The lepton‒hadron interaction

Abstract

The scattering of leptons (e, μ, ν) by hadrons at high momentum transfer has been the incisive probe of the internal dynamics of the proton. Point-like ‘partons’ are found inside the protons. These have the electromagnetic and weak properties expected of quarks, and are in dynamical equilibrium with a sea of virtual quark─antiquark pairs and gluons, the quanta of the strong force. Neutrino scattering has probed the weak coupling of the quarks, and their parity-violating handedness. Weak neutral currents (coupling with the Z 0 boson) were discovered in neutrino scattering, and parity-violating γ ─ Z 0 interference was observed in electron─deuteron scattering. The higher momentum transfers that will be available at the HERA electron─proton storage ring, now under construction, will greatly expand the kinematic region available to search for new phenomena at and beyond the W, Z energy scale.