A First Study of the Structure of the Virtual Photon at HERA

Abstract

The production of two or more jets of hadrons in photoproduction events at the HERA e+p collider has been studied using the ZEUS detector. By tagging the final state positron, two samples of event have been isolated where the photon exchanged between proton and positron is quasi-real (of virtuality p2< 0.02 GeV2) and virtual (0.1 < p2< 0.55 GeV2) respectively. It is shown that photons in both P2 ranges show resolved structure. The P2 evolution of the structure of the photon is studied by measuring the relative contribution of direct and resolved photon processes to the cross-section for the production of two or more jets. Events have been classified as direct or resolved based upon the value of the final state observable xgammaobs The data suggest that the contribution from resolved photon processes is suppressed relative to that from direct photon processes as P2 rises and are in general agreement with leading order calculations. Limited statistics in the data prevent a more quantitative study