An approach to a structure of the virtual photon using dijet events at HERA

The need of a partonic structure of the virtual photon to describe experimental data has been studied by comparing measurements of inclusive dijet cross sections of positron/proton interactions in the virtuality range 0.1 < Q2 < 120 GeV2 to NLO QCD calculations. The measurements have been performed with the ZEUS experiment at the storage ring HERA using an integrated luminosity of 38.6 pb-1. Jets were identified in the γ*p center-of-mass frame of reference using the longitudinal invariant kT cluster algorithm. The inclusive dijet cross sections were measured for jet transverse energies ET1 > 7.5 GeV and ET2 > 6.5 GeV in the pseudorapidity range -3 < η1,2 < 0, where the indices 1,2 refer to the jets with the highest and next highest transverse energy and the values refer to the γ*p center-of-mass frame of reference. The results are presented as functions of Q2, ET1, ηfor and xγobs. Here, ηfor is the pseudorapidity of the most forward jet and xγobs is an estimation of the momentum fraction of the photon entering the hard part of the interaction. Though the NLO QCD calculations including a parton density function for the virtual photon describe the experimental data better than those without, due to the large theoretical uncertainties no conclusions could be made that a partonic structure of the virtual photon is really necessary to describe the data. (orig.)