Dijets in Diffractive Photoproduction and Deep-Inelastic Scattering at HERA

Reference O Gutsche, presented at ICHEP2004, Summer 2004
Document Conference paper
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Contact S Schaetzel
Abstract Measurements are presented of differential dijet cross sections in low-|t| diffractive photoproduction (photon virtuality Q^2 < 0.01 GeV^2) and deep-inelastic scattering (Q^2 > 4 GeV^2) processes of the type ep -> eXY, in which the photon dissociation system X is separated from a leading low-mass baryonic system Y by a large rapidity gap. The measurements are based on an integrated luminosity of 18 pb^-1 and are performed in the same kinematic ranges besides Q^2. Dijet events are identified in the gamma p rest frame using the inclusive k_T cluster algorithm. The cross sections are given at the level of stable hadrons and correspond to the kinematic range xpom < 0.03, E_t^jet1 > 5 GeV and E_t^jet2 > 4 GeV. The measurements of dijet rates in photoproduction and DIS are compared with NLO QCD predictions based on diffractive parton distributions previously obtained from a NLO QCD analysis of inclusive diffractive DIS. Whereas the diffractive dijet rate in DIS is in good agreement with QCD factorisation, the dijet rate in photoproduction is suppressed by about a factor 0.5 compared to the NLO QCD prediction. The results are suggestive of a breakdown of factorisation in photoproduction for both direct and resolved photon interactions. This is reminiscent of the observed breakdown of factorisation in diffractive dijet production in p pbar collisions.
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