would have already destroyed known astronomicalobjects such as the Earth, Sun, neutron stars, or white dwarfs. Further, microscopic black holes generated from a
particle accelerator are very small in size and are expected to have a high velocity, making it impossible for them to
accrete a dangerously large amount of mass before leaving the earth for good.
Black holes in quantum theories of gravity
It is possible, in some theories of quantum gravity, to calculate the quantum corrections to ordinary, classical black
holes. Contrarily to conventional black holes which are solutions of gravitational field equations of the general
theory of relativity, quantum gravity black holes incorporate quantum gravity effects in the vicinity of the origin,
where classical a curvature singularity occurs. According to the theory employed to model quantum gravity effects,there are different kinds of quantum gravity black holes, namely loop quantum black holes, noncommutative black
holes, asympotically safe black holes. In these approaches black holes are singularity free.
Fiction
• In David Brin's novel Earth a manmade micro black hole slips into the core of the earth.
• In Dan Simmons's novels Ilium and Olympos, a major landmark is "Paris Crater", the site where a man made
micro black hole's containment field failed, and the black hole sank toward the centre of the earth before
collapsing (presumably in accordance with the Hawking radiation theory), leaving a volcanic crater in its wake.
• In the short story How We Lost the Moon, A True Story by Frank W. Allen, which is actually written by Paul J.
McAuley, a micro black hole is accidentally created on the Moon and gradually consumes it.[21]
• Larry Niven's Hugo Award-winning stories The Hole Man and The Borderland of Sol deal with "quantum black
holes".
• In Martin Caidin's novel Star Bright , an object is created during an implosion-fusion test that has essentially the
properties of a micro black hole, though it is not given that name. The object is eventually destroyed, but the
resulting explosion destroys a huge area around it.
• In Steven R. Donaldson's 5 volume Gap series of books he presents singularity grenades as anti-spaceship cosmic
weapons that release a micro black hole on impact with a ship.
• In Bungie's award-winning Halo Series, spaceships travel through space by ripping the space-time continuum by
artificially generating thousands of micro black holes that quickly evaporate via Hawking radiation.
[18] Peng, G. X.; Wen, X. J.; Chen, Y. D. (2006). "New solutions for the color-flavor locked strangelets". Physics Letters B 633 (2 –3): 314 –318.doi:10.1016/j.physletb.2005.11.081. arXiv:hep-ph/0512112.
[19] S.B. Giddings and M.L. Mangano, "Astrophysical implications of hypothetical stable TeV-scale black holes," arXiv:0806.3381 (http://
• D. Ida, K.-y. Oda & S.C.Park, (http://arxiv. org/abs/hep-th/0602188): determination of black hole's life and
extra-dimensions
• Sabine Hossenfelder: What Black Holes Can Teach Us, hep-ph/0412265 (http://www.arxiv.org/abs/hep-ph/
0412265)
• L. Modesto, PhysRevD.70.124009 (http://arxiv.org/abs/gr-qc/0407097): Disappearance of Black Hole
Singularity in Quantum Gravity
• P. Nicolini, A. Smailacic, E. Spallucci, j.physletb.2005.11.004 (http://arxiv.org/abs/gr-qc/0510112):
Noncommutative geometry inspired Schwarzschild black hole
• A. Bonanno, M. Reuter, PhysRevD.73.083005 (http://arxiv.org/abs/hep-th/0602159): Spacetime Structure of
an Evaporating Black Hole in Quantum Gravity
External links
• Astrophysical implications of hypothetical stable TeV-scale black holes (http://arxiv. org/abs/0806. 3381)
• A. Barrau & J. Grain, The Case for mini black holes (http://www. cerncourier. com/main/article/44/9/22) : a
review of the searches for new physics with micro black holes possibly formed at colliders
• Mini Black Holes Might Reveal 5th Dimension (http://www.space. com/scienceastronomy/ 060626_mystery_monday. html) - Space.com
• Doomsday Machine Large Hadron Collider? (http://www.ostina.org/content/view/3547/1077/) - A scientific
essay about energies, dimensions, black holes, and the associated public attention to CERN, by Norbert Frischauf
(also available as Podcast)
Strangelet
A strangelet is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange
quarks. Its size would be a minimum of a few femtometers across (with the mass of a light nucleus). Once the sizebecomes macroscopic (on the order of meters across), such an object is usually called a quark star or "strange star"
rather than a strangelet. An equivalent description is that a strangelet is a small fragment of strange matter. The term
"strangelet" originates with E. Farhi and R. Jaffe.[1] Strangelets have been suggested as a dark matter candidate.[2]
Theoretical possibility
Strange matter hypothesis
The known particles with strange quarks are unstable because the strange quark is heavier than the up and down
quarks, so strange particles, such as the Lambda particle, which contains an up, down, and strange quark, always losetheir strangeness, by decaying via the weak interaction to lighter particles containing only up and down quarks. But
states with a larger number of quarks might not suffer from this instability. This is the "strange matter hypothesis" of
Bodmer [3] and Witten.[2] According to this hypothesis, when a large enough number of quarks are collected
together, the lowest energy state is one which has roughly equal numbers of up, down, and strange quarks, namely a
strangelet. This stability would occur because of the Pauli exclusion principle; having three types of quarks, rather
than two as in normal nuclear matter, allows more quarks to be placed in lower energy levels.
