Particle Physics

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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
• 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//ParticleAdventure.org/ (Lawrence Berkeley
  Lab.)

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Topics

• what is particle physics, goals and issues
• historical flashback over development of the
  field
• cosmic rays
• particle discoveries
• forces
• new theories
• the standard model of particle physics

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About Units

• Energy - electron-volt
• 1 electron-volt kinetic energy of an electron
  when moving through potential difference of 1
  Volt
• 1 eV 1.6 10-19 Joules 2.1 10-6 Ws
• 1 kWhr 3.6 106 Joules 2.25 1025 eV
• 1 MeV 106 eV, 1 GeV 109 eV, 1 TeV 1012
  eV
• mass - eV/c2
• 1 eV/c2 1.78 10-36 kg
• electron mass 0.511 MeV/c2
• proton mass 938 MeV/c2 0.938 GeV/ c2
• neutron mass 939.6 MeV/c2
• momentum - eV/c
• 1 eV/c 5.3 10-28 kg m/s
• momentum of baseball at 80 mi/hr ? 5.29
  kgm/s ? 9.9 1027 eV/c
• Distance
• 1 femtometer (Fermi) 10-15 m

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Outline

• what is particle physics?
• Origins of particle physics
• Atom (p, e-), radioactivity, discovery of neutron
  (n) (1895-1932)
• Cosmic rays positron (e), muon (µ-), pion (p),
  Kaon (K, K0) (1932 1959)
• the advent of accelerators
• more and more particles discovered, patterns
  emerge (1960s and on)
• leptons and hadrons
• Electromagnetic, weak, strong interactions
• present scenario Standard Model of
  electroweak and strong interactions
• Formulation and discovery (1960s to 1980s)
• Precision experimental tests (from 1990s)
• 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 lt10-19m

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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
• in 19th century, atoms were considered smallest
  building blocks,
• early 20th century research electrons, protons,
  neutrons
• now evidence that nucleons have substructure -
  quarks
• going down the size ladder atoms -- nuclei --
  nucleons -- quarks ???... ??? (preons, toohoos,
  voohoos,.????)

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Issues of High Energy Physics

• Basic questions
• Are there irreducible building blocks?
• How many?
• What are their properties?
• mass? charge? flavor?
• What is mass?
• What is charge ?
• How do the building blocks interact?
• forces?
• Differences? similarities?
• Why more matter than antimatter?
• why is our universe the way it is?
• Coincidence?
• Theoretical necessity
• 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
• Photographed by Carl Anderson (Aug. 2,
  1932)
• Particle moving upward, as determined by
  increase in curvature
  of the top half of the
  track after it passed
  through lead plate,
• 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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Chadwicks 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

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More particles Muon

• 1937 mesotron is observed in cosmic rays
  (Carl Anderson, Seth Neddermeyer) first
  mistaken for Yukawas particle
• However it was shown
• in 1941 that mesotrons didnt
• interact 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 decay in a bubble chamber
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Bubble chamber
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p p ?? p n K0 K- ? ?- ?0 n p ?? 3 pions ?0 ??
??, ? ? e e- K0 ? ? ?-
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Particle Zoo

• 1940s to 1960s
• Plethora of new particles discovered (mainly in
  cosmic rays)
• e-, p, n, ?, µ-, p, p0, ?0, S , S0 , ?,.
• 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

• 1950s 1960s 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)
• 1962 eightfold way, flavor SU(3) symmetry
  (Gell-Mann, Neeman)
• allows classification of particles into
  multiplets
• Mass formula relating masses of particles in same
  multiplet
• Allows prediction of new particle O- , with all
  of its properties (mass, spin, expected decay
  modes,..)
• subsequent observation of O- with expected
  properties at BNL (1964)

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?-

• The bubble chamber picture of the first
  omega-minus. An incoming K- meson interacts with
  a proton in and produces an omega-minus, a K and
  a K meson. The 5.0 GeV/c K-
• beam interacts with a proton of the
  liquid hydrogen in the bubble chamber
• K- p ? ?- K K0
• The omega minus then decays ?- ?gt ?- p-,
  with subsequent decay ?- ?gt ? p0
• ? ?gt p p- p0 ?gt ? ? ? ?gt e e-

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?-
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Particle nomenclature

• by mass
• baryons heavy particles .. p, n, ?, S, O-,
  ? .
• nucleons and their excited states p, n, N, ?
  ,
• hyperons ?, S, ?, O- , and their excited
  states
• mesons medium-heavy particles p, K, K, ?,
  ?
• leptons light particles e, µ, ?e, ??
• by spin
• fermions spin odd-integer multiple of ½
  ½, 3/2, 5/2,
• 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 gt 10-17 sec
  (decay by weak or electromagnetic
  interaction)
• unstable particles (resonances) lifetime
  lt10-20 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 gt
  - f2, f1 , f3 gt
• 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 fermions
  (wave function stays the same) b1, b2 ,
  b3 gt b2, b1 , b3 gt
• 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
• u 2/3, d -1/3, s -1/3
• baryons are made of 3 quarks
• p uud, n udd, ? uds, S uus ?
  uss, O- sss
• mesons are made of quark-antiquark pairs
• p ud, p- u d, p0 u u d d, ..

