LHC: unbelievable pursuit for the unimaginablemazumdar/talks/Vashi_LHC.pdf · • Ancient civilizations ... Reflection/Bilateral symmetry Five fold symmetry Radial symmetry Dogma

LHC: the unbelievable pursuit for the unimaginable

Kajari Mazumdar

Department of High Energy Physics

Tata Institute of Fundamental Research

Mumbai

http://www.tifr.res.in/~mazumdar

[email protected]

Evening lecture, IWSA, Vashi, Mumbai. 20 December, 2014

• Ancient civilizations constituents of the universe:

Earth, Air, Fire, Water

• By 1900, nearly 100 elements!

• 1936 three basic particles: proton, neutron, electron

The eternal question of mankind

• Which principles govern energy, matter, space and time at the most elementary level?

• What is the world made up of? • How does it work? High Energy Physics tries to answer these. It also brings in technological spin-offs.

Today

What lies within…? 12/20/2014 2

Essential tool: Microscope

The probe wavelength should be smaller than the distance scale to be probed higher energy needed:

xE

41 mm 10 eV21 nm 10 eV

20 1310 m 10 eV

10 TeV

(1 eV = 1.6 * 10 -19 Joule)

Typical energy scales:

• Motion of air atom, room temp: 0.04 eV

• Chemical reactions/atom, visible photons

of light:: 1 to a few eV

• Nuclear reactions, per atom: 1 MeV

• Rest energy mc2 of proton

~1 Billion eV (1 GeV)

• Each proton in each LHC beam:

4 Trillion eV (4 TeV) during 2010-2012,

6.5 TeV from 2015

• We have already probed matter up to a distance

scale of ~ 10-18 m with probes of energy ~ 100 GeV.

• LHC probes length scales ~ 10-19 to 10-20 m

12/20/2014 3

Atom Proton

Big Bang

Radius of Earth

Radius of Galaxies

Earth to Sun

Universe

Study physics laws of first moments after Big Bang

increasing symbiosis between Particle Physics,

Astrophysics and Cosmology.

Super-Microscope

LHC

Hubble ALMA

VLT AMS

Dimensions in Physics

12/20/2014 4

Technological advancements make it

possible to stretch the limits of our

knowledge for the smallest and the largest

tera

sc

ale

Superstrings ?

Unified

Forces

Inflationary

Expansion

Separation

of Forces Nucleon

Formation Formation

of Atoms Formation

of Stars Today

Big Bang

Time 10-43 s 10-35 s 10-10 s 10-5 s 300 000 years 109 years 14∙109 years

Energy 1017 TeV 1013 TeV 1 TeV 150 MeV 1 eV 4 meV 0.7 meV

Travelling back in time

30 Kelvin

12/20/2014 5

10 thousand km

Connection with our Universe

6 12/20/2014

We belong to Milkyway galaxy

7 12/20/2014

A typical galaxy: 100 million (10 8) larger than the earth!

Contains a million million of stars!

Our universe contains hundred thousand million (10 11) galaxies 10 23 stars: all made up of same elements of matter!

8 12/20/2014

20

Institute of Physics Peter Kalmus Particles and the Universe

Forces

Gravity

falling

objects

planet

orbits

stars

galaxies

inverse

square law

graviton

inverse

square law

photon

short

range

W±, Z0

Electro-

magnetic

atoms

molecules

optics

electronics

telecom.

Weak

beta

decay

solar

fusion

Strong

nuclei

particles

short

range

gluon

BUT MATTER IN THE UNIVERSE IS NEUTRAL, because positive and negative charges cancel each other precisely. Gravitation is the dominant force in the Universe

Relative strength of gravitation, weak, electromagnetic, strong interactions ~10-40 : 10-5 : 10-2 : 1

Fundamental Forces

12/20/2014 9

The messengers of the forces: 1) g for electromagnetic interaction 2) W ±, Z for weak .. 3) 8 gluons for strong ..

Fundamental particles

• Experiments measure the masses of all the elementary particles which are basic inputs to theory. • Once the mass, electric charge etc. are known, theory can predict the results of experiments.

Matter particles interact with various forces via the carrier particles.

constituents of everyday matter

Matter particles

Newton: F = ma

Einstein: E = mc2 12/20/2014 10

LHC

Present Wisdom

11

• All behaviour of the matter particles (fermions) can be explained in terms

of few forces carried by the exchange / carrier particles (bosons):

simplistic nature of basic rules.

