Diffractive Dijet Production with a Leading Proton in ep Collisions at HERA

The production of dijets with a tagged forward proton is measured at HERA. The data were recorded with the H1 detector at DESY in the years 2006-2007. Events with a leading proton are detected using the very forward proton spectrometer of the H1 detector. Two jets are selected with transverse momenta transverse momenta in the hadronic-centre-of-mass larger than 4 and 5.5GeV, respectively. The analysis is performed both in the regime of deep-inelastic scattering (DIS), with momentum transfer $Q{^2} > 4$ GeV${^2}$ and for photoproduction ({\gamma}p), with $Q{^2} < 2$ GeV${^2}$. Cross sections are measured single-differentially in various kinematic quantities. For DIS, the data are found to be in good agreement with NLO QCD calculations based on diffractive parton densities determined from inclusive diffractive cross section measurements. For {\gamma}p, the cross sections are found to be overestimated by approximately a factor of two.