Particle physics is the study of the fundamental constituents of matter and their interactions. However, which particles are regarded as fundamental has changed with time as physicists’ knowledge has improved. Modern theory – called the standard model – attempts to explain all the phenomena of particle physics in terms of the properties and interactions of a small number of particles of three distinct types: two spin– 1 2 families of fermions called leptons and quarks, and one family of spin-1 bosons – called gauge bosons – which act as ‘force carriers’ in the theory. In addition, at least one spin-0 particle, called the Higgs boson, is postulated to explain the origin of mass within the theory, since without it all the particles in the model are predicted to have zero mass. All the particles of the standard model are assumed to be elementary; i.e. they are treated as point particles, without internal structure or excited states.
The most familiar example of a lepton is the electron e− (the superscript denotes the electric charge), which is bound in atoms by the electromagnetic interaction, one of the four fundamental forces of nature.Asecond well-known lepton is the electron neutrino ve, which is a light, neutral particle observed in the decay products of some unstable nuclei (the so-called β-decays). The force responsible for the β-decay of nuclei is called the weak interaction.