• All interactions

behaved as a single

one when the universe

was much younger

and hotter

idea of unification!

How did we achieve this picture?

Connects the largest with the smallest entities in the universe.

12/20/2014

Five fold symmetry Radial symmetry Reflection/Bilateral symmetry

Dogma of Symmetry

• Beauty in symmetry has been appreciated by mankind.

• Symmetry considerations have practical applications too!

eg., position of eyes, weighing balance.etc 12/20/2014 12

Highlights of 20th century physics

• Special relativity

• General Relativity

• Quantum Mechanics

• Quantum Field Theory

• Standard Model of elementary

particles and their interactions

First example of embedded symmetry in physical law: Newton’s law:

Covariant under rotations F, a changes same way under rotation. Invariant under Galilean transformations F, a does not change in a boosted reference frame.

F = m a

Mathematics Physics • Calculus • Complex numbers/functions • Differential geometry • Group theory • Hermitian operators, Hilbert space •….

Symmetry considerations play big role in physics

and in mathematical formulations.

Symmetry of a physical system is a physical or mathematical

feature of the system (observed or intrinsic)

that is "preserved" under some change. 12/20/2014 13

Ex.1: The temperature of a room translational symmetry.

2: Symmetry of a mathematical function: a2c + 3ab + b2c

Manifestation of symmetry

Electromagnetism: Maxwell’s equations

• First attempt to combine electricity and magnetism.

• Invariant under Lorentz transformation.

• Also invariant under gauge transformation.

• Define scalar (f) and vector (A) potentials related to electric & magnetic fields : E = −∇f − ∂A/∂t, and B = ∇ × A • E and B remains unchanged even if we change the potentials as : f′ = f − ∂L/∂t and A′ = A + ∇L, where L is a function of (x, t).

Gauge transformation

14 12/20/2014

Quantum theory is invariant under constant phase

transformations of wave function This symmetry leads to charge conservation. If the phase is a function of space-time, the phase invariance is lost.

Gauge symmetry demands quantum theory should be invariant under

space-time dependent phase transformations.

• To ensure the invariance, one has to introduce a vector field into theory.

• This vector field corresponds to the photon (g), carrier of EM interaction.

The role of photon as carrier particle logically defined.

Gauge Symmetry

Gauge Invariance, proposed by Hermann Weyl, is

the cornerstone of modern day particle physics.

12/20/2014 15

Electromagnetic and weak interactions are similar in nature.

Electroweak symmetry : basic idea

16

Enrico Fermi was the first to write down a theory of beta

decay (1934), with the name neutrino coined by Pauli.

Improved theory (1956): Intermediate Vector Bosons:

proposed by Sudarshan and Marshak,

and, independently, by Feynman and Gell-Mann.

Neutrino

electromagnetic weak

n p e- n

d -1/3 u +2/3 W-1 ( e-1 n) d

u

W

Weak Interaction; • Responsible for some kinds of radioactivity (b decay)

• Only force neutrinos (n) can feel, makes sun burn.

• Typically very slow process: C14 decay takes 6,000 years!

• Carried by weak bosons : charged ( W±) or neutral (Z0)

12/20/2014

• Gauge symmetry is required to correctly describe the interaction of matter

• Quantum electrodynamics is the most complete and successful theory.

• The theoretical predictions match very well with experimental results.

Eg. , value of fine structure constant matches upto 9 digits after decimal!

• Carrier of EM interaction, Photon, is massless.

• Carriers of weak interaction, W+, W- should couple to photon as

electron does : couplings are universal unification of two forces

Electroweak theory 4 carriers particles (spin-1 boson): g, W+, W- , Z0 .

• Carriers of weak interaction needed to be massless to respect symmetry

with electromagnetic interaction infinite range expected like in EM.

• Experimentally observed: weak interaction has short range

the carriers must be massive!

Unification of two forces?

However there is a big problem

Several scientists, in independent groups, worked the way out for the mass

generation of carrier particles of weak interaction during early 1960s. 12/20/2014 17

Nambu-san, in 1960, suggested how

it is possible to have our cake and eat it.

• The equations describing the system

are invariant under some symmetry.

• The ground state of a system breaks

the symmetry in the dynamics. phase transition

Spontaneous Symmetry Breaking

In nature, spontaneous symmetry breaking

is not very uncommon. eg., a ferromagnet

below and above Curie temperature

Symmetry disappears at low energy, reappears at high energy.

