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Particle Physics


Particle Physics Particle physics what is it? why do it? Standard model Quantum field theory Constituents, forces Milestones of particle physics – PowerPoint PPT presentation

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Title: Particle Physics

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// (Fermilab homepage)
  • http// (DØ homepage)
  • http// (CERN -- European
    Laboratory for Particle Physics)
  • http// (CMS)
  • http//
    (has links to many particle physics
    sites and other sites of interest)
  • http// (Fermilab
    particle physics tour)
  • http// (Lawrence Berkeley

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

About Units
  • Energy - electron-volt
  • 1 electron-volt kinetic energy of an electron
    when moving through potential difference of 1
  • 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
  • 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

  • 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
  • Open questions, possible strategies
  • Present and future experiments, facilities
  • Search for Higgs particle
  • outlook

Sizes and distance scales
  • virus 10-7
  • Molecule 10-9m
  • Atom 10-10m
  • nucleus 10-14 m
  • nucleon 10-15m
  • Quark lt10-19m

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 are leptons that are bound to the
    nucleus by photons, which are bosons.
  • The nucleus of a hydrogen atom is just a single
  • 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   

Contemporary Physics Education Project
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
  • 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
  • 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,
  • now evidence that nucleons have substructure -
  • going down the size ladder atoms -- nuclei --
    nucleons -- quarks ???... ??? (preons, toohoos,

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

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

Victor Hess Balloon ride 1912
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  • 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,
  • 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

Anderson and his cloud chamber
  • 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
  • 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 assumed to be proton and electron
    in close association

Chadwicks experiment
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
  • 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

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.

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

  • 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

Strange particles
  • Kaon discovered 1946 first called V particles

K0 production and decay in a bubble chamber
Bubble chamber
p p ?? p n K0 K- ? ?- ?0 n p ?? 3 pions ?0 ??
??, ? ? e e- K0 ? ? ?-
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
  • or can we recognize patterns or symmetries in
    their nature (charge, mass, flavor) or the way
    they behave (decays)?

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
  • Mass formula relating masses of particles in same
  • Allows prediction of new particle O- , with all
    of its properties (mass, spin, expected decay
  • subsequent observation of O- with expected
    properties at BNL (1964)

  • 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-

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
  • 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
  • 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
  • unstable particles (resonances) lifetime
    lt10-20 sec (decay by strong interaction)

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
  • 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)

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
  • 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, ..

The 8-fold way
baryons qqq
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

Elementary particles?
  • leptons (electron, muon and their neutrinos)
    are fundamental, interact electromagnetically and
  • hadrons (p, n, ?, S, O-, ? , p, K, K, ?,
    ?,) are not fundamental particles are made of
    quarks, interact electromagnetically, weakly,
    and strongly

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

Matter and forces
  • Fundamental forces (mediated by force
  • 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)

(No Transcript)
From Contemporary Physics Education Project
From Contemporary Physics Education Project
(No Transcript)
From Contemporary Physics Education Project
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

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)

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,
  • top quark (1995) (DØ, CDF, Fermilab)

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
  • 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?
  • 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)

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,
  • 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
  • 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)

Brief History of the Standard Model - 3
  • 1974 J/? discovered at BNL/SLAC
  • 1975 J/? interpreted as cc bound state
  • 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)

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

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
  • 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

Study of high energy interactions -- going back
in time
Homework set 5
  • HW 5.1
  • go to Particle Data Group website
  • 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-

  • 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
  • precision tests of standard model ongoing,
    looking for hints of new physics
  • next
  • symmetries
  • tests of standard model, experiments,
  • problems and shortcomings of standard model
  • new projects, outlook
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