1 Particle Physics Particle physics – what is it? – why do it? Standard model Quantum field theory Constituents, forces Milestones of particle physics Particle physics experiments shortcomings of standard model Summary Webpages of interest http://www.fnal.gov (Fermilab homepage) http://www-d0.fnal.gov (DØ homepage) http://www.cern.ch (CERN -- European Laboratory for Particle Physics) http://cms.web.cern.ch/cms/ (CMS) http://www.hep.fsu.edu/~wahl/Quarknet/links.html (has links to many particle physics sites and other sites of interest) http://www.fnal.gov/pub/tour.html (Fermilab particle physics tour) http://www.cpepweb.org/ Contemporary Physics Education Project
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1 Particle Physics Particle physics – what is it? – why do it? Standard model l Quantum field theory lConstituents, forces lMilestones of particle.
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Particle Physics Particle physics – what is it? – why do it? Standard model
Quantum field theory Constituents, forces Milestones of particle physics
Particle physics experiments shortcomings of standard model Summary Webpages of interest
more and more particles discovered, patterns emerge (1960’s and on):
o leptons and hadronso Electromagnetic, weak, strong interactions
present scenario: Standard Model of electroweak and strong interactions Formulation and discovery (1960’s to 1980’s) Precision experimental tests (from 1990’s)
quest for new physics (beyond the standard model) Open questions, possible strategies Present and future experiments, facilities Search for Higgs particle
outlook
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Sizes and distance scales
virus 10-7
Molecule 10-9m Atom 10-10m nucleus 10-14
m nucleon 10-15m Quark <10-
19m
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The Building Blocks of a Dew Drop A dew drop is made up of
1021 molecules of water. Each molecule = one
oxygen atom and two hydrogen atoms (H2O).
Each atom consists of a nucleus surrounded by electrons.
Electrons are leptons that are bound to the nucleus by photons, which are bosons.
The nucleus of a hydrogen atom is just a single proton.
Protons consist of three quarks. In the proton, gluons hold the quarks together just as photons hold the electron to the nucleus in the atom
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Contemporary Physics
Education Project
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Goals of particle physics particle physics or high energy physics
is looking for the smallest constituents of matter (the “ultimate building blocks”) and for the fundamental forces between them (“interactions”);
aim is to find description in terms of the smallest number of particles and forces
Try to describe matter in terms of specific set of constituents which can be treated as fundamental;
With deeper probing (at shorter length scale), these fundamental constituents may turn out to consist of smaller parts (be “composite”).
“Smallest constituents” vs time:o in 19th century, atoms were considered smallest building
blocks,o early 20th century research: electrons, protons, neutrons;o now evidence that nucleons have substructure - quarks; o going down the size ladder: atoms -- nuclei -- nucleons --
Are there irreducible building blocks?o How many?o What are their properties?
mass? charge? flavor? What is mass? What is charge ? How do the building blocks interact?
o forces? o Differences? similarities?
Why more matter than antimatter? why is our universe the way it is?
o Coincidence? o Theoretical necessityo Design?
Why do we want to know? Curiosity Understanding constituents may help in understanding composites Implications for origin and destiny of Universe
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Cosmic rays Discovered by Victor Hess (1912) Observations on mountains and in balloon: intensity of cosmic
radiation increases with height above surface of Earth – must come from “outer space”
Much of cosmic radiation from sun (rather low energy protons) Very high energy radiation from outside solar system,
but probably from within galaxy
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Victor Hess’ Balloon ride 1912
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Positron Positron (anti-electron)
Predicted by Dirac (1928) -- needed for relativistic quantum mechanics
existence of antiparticles doubled the number of known particles!!!
Positron track going upward through lead plate
o Photographed by Carl Anderson (Aug. 2, 1932)
o Particle moving upward, as determined by increase in curvature
of the top half of the track after it passed through lead plate,
o and curving to the left, meaning its charge is positive
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Anderson and his cloud chamber
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Neutron Bothe + Becker (1930):
Some light elements (e.g. Be), when bombarded with alpha particles, emit neutral radiation, “penetrating”– gamma?