Electromagnetic and weak interactions are combined together in a single

Electroweak theory where symmetry is spontaneously broken at lower energy

between EM mediator g and mediators of weak interaction W+, W-, Z0 .

Nobel prize in 2008

12/20/2014 18

Nambu’s idea was taken up in earnest in 1964 by three groups to propose thereotical argument for mass generation in weak interaction:

1. Francois Englert and Robert Brout at Brussels

2. Peter Higgs in Edinburgh

3. Gerald Guralnik, Carl Hagen and Tom Kibble at London

The spontaneous symmetry breaking leads to the existence of an

additional, spin-0, elementary particle called the Higgs boson.

Electroweak symmetry breaking and its consequences

Symmetric position is not the ground state!

• At high energy W boson appears massless, ie., at

short distance scale of less than ~1 fermi (10-13 cm).

• Close to the ground state, at low energy, symmetry

disappears and, the effect corresponds to massive

W, Z bosons.

Experimental verification of this hypothesis was essential. 12/20/2014 19

The drag in movement

• Simply put, there is an all pervading Higgs field.

• All particles necessarily interact with this field and thereby acquire mass.

• Coupling of a particle with Higgs boson depends on its mass.

• The Higgs field also interacts with itself as well resulting in its own mass!

20 12/20/2014

Mass spectrum of elementary particles

The missing piece till now

Mass of the Higgs boson could have

been anything up to ~ 1 TeV

equivalent energy density is needed

for Higgs boson to be created in the

laboratory.

• LHC has been built to discover the

Higgs particle and study EWSB

• LHC experiments discovered the

Higgs boson in 2012,

hypothesized ~ 50 years back!

Great triumph of human intellect

• Hunt for the particle started

seriously during 1980s.

LHC project: largest human endeavour

ever taken up. 12/20/2014 21

22

The recognized giants of the standard model

• 1967: Steven Weinberg and Abdus Salam used their ideas to build a model of electron mass and weak boson mass.

• Also earlier ideas by Sheldon Glashow.

Nobel prize in 1979

Nobel prize in 1999

• Not much attention paid to 1967 work until 1972 paper by Gerard ’t Hooft and Martinus Veltman,

which relied even more explicitly on Higgs’ idea.

It was indeed time for more attention towards Higgs sector!

12/20/2014

• Results of all the past experiments matched well to the theoretical prediction

of Standard Model corresponding theoretical description is correct.

23

Why was the Higgs discovery so important

• Success of standard model crucially depended on the experimental

confirmation of the “Higgs mechanism” the existence of the Higgs boson.

• We have now all the essential members of the particle zoo in SM.

Need to appreciate the elegance of the idea of unification

and other hall marks achieved in the field during the last century.

Gigantic efforts in many fronts led to the success at

the LHC, in first phase of its operation (2010-13).

-- 2012: “a Higgs like new particle”

-- 2013: “a Higgs boson”

Physics Nobel prize to P. Higgs and F. Englert

After the discovery: Is it the Higgs boson of standard model ?

• Is there any other Higgs particle?

• Is the resonance a window to new physics?

12/20/2014

• LHC is the world’s most powerful microscope doing nanonano physics.

• Collide these protons to accumulate sufficient energy so that heavy

particles could be produced in the lab E = mc2

Perfect role of Large Hadron Collider

Accelerates charged particles (protons or lead ions) using electric field and

bending them in circular arc using magnetic field, repeatedly.

• Mass of particles comes from energy of the short-distance reaction.

• The larger the energy the greater the variety of particles produced.

• Otherwise, equal amount of matter and antimatter can also be

produced when energy is converted to matter.

LHC: the giant marvel of technology

• 100-150 m under the surface

• 27 km at 1.9 K (superfluid He)

• Vaccuum ~ 10-13 Atmosphere

• SuperConducting coils: 12000 tonnes/7600

• Temperature generated at LHC due to proton-proton collision ~1016 0c, compare with sun: 5506 0c, a matchstick: 250 0c

• LHC machine maintained at -271 0c vs. home freezer is at -8 0c Boomerang nebula: -272 0c, antarctica: -89.2 0c,

Indian contributions in LHC:

• Main accelerator magnet components

• CMS and ALICE experiments

• Grid computing

• One of the fastest race tracks: protons zipping past with 99.999999% of velocity of light around 27 km of LHC ring 11000 times/sec.

