Particle Production in e⁺e^- Collisions

Abstract

So far, we have only discussed the light quarks, u and d, and those hadrons composed of these two quarks. The easiest way to produce hadrons with heavier quarks is in e⁺e^- collisions. Free electrons and positrons may be produced rather easily. They can be accelerated, stored and made to collide in accelerators. In an electron-positron collision process, all particles which interact electromagnetically and weakly can be produced, as long as the energy of the beam particles is sufficiently high. In an electron-positron electromagnetic annihilation, a virtual photon is produced, which immediately decays into a pair of charged elementary particles. In a weak interaction,the exchanged particle is the heavy vector boson Z⁰ (cf. the diagram and see Chap. 11). The symbol f denotes an elementary fermion (quark or lepton) and f its antiparticle. The f$\bar f$ system must have the quantum numbers of the photon or the Z⁰, respectively. In these reactions all fundamental, charged particle-antiparticle pairs can be produced; lepton-antilepton and quark-antiquark pairs. Neutrinos are electrically neutral; hence, neutrino-antineutrino pairs can only be produced by Z⁰ exchange.