III.

Classification

Several hundred elementary particles are now known experimentally. They can be divided into several broad classes. Hadrons and leptons are defined according to the types of force that they are subject to (see below). The forces are transmitted by further types of particles, called exchange, or messenger, particles. Examples are listed in the accompanying table.

Protons and neutrons are the basic constituents of atomic nuclei, which, combined with electrons, form atoms. Photons are the fundamental units of electromagnetic radiation, which includes radio waves, visible light, and X-rays. The neutron is unstable as an isolated particle, disintegrating into a proton, an electron, and a type of antineutrino called an electron-antineutrino. This process is symbolized thus:

n → p + e + e

This process should not be thought of as the separation of three particles that were originally all present together in the neutron. The neutron ceases to exist, while the proton, electron, and electron-antineutrino are created.

The neutron has an average life of 917 seconds. When combined with protons, however, to form certain atomic nuclei, such as oxygen-16 or iron-56, the neutrons are stabilized. Most of the known elementary particles have been discovered since 1945, some in cosmic rays, the remainder in experiments using high-energy accelerators (see Particle Accelerators). The existence of a variety of other particles has been proposed, such as the graviton, thought to transmit the gravitational force.

In 1930 the British physicist Paul A. M. Dirac predicted on theoretical grounds that, for every type of elementary particle, there is another type called its antiparticle. The antiparticle of the electron was found in 1932 by the American physicist Carl D. Anderson, who called it the positron. The antiproton was found in 1955 by the American physicists Owen Chamberlain and Emilio Segrè. It is now known that Dirac’s prediction is valid for all elementary particles, though some elementary particles, such as the photon, are their own antiparticles. Physicists generally use a bar to denote an antiparticle; thus _e (the electron-antineutrino) is the antiparticle of v_u (the electron-neutrino).

Particles may also be classified in terms of their spin, or intrinsic angular momentum, as bosons or fermions. Bosons have a spin that is a whole-number multiple of h/2p, where h is Planck’s constant; fermions have a spin that is an odd-half-integer multiple of h/2p, such as ” (h/2p). Fermions obey the Pauli exclusion principle.