Notes

Chapter 9: Fundamental Physics

Section 14: Elementary Particles


Types of [elementary] particles

Current particle physics identifies three basic types of known elementary particles: leptons, quarks and gauge bosons. The known leptons are the electron (e), muon (μ) and and lepton (τ), and their corresponding neutrinos (ne, nμ, nτ). Quarks exist inside hadrons like the proton and pion, but never seem to occur as ordinary free particles. Six types are known: u, d, c (charm), s (strange), t (top), b. Gauge bosons are associated with forces. Those currently known are the photon (γ) for electromagnetism (QED), W and Z for so-called weak interactions, and the gluon (g) for QCD interactions between quarks. Gravitons associated with gravitational forces presumably also exist. In ordinary matter, the only particles that contribute in direct ways to everyday physical, chemical and even nuclear properties are electrons, photons and effectively u and d quarks, and gluons. (These, together presumably with some type of neutrino, are the only types of particles that never seem to decay.) The first reasonably direct observations of the various types of particles were as follows (some were predicted in advance): e (1897), γ (~1905), u, d (1914/~1970), μ (1937), s (1946), ne (1956), nμ (1962), c (1974), τ, nτ (1975), b (1977), g (~1979), W (1983), Z (1983), t (1995).

Most particles exist in several variations. Apart from the photon (and graviton), all have distinct antiparticles. Each quark has 3 possible color configurations; the gluon has 8. Most particles also have multiple spin states. Quarks and leptons have spin 1/2, yielding 2 spin states (neutrinos could have only 1 if they were massless). Gauge bosons normally have spin 1 (the graviton would have spin 2) yielding 3 spin states for massive ones. Real massless ones such as the photon always have just 2. (See page 1046.)

In the Standard Model the idea of spontaneous symmetry breaking (see page 1047) allows particles with different masses to be viewed as manifestations of single particles, and this is effectively done for W, Z, g, as well as for each of the 3 so-called families of quarks and leptons: u, d; c, s; t, b and e, ne; μ, nμ; τ, nτ. Grand unified models typically do this for all known gauge bosons (except gravitons) and for corresponding families of quarks and leptons—and inevitably imply the existence of various additional particles more massive than those known, but with properties that are somehow intermediate. Some models also unify different families, and supersymmetric models unify quarks and leptons with gauge bosons.


From Stephen Wolfram: A New Kind of Science [citation]