A nucleus is a collection of a large number of up and down quarks, confined into triplets (neutrons and protons).
According to the strange matter hypothesis, strangelets are more stable than nuclei, so nuclei are expected to decay
into strangelets. But this process may be extremely slow because there is a large energy barrier to overcome: as the
weak interaction starts making a nucleus into a strangelet, the first few strange quarks form strange baryons, such as
the Lambda, which are heavy. Only if many conversions occur almost simultaneously will the number of strangequarks reach the critical proportion required to achieve a lower energy state. This is very unlikely to happen, so even
if the strange matter hypothesis were correct, nuclei would never be seen to decay to strangelets because their
lifetime would be longer than the age of the universe.
Size
The stability of strangelets depends on their size. This is because of (a) surface tension at the interface between quark
matter and vacuum (which affects small strangelets more than big ones), and (b) screening of charges, which allows
small strangelets to be charged, with a neutralizing cloud of electrons/positrons around them, but requires large
strangelets, like any large piece of matter, to be electrically neutral in their interior. The charge screening distance
tends to be of the order of a few femtometers, so only the outer few femtometers of a strangelet can carry charge.[4]
The surface tension of strange matter is unknown. If it is smaller than a critical value (a few MeV per square
femtometer[5] ) then large strangelets are unstable and will tend to fission into smaller strangelets (strange stars
would still be stabilized by gravity). If it is larger than the critical value, then strangelets become more stable as they
get bigger.
Natural or artificial occurrence
Although nuclei do not decay to strangelets, there are other ways to create strangelets, so if the strange matter
hypothesis is correct there should be strangelets in the universe. There are at least three ways they might be created
in nature:• Cosmogonically, i.e., in the early universe when the QCD confinement phase transition occurred. It is possible
that strangelets were created along with the neutrons and protons which form ordinary matter.
• High energy processes. The universe is full of very high-energy particles (cosmic rays). It is possible that when
these collide with each other or with neutron stars they may provide enough energy to overcome the energy
barrier and create strangelets from nuclear matter.
• Cosmic ray impacts. In addition to head-on collisions of cosmic rays, ultra high energy cosmic rays impacting on
Earth's atmosphere may create strangelets.
These scenarios offer possibilities for observing strangelets. If there are strangelets flying around the universe, then
occasionally a strangelet should hit Earth, where it would appear as an exotic type of cosmic ray. If strangelets can
be produced in high energy collisions, then we might make them at heavy-ion colliders.
Accelerator production
At heavy ion accelerators like RHIC, nuclei are collided at relativistic speeds, creating strange and antistrange quarks
which could conceivably lead to strangelet production. The experimental signature of a strangelet would be its very
high ratio of mass to charge, which would cause its trajectory in a magnetic field to be extremely straight. The STAR
collaboration has searched for strangelets produced at the Relativistic Heavy Ion Collider, [6] but none were found.
The Large Hadron Collider (LHC) is even less likely to produce strangelets,[7] but searches are planned[8] for the
The Alpha Magnetic Spectrometer (AMS), an instrument which is planned to be mounted on the International Space
Station, could detect strangelets.[9]
Possible seismic detection
In May 2002, a group of researchers at Southern Methodist University reported the possibility that strangelets mayhave been responsible for a seismic event recorded on October 22 and November 24 in 1993.[10] The authors later
retracted their claim, after finding that the clock of one of the seismic stations had a large error during the relevant
period.[11]
It has been suggested that the International Monitoring System being set up to verify the Comprehensive Nuclear
Test Ban Treaty (CTBT) may be useful as a sort of "strangelet observatory" using the entire Earth as its detector. The
IMS will be designed to detect anomalous seismic disturbances down to 1 kiloton of TNT's equivalent energy release
or less, and could be able to track strangelets passing through Earth in real time if properly exploited.
