elementary particle

Any of those particles that combine to form atoms and all matter, the most familiar being the electron, proton, and neutron. More than 200 particles have now been identified by physicists, categorized into several classes as characterized by their mass, electric charge, spin, magnetic moment, and interaction. Although many particles were thought to be nondivisible and permanent, now most are known to be combinations of more basic particles.

Since the 1890s, inquiry has progressed from the atom to its nucleus to the hadrons that make up the nucleus to the quarks that make up the hadrons. Since the 1960s, the main interest of particle physicists has been the elucidation of hadron structure. Only a small number of the 200 known particles are nondivisible - basic and stable - those that cannot be subdivided. They include eight of the 12 leptons (the electron, the positron, and six kinds of neutrinos); the quarks (36 varieties); and the gauge bosons - the particles that carry the four fundamental forces (the photon, the graviton, eight kinds of gluons, and three kinds of weakons). All matter in the universe is now seen as being made up of various combinations and interactions of these particles.