Particle Physics

The first discovered of what we would now call an elementary particle is the electron; although its discovery was a long and complicated process, J. J. Thomson's experiments of 1897 played a decisive role, since he was the first to obtain a quantitative value for e. Remarkably, precision experiments conducted last year (2006) [1] show that the electron still possesses no structure other than that demanded by quantum mechanics and relativity — it is a point particle. The proton, as the nucleus of the hydrogen atom, was identified as soon as the Rutherford scattering experiment demonstrated the ► model of the atom (► large-angle scattering); its partner in the nucleus, the neutron, was discovered by Chadwick in 1932. (For a review of the history of particle physics told in words of some of its creators, see Ref. [13]. See also Refs. [14,15].)

Antiparticles were theoretically predicted by P.A.M. Dirac in 1928 on the basis of his famous ► Dirac equation describing the relativistic electron, or more generally, any particle carrying ► spin ħ/2 [2]. At first he thought the positive proton was the antiparticle to the negative electron, but then he was convinced that the antipar-ticle had to have the same mass as the particle (this is now seen as a consequence of the famous ► CPT theorem). The positron was actually discovered in 1933 by Anderson, Blackett, and Occhialini [3]. The antiproton was found in 1955 [4].