Curie-Joliot and Joliot (1932): This radiation from Be and B able to eject protons from
material containing hydrogen Chadwick (1932)
Doubts interpretation of this radiation as gamma Performs new experiments; protons ejected not only
from hydrogen, but also from other light elements; measures energy of ejected protons
(by measuring their range), results not compatible with assumption that unknown
radiation consists of gamma radiation (contradiction with energy-momentum conservation), but are compatible with assumption of neutral particles with mass approximately equal to that of proton – calls it “neutron”
Neutron assumed to be “proton and electron in close
association”
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Chadwick’s experiment
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Nuclear force – field quantum photon carries the electromagnetic force. Analogy: postulate particle as carrier of nuclear
force (Hideki Yukawa, 1935) with mass intermediate between the electron and the proton
This particle also to be responsible for beta-decay.
potential energy between the nucleon field quanta has the form
m = mass of the exchanged quantum; from observed range of nuclear force:
mass of the exchanged particle 200MeV
2
2
/exp
4 mcc
R
R
gU
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More particles: Muon 1937: “mesotron” is
observed in cosmic rays (Carl Anderson, Seth Neddermeyer) – first mistaken for Yukawa’s particle
However it was shown in 1941 that mesotrons didn’tinteract strongly with matter.
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Discovery of pion Lattes, Occhialini and Powell (Bristol, 1947)
(+ graduate student Hugh Muirhead): observed decay of a new particle into two particles
decay products: muon (discovered by Neddermeyer), the other is invisible (Pauli's neutrino).
muon in turn also decays into electron and neutrino
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Kaons First observation of
Kaons: Experiment
by Clifford Butler and George Rochester at Manchester
Cloud chamber exposed to cosmic rays Left picture: neutral Kaon decay (1946) Right picture: charged Kaon decay
into muon and neutrino Kaons first called “V” particles Called “strange” because they behaved
differently from others
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“Strange particles”
Kaon: discovered 1946; first called “V” particles
K0 production and decayin a bubble chamber
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Bubble chamber
p p p n K0 K- + - 0
n + p 3 pions0 , e+ e-K0 + -
-
---
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Particle Zoo
1940’s to 1960’s : Plethora of new particles discovered
(mainly in cosmic rays): e-, p, n, ν, μ-, π±, π0, Λ0, Σ+ , Σ0 , Ξ,….
question: Can nature be so messy? are all these particles really
intrinsically different? or can we recognize patterns or
symmetries in their nature (charge, mass, flavor) or the way they behave (decays)?
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Particle spectroscopy era 1950’s – 1960’s: accelerators, better detectors even more new particles are found, many of them
extremely short-lived (decay after 10-21 sec) “particle spectroscopy era”
Bubble chamber allows detailed study of reactions, reconstruction of all particles created in the reactions
find that often observed particles actually originate from decay of very short-lived particles (“resonances”)
allows classification of particles into “multiplets” Mass formula relating masses of particles in same
multiplet Allows prediction of new particle Ω- , with all of its
properties (mass, spin, expected decay modes,..) subsequent observation of Ω- with expected properties
at BNL (1964)
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Ω-
The bubble chamber picture of the first omega-minus. An incoming K- meson ( of momentum 5.0 GeV/c ) interacts with a proton of the liquid hydrogen in the bubble chamber and produces an omega-minus, a K° and a K+ meson:
K- + p ―› Ω- + K+ + K0
The omega minus then decays: Ω- ―> Ξ0 + π-, with subsequent decay Ξ0 ―> Λ + π0; Λ ―> p + π- π0 ―> γ γ γ ―> e+ e-
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Ω-
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Particle nomenclature by mass:
baryons – heavy particles .. p, n, Λ, Σ, Ω-, ++ ….
o nucleons and their excited states: p, n, N*, ++ , ……
o hyperons: Λ, Σ, Ξ, Ω- , and their excited states mesons – medium-heavy particles … π, K, K*, ρ, ω … leptons – light particles e, μ, νe, ν …
by spin:
fermions: spin = odd-integer multiple of ½: ½, 3/2, 5/2,……
o leptons and baryons bosons: integer spin 0, 1, 2, …. – mesons are bosons
by interaction: hadrons: partake in strong interactions leptons: no strong interaction
by lifetime: stable particles: lifetime > 10-17 sec
(decay by weak or electromagnetic interaction)
unstable particles (“resonances”) lifetime <10-20 sec (decay by strong interaction)
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Spin and statistics Fermions obey “Fermi-Dirac” statistics:
multi-fermion states are antisymmetric with respect to exchange of any two identical fermions (wave function changes sign)
|f1, f2, f3 > = - |f2, f1 , f3 >
Pauli exclusion principle is special case of this Bosons obey “Bose-Einstein” statistics:
multi-boson states are symmetric with respect to exchange of any two identical bosons (wave function stays the same) |b1, b2 , b3 > = |b2, b1 , b3 >
consequence: bosons like to “stick together” (e.g. Bose-Einstein
condensate)
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Towards the standard model
Quark Model (Gell-Mann, Zweig, 1964) observed SU(3) symmetry can be explained by
assuming that all hadrons are made of “quarks” There are 3 quarks: u (up), d (down), s
(strange); quarks have non-integer charge:
o u 2/3, d -1/3, s -1/3 baryons are made of 3 quarks:
o p = uud, n = udd, Λ = uds, Σ = uus Ξ = uss, Ω- = sss
mesons are made of quark-antiquark pairs:o π+ = ud, π- = u d, π0 = u u + d d, ……..
