Quarks and Leptons

Matter, according to the "standard model", consists of quarks and leptons, collectively (with their antiparticles) called fermions. Their common property is a half integral spin (s = 1/2, 3/2. 5/2, ...). The leptons are the electrons, muons and tau particles, along with their corresponding neutrinos (and, as always, their antiparticles). The quarks are six in number, labeled up, down (as a pair), charm, strange (another pair), and top, bottom (the third pair). These combine in groups of three to form protons and neutrons. For example, a neutron consists of one up quark (charge +2/3) and two down quarks (charge -1/3 each). When these particles interact the forces are carried by Bosons (characterized by integral spin quantum numbers (s = 0, 1, 2, ...). Bosons are photons, W particles (W⁺ and W^-) and Z particles.

A typical interaction is that of beta decay, when a neutron decays into a proton, an electron and an antineutrino. The force that carried this interaction is the W-. A pictorial representation of this interaction is shown to the right. Below are two charts showing the basic constituents of matter (the particles) and the forces that mediate interactions between these particles. The photon mediates electromagnetic interactions; W and Z particles are involved with interactions involving nuclear decay. For a complete chart of the standard model click here. Back to the previous page.