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# Particle physics

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Particle physics, or quantum physics, studies the elementary constituents of matter and the interactions between them. In modern Quantum Mechanics, particles and field quanta are interchangeable - for instance electrons are just the quanta of an electron field, and the electromagnetic field is carried by particles called photons. The name particle is somewhat of a misnomer, then, as they are really non-linear waves that only look particle-like in certain situations.

Fundamental particles are subdivided into boson?s and fermion?s, the former with [integer spin]? (for example, 0, 1, 2) and the latter with [half-integer spin]? (for example, 1/2, 1 1/2). Fermions are subject to the [Pauli Exclusion Principle]? which states that no two particles can share the same quantum numbers--that is, two identical fermions can't be in the same position, momentum, energy, and angular momentum state at the same time, whereas bosons can. At extremely low energies, fermions pair up to form pseudo-boson?s which can then share the same [quantum number]?s. This state of matter has been achieved in [Bose-Einstein condensates], superfluids and superconductors.

The field quanta of the fundamental forces are all bosons:

- Photons carry the [electromagnetic interaction]?
- The [W boson]?s and [Z boson]? carry the [weak interaction]?
- The 8 gluon?s carry the [strong interaction]?
- The gravitons are believed to carry the [gravitational interaction]? (that is, if it is quantized)
- Various high mass bosons are predicted by unification models, notably the [Higgs boson]?s and [X boson]?s.

The basic constituents of matter are fermions, including the well-known proton, neutron, and electron. Of these, though, only the electron is really elementary, the other two being aggregates of smaller particles held together by the strong interaction. What appear to be elementary fermions come in four basic varieties, each of which come in three generations with different masses, for a total of twelve different "flavors":

                            Symbol   Electromagnetic charge   Weak charge*   Strong charge (color)   Mass
     Electron                 e-            -1                  -1/2             0
     Muon                     μ-            -1                  -1/2             0
     Tau                      τ-            -1                  -1/2             0

     Up quark                 u            +2/3                 +1/2           R/G/B
     Charm quark              c            +2/3                 +1/2           R/G/B
     Bottom quark             b            +2/3                 +1/2           R/G/B

     Down quark               d            -1/3                 -1/2           R/G/B
     Strange quark            s            -1/3                 -1/2           R/G/B
     Top quark                t            -1/3                 -1/2           R/G/B

     Electron neutrino        Ve             0                  +1/2             0
     Muon neutrino            Vμ             0                  +1/2             0
     Tau neutrino             Vτ             0                  +1/2             0

     * - particles on the table only have a weak charge when they have left-handed spin, and their antiparticles, right-handed.

Colorless particles (leptons) occur free, but because gluons are charged, the strong force gets stronger with distance, and so colored particles (quarks) are always found in colorless combinations called hadrons. These are either fermionic baryon?s composed of three quarks (for example, protons and neutrons) or bosonic meson?s composed of a quark-antiquark pair (for example, pions). The total mass of such aggregates exceeds that of the components thanks to the binding energy and in fact each comes in a series of energy states.

Different generations of particles are really just different energy eigenstates too, and so particles can occur in a superposition? of them. This happens mainly inside hadrons where two quarks have the same quantum numbers, but experiments have recently suggested neutrinos might fluctuate between generations, which could explain the solar neutrino problem. So far no experiments have suggested extra generations, but why this should be so is not yet entirely clear. As always, higher energy states tend to be unstable, and aside from electrons and neutrinos the only reasonably stable particles are protons and neutrons (neutrons have a half-life of fifteen minutes outside a nucleus but inside are stabilized by resonance with protons). The decay of a proton has been predicted by most models but has yet to be observed.

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Last edited August 6, 2001 5:41 pm (diff)
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