# Walter Greiner

• About 70 years ago, only a small number of “elementary particles”, thought to be the basic building blocks of matter, were known: the proton, the electron, and the photon as the quantum of radiation. All these particles are stable (the neutron is stable only in nuclear matter, the free neutron decays by beta decay: n→p+e+${\displaystyle {\overline {\nu }}}$). Owing to the availability of large accelerators, this picture of a few elementary particles has profoundly changed: today, the standard reference Review of Particle Properties lists more than 100 particles. The number is still growing as the energies and luminosities of accelerators are increased.