_ _ __
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The 8-fold way
K0
-
K+
+0
K- K0sd
ud
su
du
ds us
uu,dd,ss
0
-
+
+0
- 0uss
uus
dss
dds
udd uud
uds
-
ddd++
uuu
-
sss
n p
mesonsqq
baryons qqq
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color charge problem with quark model
Quarks have spin ½, i.e. are fermions must obey Pauli principle
Ω- = sss, has spin 3/2; spins of 3 s quarks must be aligned, i.e. Ω- has 3 quarks in identical state --- forbidden
similarly for ++ = uuu, spin 3/2
way out: quarks have additional hidden property – “color charge”
3 colors: green, red, blue each quark can carry one of three colors
o red blue green antiquarks carry anticolor
o anti-red anti-blue anti-green observed particles are “color-neutral”; only colorless (“white”) combinations of
quarks and antiquarks can form particles:
qqq qq
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“Elementary” particles?
“leptons” (electron, muon and their neutrinos) are fundamental, interact electromagnetically and “weakly”
“hadrons” (p, n, Λ, Σ, Ω-, ++ , π, K, K*, ρ, ω,…) are not fundamental particles – are made of quarks, interact electromagnetically, “weakly”, and “strongly”
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A theoretical model of interactions of elementary particles, based on quantum field theory
1967: Weinberg & Salam propose a unified Gauge Theory of electroweak interactions, introducing the W±,Z as force carriers and the Higgs field to provide the symmetry breaking mechanism.
1971-1972: Gauge theories are renormalizable (even when symmetry is spontaneoulsy broken) (t’Hooft, Veltman, Lee, Zinn-Justin..)
1972: high pt pions observed at the CERN ISR
1973: Quantum Chromodynamics (Gross, Wilczek, Politzer, Gell-Mann & Fritzsch) : quarks are held together by a Gauge-Field whose quanta, gluons, mediate the strong force
1973: “neutral currents” observed (Gargamelle bubble chamber at
CERN)
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Brief History of the Standard Model - 3
1974: J/ discovered at BNL/SLAC; 1975: J/ interpreted as cc bound state (“charmonium”) 1976: t lepton discovered at SLAC 1977: discovered at Fermilab in 1977, interpreted as bb
bound state (“bottomonium”) 3rd generation
1979: gluon – jets observed at DESY 1982: direct evidence for jets in hadron hadron interactions at
CERN (pp collider)
1983: W±, Z observed at CERN (pp collider built for that purpose)
1995: top quark found at Fermilab (DØ, CDF) 1999: indications for “neutrino oscillations” (Super-
Kamiokande experiment) 2000: direct evidence for tau neutrino () at Fermilab
(DONUT experiment) 2005: clear evidence for neutrino oscillations (Kamiokande,
SNO)
-
-
-
-
-
-
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Feynman diagrams Feynman Diagrams
In our current understanding, all interactions arise from the exchange of quanta
The mathematics describing such interactions can be represented by a diagram, called a Feynman diagram
ee
Feynmandiagram
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Present scenario
Most of what’s around us is made of very few particles: electrons, protons, neutrons (e, u, d)
this is because our world lives at very low energy
all other particles were created at high energies during very early stages of our universe
can recreate some of them (albeit for very short time) in our laboratories (high energy accelerators and colliders)
this allows us to study their nature, test the standard model, and discover direct or indirect signals for new physics
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Study of high energy interactions -- going back in time
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Homework set 5
HW 5.1 go to Particle Data Group website:
http://pdg.lbl.gov find masses (in MeV or GeV) ,
principal decay modes and lifetimes of the following particles: π±, π0, K0, J/Ψ, p, n, Λ0, Σ+ , Σ0, Ξ, Ω-
give the quark composition of π±, π0, K0, J/Ψ, p, n, Λ0, Ω-
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Summary
Particle physics was born during last century, grew out of atomic and nuclear physics
huge progress in understanding over last 50 years, due to revolutionary ideas in both theory and experiment
intense dialog between experimenters and theorists precision tests of standard model ongoing, looking
for hints of new physics next:
symmetries tests of standard model, experiments,
accelerators problems and shortcomings of standard model new projects, outlook