2 major, multipurpose experiments which discovered the Higgs boson: ATLAS and CMS

12/20/2014 25

What happens in LHC experiment

Proton-Proton 1380 bunch/beam Protons/bunch 2. 1011

Beam energy 4 TeV Luminosity 7.5*1033 /cm 2/s Crossing rate 20 MHz Total event rate 5.4*108 Hz Higgs production <1 Hz

Summer, 2012

• 80 Million electronic channels per experiment, ready for data/25 ns.

20 years to build, 30 years to operate (in phases)

26 12/20/2014

Cartoon of ATLAS detector

12/20/2014 27

CMS Collaboration: 1740 Ph.D.s + 1535 students (845 for Ph.D.) + 790 engineers from 179 institutes in 41 countries.

Only a small fraction of ~4500 people who made CMS possible

~ 120 Indians 12/20/2014 28

C CMS experiment

Cartoon of current CMS detector

HO TIFR, U.Panjab

Silicon preshower BARC, U.Delhi

RPC detector BARC, U.Panjab 12/20/2014 29

The missing piece

we have been after

Slice of CMS detector

Measure the position and momentum of photons and leptons (electron, muon, tau) with high accuracy and reliability. Measure hadronic shower (jets of particles, like pion, kaon etc.) and missing energy. 12/20/2014 30

Event rate of a physical process: R = s L = cross section*instantaneous luminosity

Cross-section of Higgs production for m H = 125 GeV at LHC energy of 8TeV 22 pb = 22* 10-36 cm2

• 0.5 Million Higgs particles produced till now in CMS/ATLAS expt.

Each constituent of proton (quark/gluon) carries only a fraction of the parent energy. Effective energy in a violent collision varies on event-by-event basis possibility of producing particles of different masses Higgs of any mass within allowed range could be produced at LHC

LHC motivations: explore, search, measure

Background rate is ~1012 times higher

efficient and diligent strategy needed. Any event 109/s Higgs event: 0.2 /s 12/20/2014 31

• Higgs boson decays within ~ 10 -24 s. • Decay modes of an unstable, heavy particle X X A : a% of total decay events X B : b% of total decay events Not all decay channels are experimentally suitable. Discovery mode H gg , Branching ratio= 0.23% • Experimental signature is simple and easy to identify final state with 2 energetic, isolated photons. But there are many process which can produce similar signature in the detector. Search for resonance structure in diphoton mass distribution

Crucial for mass resolution: • individual energy measurement for each photon • angle between 2 photons huge investments in every sense paid very well.

m2γγ= 2 E1 E2 (1-cosα)

Higgs decaying to a pair of photons

12/20/2014 32

Golden decay channel H ZZ* 4 leptons

• Signal: 4 energetic, isolated leptons (electron or muon)

(2 pairs of opposite sign, same flavour)

Use kinematic properties of final state leptons to discriminate

signal vs. background on event-by-event basis.

Used for discovery & determination of properties

mass, width, spin, parity, couplings. Z 4l

H 4l

4l continuum

Backgrounds:

Interference of diagrams for

off-shell resonance and

continuum background must

be taken into account.

12/20/2014 34

Lot of investments in measuring decay

leptons accurately

extract multiple information about the

properties. of Higgs boson to identify

Its exact nature.

Work of a detective

• Measurement of mass Fundamental property, not predicted by theory

Once measured, SM predictions are completely determined

Use resonance structure in high resolution channels H gg, H 4leptons

CMS : 125.03 ± 0.26 (stat.) ± 0.14 (syst.) GeV ATLAS: 125.36 ± 0.37 (stat.) ± 0.18 (syst.) GeV

GH < 22 MeV ~ 5GH SM

Compatible within errors

• Total width of Higgs boson

• Spin-Parity JP = 0+

• there is no other particle of different spin or similar mass

• Couplings of Higgs to various particles are similar to as in standard model 12/20/2014 35

• Background distribution mostly Gaussian

stability of result expressed in terms of width s of the Gaussian.

• Characterization of excess using test statistic

Significance where

• Greater the significance (s) minor the p-value lower is the chance

that the observed excess is due to background fluctuation.

Statistical analysis

Signal strength for H gg

12/20/2014 36

Individual analyses

Significance of observation of 125 GeV Higgs boson: CMS summary

sobs /sSM = m

measure of signal strength compared to SM expectation for Higgs mass at the fitted value.