DangersIf the strange matter hypothesis is correct and a strangelet comes in contact with a lump of ordinary matter such as
Earth, it could convert the ordinary matter to strange matter.[12] [13] This "ice-nine" disaster scenario is as follows:
one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, producing
a larger, more stable strangelet, which in turn hits another nucleus, catalyzing its conversion to strange matter. In the
end, all the nuclei of all the atoms of Earth are converted, and Earth is reduced to a hot, large lump of strange matter.
This is not a concern for strangelets in cosmic rays because they are produced far from Earth and have had time to
decay to their ground state, which is predicted by most models to be positively charged, so they are electrostatically
repelled by nuclei, and would rarely merge with them.[14] [15] But high-energy collisions could produce negatively
charged strangelet states which live long enough to interact with the nuclei of ordinary matter.[16]
The danger of catalyzed conversion by strangelets produced in heavy-ion colliders has received some media
attention,[17] [18] and concerns of this type were raised[12] [19] at the commencement of the Relativistic Heavy Ion
Collider (RHIC) experiment at Brookhaven, which could potentially have created strangelets. A detailed analysis[13]
concluded that the RHIC collisions were comparable to ones which naturally occur as cosmic rays traverse the solar
system, so we would already have seen such a disaster if it were possible. RHIC has been operating since 2000
without incident. Similar concerns have been raised about the operation of the Large Hadron Collider (LHC) at
CERN[20] but such fears are dismissed as far-fetched by scientists.[20] [21] [22]
In the case of a neutron star, the conversion scenario seems much more plausible. A neutron star is in a sense a giant
nucleus (20 km across), held together by gravity, but it is electrically neutral and so does not electrostatically repel
strangelets. If a strangelet hit a neutron star, it could convert a small region of it, and that region would grow toconsume the entire star, creating a quark star.[23]
All the issues discussed above relating to the conversion of ordinary matter to strange matter only arise if the strange
matter hypothesis is true, and its surface tension is larger than the aforementioned critical value.
Debate about the strange matter hypothesis
The strange matter hypothesis remains unproven. No direct search for strangelets in cosmic rays or particle
accelerators has seen a strangelet (see references in earlier sections). If any of the objects we call neutron stars could
be shown to have a surface made of strange matter, this would indicate that strange matter is stable at zero pressure,
which would vindicate the strange matter hypothesis. But there is no strong evidence for strange matter surfaces on
pdf?request-id=1973667e-34da-47a4-b75a-08624558a81b)" (PDF, 586 KiB). ''Journal of Physics G: Nuclear and Particle Physics. 35,
115004 (18pp). doi:10.1088/0954-3899/35/11/115004. arXiv:0806.3414. CERN record (http://cdsweb.
cern.
ch/record/1111112?ln=fr).[8] A. Angelis et al., "Model of Centauro and strangelet production in heavy ion collisions", Phys. Atom. Nucl. 67:396-405 (2004)
[17] New Scientist, 28 August 1999: "A Black Hole Ate My Planet" (http://www.kressworks. com/Science/A_black_hole_ate_my_planet.
htm)
[18] Horizon: End Days, an episode of the BBC television series Horizon
[19] W. Wagner, "Black holes at Brookhaven?" and reply by F. Wilzcek, Letters to the Editor, Scientific American July 1999[20] Dennis Overbye, Asking a Judge to Save the World, and Maybe a Whole Lot More, NY Times, 29 March 2008 (http://www. nytimes. com/
2008/03/29/science/29collider. html?ref=us)
[21] "Safety at the LHC" (http://public. web. cern.ch/Public/en/LHC/Safety-en.html). .
[22] J. Blaizot et al., "Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC", CERN library record (http://cdsweb.
[27] J. Madsen, "Reply to Comment on Strangelets as Cosmic Rays beyond the Greisen-Zatsepin-Kuzmin Cutoff", Phys. Rev.Lett. 92:119002(2004), arXiv:astro-ph/0403515 (http://www.arxiv. org/abs/astro-ph/0403515)
[28] J. Madsen, "Strangelet propagation and cosmic ray flux",Phys. Rev. D71, 014026 (2005) arXiv:astro-ph/0411538 (http://www. arxiv. org/
abs/astro-ph/0411538)
[29] A. Heger, A. Cumming, D. Galloway, S. Woosley, "Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen
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