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 (``interactions'')
• at given length scale, it is useful to describe matter in terms of specific set of constituents which can be treated as fundamental; at shorter length scale, these fundamental constituents may turn out to consist of smaller parts (be ``composite'').
• 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 ???... ???

  

  (``energy'' means energy necessary to probe structure)

• Particle physics experiments collide particles to
  • produce new particles
  • reveal their internal structure and laws of their interactions by observing regularities, measuring cross sections,...
• particle physics = high energy physics: needs high energies
  • to make objects of large mass
  • to resolve structure at small distances

• in particle physics, mass-energy equivalence plays an important role; ``relativistic'' formulation necessary; relation between kinetic energy K, total energy E and momentum p :

  

  to simplify calculations, measure mass in and momenta in eV/c

• to study structure of small objects, need probe with short wavelength: use particles with high momentum to get short wavelength
• remember de Broglie wavelength of a particle

  

• de Broglie wavelengths for an electron vs its kinetic energy:

  

• cosmic rays:
  • early particle physics (1920 to 1940) used ``cosmic rays'' as source of high energy particles;
  • cosmic rays = particles (mainly protons) coming from the sun and also from other stars within and outside of our galaxy;
  • cosmic rays interact with nuclei in atmosphere create other particles, which in turn interact and create more particles development of ``air shower'' or ``cascade''
  • many new particles were discovered in studies using cosmic rays
• later (and now), used accelerators to ``accelerate'' particles to high energy, so they can be used as fine probes of structure of protons etc.
  use of accelerators allowed more systematic studies (can select energy, type of particle; not dependent on uncontrollable particle source)
• hundreds of particles discovered, wide variety of properties (mass, charge, spin, lifetimes, decay patterns,..) -- ``particle zoo''
• CLASSIFICATION OF PARTICLES:
  • according to mass: leptons (lightest), mesons (medium heavy), baryons (heavy);
  • according to spin:
    fermions (Fermi-Dirac particles) with half-integer spin
    bosons (Bose-Einstein particles) with integer spin
  • according to interactions:
    hadrons: feel strong interactions
    leptons: no strong interaction

• QUARK MODEL

  attempts to find order in the particle zoo led to development of the ``quark model''(1962-1964):

  • all ``hadrons'' (strongly interacting particles) are made of combinations of quarks;
  • there are three different kinds of quarks (``flavors'' of quark), called ``up'', ``down'', ``strange'' (u,d,s)
  • baryons made of three quarks, mesons of quark-antiquark
  • e.g. proton = , neutron = ,
  • quark model provided a kind of ``periodic table'' of the particles; explained the pattern of observed particles:
  • particles arranged in ``multiplets'',
  • predicted existence of particle, its mass and other properties;
  • observed in bubble chamber at BNL in 1964;
  • late 60's - early 70's: ``deep inelastic scattering'' experiments of electrons on protons, and ``hard scattering'' of protons on protons shows first evidence for proton substructure -- ``partons''
    (in pp scattering, saw more scattered particles at large angles than expected from uniform ball of nuclear matter - analogy with Rutherford's scattering experiment)
  • jets (manifestation of quarks) seen in collisions at SLAC and DESY 1975/1976
  • mid 70's: deep inelastic scattering of neutrinos and muons on protons and deuterons show that partons behave like quarks
    ``quark-parton model''

• to explain difficulties with quark model of baryons quarks are postulated to have property called ``color'' (1972-1973)
• quarks come in three colors (``red, blue, green'')
• development of theory of strong interactions
  called ``quantum chromodynamics''
• in parallel (late 60's), unification of electromagnetic and weak interactions -- ``electroweak interactions''
  predict existence of ``neutral currents'' in neutrino scattering,
  and existence of ``intermediate vector bosons W, Z
• neutral currents observed 1973
  (``Gargamelle'' bubble chamber at CERN);
• particle containing ``charmed'' quarks
  observed 1974 at Brookhaven and Stanford;
• particle containing ``beauty'' or ``bottom'' quarks
  observed 1977 at Fermilab
• W,Z observed 1983 (UA1, UA2 experiments at CERN);
• top quark
  observed 1995 by CDF and DØ experiments at Fermilab;

• the ``STANDARD MODEL''
  • fundamental constituents of matter are fermions
    - quarks and leptons;
  • they interact by the exchange of ``gauge bosons''
    hspace*1cm which are manifestations of ``quantum fields''
    described by ``quantum field theory''.
  • interactions:

    

    note:

    • range of strong interaction in table is range of strong ``Van der Waals force'' between hadrons, not that between quarks within hadrons
    • electromagnetic and weak interactions are ``unified'', are really different aspects of one force - ``electroweak interaction'';
    • weakness and short range of weak interaction
      is due to large mass of W,Z (80, 91 GeV)
  • matter particles come in three ``generations'':

    

SOME OF THE PARTICLES