Particle physics

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Particle physics, also called high energy physics, is that branch of physics which 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. They are actually subject to Wave-Particle duality: objects that appear to be particles in some circumstances can be interpreted as waves in others.

Fundamental particles are subdivided into bosons and fermions, 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-bosons which can then share the same quantum numbers. 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 are responsible for generating the electromagnetic interaction
  • The W+ and W- bosons together with the Z0 boson are responsible for generating the weak nuclear force (weak interaction).
  • The 8 colorful gluons are responsible for the strong nuclear force (strong interaction). Six of these gluons come in color pairs of colors and anti-colors (for example, a gluon can carry red and anti-green)while the other two are a complicated mix of colors and anti-colors.
  • The graviton is believed to be responsible for the gravitational interaction, No gravitons have thus far been discovered. They remain theoretical objects until some evidence of their existence is found.
  • Various high mass bosons are predicted by unification models, notably the Higgs bosons or the higgson.

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":

SymbolElectromagnetic chargeWeak charge*Strong charge (color)Mass
Electrone--1-1/200.511 MeV
Muonμ--1-1/20105.6 MeV
Tauτ--1-1/201.784 GeV
Up quarku+2/3+1/2R/G/B~5 MeV
Charm quarkc+2/3+1/2R/G/B~1.5 GeV
Top quarkt+2/3+1/2R/G/B> 30 GeV
Down quarkd-1/3-1/2R/G/B~10 MeV
Strange quarks-1/3-1/2R/G/B~100 MeV
Bottom quarkb-1/3-1/2R/G/B~4.7 GeV
Electron neutrinoVe0+1/20< 50 eV
Muon neutrinoVμ0+1/20< 0.5 MeV
Tau neutrinoVτ0+1/20< 70 MeV

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

These particles can be arranged in three "generations", the first one consisting of the electron, the up and down quarks, and the electron neutrino. All ordinary matter is made from first generation particles; the higher generation particles decay quickly into the first generation ones and can only be generated for a short time in high-energy experiments.

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 baryons composed of three quarks (for example, protons and neutrons) or bosonic mesons 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.

Is it just me, or does the above paragraph say that there can be superpositions of, say, an electron and a neutrino? I'll try to clean this up, unless someone thinks I'm just not getting something here. The neutrino mention needs some cleaning too. --StephenFuqua

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