Generally all particles other than field particles can be classified into two broad categories:
- Hadrons
- Leptons
Hadrons
Particles that interact through the strong force are known as hadrons. The two classes of hadrons, mesons and baryons are distinguished by their masses and spins.
Mesons all have spin 0 and 1, with masses between that of the electron and that of the proton. All mesons are known to decay ultimately into electrons, positrons, neutrinos, and photons. The pion is the lightest of known mesons; it has a mass of approximately 140 MeV/c2 and a spin of 0. One more is the K meson, with a mass of approximately 500 MeV/c2 and a spin of 0.
Baryons, have masses equal to or greater than the proton mass and their spins are always odd half-integer values (½, 3/2, 5/2, etc.). Protons and neutrons are baryons, as are many other particles. With the exception of the proton, all baryons decay in such a way that the end products include a proton.
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It is imperative to note that hadrons are composite particles, not point particles, and have a measurable size of about 1 fm (10-15 m). Hadrons are composed of more elemental units called quarks, which are believed to be truly structure-less point particles. Mesons consist of two quarks and baryons of three.
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Leptons
Leptons are a group of particles that participate in the electromagnetic and weak interactions. All leptons have spins of ½. Unlike hadrons, which have size and structure, leptons appear to be truly elementary point-like particles with no structure. Furthermore, unlike hadrons, the number of known leptons is small. Presently, scientists believe there are only six leptons: electron, muon, tau, and associated with each of these particles, three different neutrinos: electron neutrino (ve), muon neutrino (vμ) and the tau neutrino (vτ).
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