Illustration

12/20/2014 37 CMS: m = 1.0 ± 0.09(stat. )+.08-.07 (theo.)± 0.07(syst) compatible with SM

38

Scattering of longitudinal vector bosons

Each diagram ~ s2

s(ppWW) > s(pp anything)!

Unitarity restored by scalar Higgs

Cancellation also requires Higgs < 800 GeV

• Taming the rate could be managed

by alternative EWSB mechanism

Search for possible resonances

LSB > 1TeV

SB sector

strongly coupled d

s/d

M(V

V)

LSB < 1TeV

SB sector

weakly coupled

12/20/2014

VV Scattering spectrum, σ(VVVV) vs M(VV)

Fundamental probe to test the nature of Higgs boson and its role in EWSB

Energy

Data rates @ CMS as foreseen for design parameters

data collection and archiving rate ~ few hundred Hz 12/20/2014 39

LHC collides 6-8 hundred million proton

on-proton /second for several years.

Only 1 in 20 thousand collisions has an

important tale to tell, but we do not

know which one!

need to search through all of them!

15 PBytes (1015 bytes) of data/year

Analysis requires ~100,000 computers

to get results in reasonable time.

Distributed computing is essential

Science without borders

LHC computing in hard numbers

World wide LHC computing GRID was the

natural evolution of internet technology.

12/20/2014 40

1. Share more than information Data, computing power, applications in dynamic, multi-institutional, virtual

organizations.

2. Efficient use of resources at many institutes.

People from many institutions working to solve a common problem.

3. Join local communities. Need comparatively large hardware resources with

high speed connectivity

4. Interactions with the underneath layers transparent & seemless to the user.

From Web to Grid Computing

CMS and ALICE Tier2 GRID computing

centers in TIFR (Mumbai) and VECC (Kolkata).

• Today ~ 200 sites • ~40k CPU cores • ~100 PB disk

WWW was born in early 1990s to satisfy the

needs of previous generation HEP experiments.

12/20/2014 41

mu = 0.003 mt = 184 mb = 5.0 me = 0.0005446 mm = 0.1126

For example, it does not explain this bizarre set of numbers for mass (in GeV)

Is our job over?

By no chance! Higgs boson fixes a crucial problem, and accounts for the origin of mass, but it leaves a lot unexplained

There are many reasons to believe that there is lot of unknown,

new physics at higher energy densities.

We are able explain the evolution up to an epoch of ~ 10 -11 s after big bang.

Future operations of LHC will take us more backward in time.

• 2015 – 2030: LHC operates with intermittent stops for ~2 -3 years

• centre of mass energy 13 TeV, but gradually increasing luminosity

helps in exploring physics beyond standard model upto energy ~ few TeV

12/20/2014 42

Seeing the dark!

Passage of 2 galaxies 100 M years back

Rotation curve of a galaxy

• With increasing distance the gravitational

pull should decrease.

• Observations suggested gravitational pull

from additional heavy objects which are

not yet detected (through EM interaction).

• Identification of the dark matter is one of

the most intriguing problem at present.

LHC can shed light on the nature of the dark matter

Constituents of the universe

12/20/2014 43

LHC experiments have discovered Higgs particle of mass 125 GeV

• Current measurements are in agreement with minimal Higgs mechanism.

• No exotic discovery as yet

• Established : Origin of mass (scalar field BEH mechanism) of particles in a

quantum field theory with local (point-like) gauge interaction.

• Starting from a reductionism strategy: question of structure of matter

evolved into the question of origin of interactions (local gauge symmetries)

and matter (interaction with Higgs field)

• The rise in centre of mass energy at LHC in next run, gives access to new

territory for the search of the unexpected lot of potential!

Miles to go before we sleep!

• However , we shall always manage to know only a drop of the ocean!

Summing it up

44 Stay tuned!

12/20/2014

backup

12/20/2014 45

1. At the core is a device called the inner tracker detects and analyzes the momentum of particles passing through the detector.

2. Surrounding the inner tracker is a calorimeter measure the energy of particles by absorbing them. 3. The outermost subdetector is muon spectrometer measures muon position and momentum. Scientists look at the path the particles took and extrapolate information about them. Reconstruct 20K charged tracks in a single event (lead-lead collisions at LHC)

Essential components of a detector

Data collected by an experiment in a year ~ Peta Byte how to handle? 12/20/2014 46