Measurement of Di-jet Cross-Sections in Photoproduction and Photon Structure

The photon has been shown to often assume the characteristics of a hadronic fluctuation when interacting with hadronic matter. The hadronic fluctuation may form a bound meson carrying the quantum number of the photon or lead to quasi free partons, i.e. quarks and gluons, interacting separately with the hadronic target. The latter process is commonly described as the resolved process. In this picture it is meaningful to determine the partonic content of the photon. This measurement describes the extraction of the gluonic component of the photon thus complementing measurements at e+e- colliders which probe its quark content.

To achieve this the H1 experiment has used a sample of photoproduction events with an explicit di-jet structure. In order to ensure sufficiently energetic interactions between a quasireal photon and a proton, the electron was tagged under very small angles and selected to carry roughly half the energy of the incident lepton beam. At lowest order the two jets originate from the hard interaction between a parton of the proton and a parton with momentum fraction xgamma in the photon. Since the partonic content of the proton has been well measured the partonic content of the photon can be inferred by an unfolding procedure.
The extraction procedure is severely hampered at small transverse momenta of the jets by the influence of multiple partonic interactions which are not easily included in the theoretical description. However, at larger ET perturbative QCD is able to successfully describe the measured distributions without being dominated by such disturbing influences. H1 has measured the influence of the underlying events and hence could extract the gluonic component in the photon (see figure).

The number of gluons in the photon is seen to rise with decreasing xgamma, an observation that has also been made for the gluon component in the proton. This feature seems to be a general property of quarkonic matter surrounded by gluons. The measured distribution is well described by a QCD model (in leading order perturbation theory).

Last Update Feb 29, 2000, E. Elsen