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The 8-fold way
baryons qqq
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color charge

• problem with quark model
• Quarks have spin ½, i.e. are fermions ? must
  obey Pauli principle
• O- sss, has spin 3/2 spins of 3 s quarks must
  be aligned, i.e. O- 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
• red blue green
• antiquarks carry anticolor
• 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, ?, S, O-, ? , p, K, K, ?,
  ?,) are not fundamental particles are made of
  quarks, interact electromagnetically, weakly,
  and strongly

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Standard Model

• A theoretical model of interactions of elementary
  particles, based on quantum field theory
• Symmetry
• SU(3) x SU(2) x U(1)
• Matter particles
• Quarks up, down, charm,strange, top, bottom
• Leptons electron, muon, tau, neutrinos
• Force particles
• Gauge Bosons
• ? (electromagnetic force)
• W?, Z (weak, electromagnetic)
• g gluons (strong force)
• Higgs boson
• spontaneous symmetry breaking of SU(2)
• mass

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Matter and forces

• Fundamental forces (mediated by force
  particles)
• strong interaction between quarks, mediated by
  gluons (which themselves feel the force) (QCD)
• leads to all sorts of interesting behavior, like
  the existence of hadrons (proton, neutron) and
  the failure to find free quarks
• Electroweak interaction between quarks and
  leptons, mediated by photons (electromagnetism)
  and W and Z bosons (weak force)
• Fundamental constituent particles
• Leptons q quarks q
• -1 e ? ? 2/3 u c t
• 0 ?e ?? ?? 1/3 d s b
• Role of symmetry
• Symmetry (invariance under certain
  transformations) governs
  behavior of physical systems
• Invariance ? conservation laws (Noether)
• Invariance under local gauge transformations
  ? interactions (forces)

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From Contemporary Physics Education Project
http//www.cpepweb.org/particles.html
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From Contemporary Physics Education Project
http//www.cpepweb.org/particles.html
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From Contemporary Physics Education Project
http//www.cpepweb.org/particles.html
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Strong quark interactions

• quarks carry color charge (red, blue, green)
  and interact exchanging gluons, the carriers of
  the strong force
• theory of strong interaction is gauge theory
  form of interaction governed by invariance under
  local SU(3)c (color SU(3))
• 8 gluons carry color charge ? interact with
  each other

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Electroweak interactions

• leptons (and also quarks) carry a weak charge
  (in addition to usual electric charge)
• they interact exchanging
• neutral EW force carriers photon ?, Z0
• charged EW force carriers W
• theory describing EW interaction is gauge
  theory gauge symmetry group SU(2)xU(1)

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Some milestones

• Quantum electrodynamics (QED) (1950s)
  (Feynman, Schwinger, Tomonaga)
• electroweak unification the standard model
  (1960s) (Glashow, Weinberg, Salam)
• deep inelastic scattering experiments partons
  (SLAC/MIT) (1956 1973)
• Quark Model (1964) (Gell-Mann, Zweig)
• Quantum Chromodynamics (1970s) (Gross,
  Wilczek, Politzer)
• neutral weak current (1973) (Gargamelle, CERN)
• Charm discovery (1974) (S. Ting, B. Richter)
• Bottom quark discovery (1977) (L. Lederman)
• gluon observation (1979) (DESY)
• W,Z observation (1983) (UA1, UA2, C. Rubbia,
  CERN)
• top quark (1995) (DØ, CDF, Fermilab)

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Brief History of the Standard Model

• Late 1920s - early 1930s Dirac, Heisenberg,
  Pauli, others extend Maxwells theory of EM to
  include Special Relativity QM (QED) - but it
  only works to lowest order!
• 1933 Enrico Fermi introduces 1st theory of weak
  interactions, analogous to QED, to explain b
  decay.
• 1935 Hideki Yukawa predicts the pion as carrier
  of a new, strong force to explain recently
  observed hadronic resonances.
• 1937 muon is observed in cosmic rays first
  mistaken for Yukawas particle
• 1938 heavy W as mediator of weak interactions?
  (Klein)
• 1947 pion is observed in cosmic rays
• 1949 Dyson, Feynman, Schwinger, and Tomonaga
  introduce renormalization into QED - most
  accurate theory to date!
• 1954 Yang and Mills develop Gauge Theories
• 1950s - early 1960s more than 100 hadronic
  resonances have been observed !
• 1962 two neutrinos!
• 1964 Gell-Mann Zweig propose a scheme whereby
  resonances are interpreted as composites of 3
  quarks. (up, down, strange)

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Brief History of the Standard Model - 2

• 1970 Glashow, Iliopoulos, Maiani 4th quark
  (charm) explains suppression of K decay into ??
• 1964-1967 spontaneous symmetry breaking (Higgs,
  Kibble)
• 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.
• 1967 deep inelastic scattering shows Bjorken
  scaling
• 1969 parton picture (Feynman, Bjorken)
• 1971-1972 Gauge theories are renormalizable
  (even when symmetry is spontaneoulsy broken)
  (tHooft, 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

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Present scenario

• Most of whats 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
  p, p0, K0, J/?, p, n, ?0, S , S0, ?, O-
• give the quark composition of p, p0, K0, J/?,
  p, n, ?0, O-

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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