Direct Photons at the CERN ISR

Abstract

A hard scatter between constituent partons in hadronic interactions usually results in a system of two jets at large angles accompanied by two spectator jets. The resulting high multiplicity and the variety of contributing diagrams make the comparison with theory difficult. However, processes where one of the out-going jets is replaced by a photon often offer a more incisive comparison between experiment and theory. Photons can be produced as a result of quark-gluon collisions (Compton process) or by quark-antiquark annihilation. There are several higher order processes involving the radiation of photons from interacting quarks (bremsstrahlung process) where the photon will be close to an accompanying jet. In proton-proton collisions at ISR energies, direct photon production is predicted to be dominated by the Compton process. This implies that the recoil (away-side) jet will be a quark jet and study of the recoil jet will thus be a study of quark fragmentation. Further, it would be expected that there will be no accompanying (same-side) jet, and a measurement of the direct photon cross section will measure the gluon structure function in the interacting protons (modulo soft gluon corrections).