Photoproduction with a leading proton at HERA

Abstract The total cross section for the photoproduction process with a leading proton in the final state has been measured at γp  centre-of-mass energies  W of 91, 181 and 231 GeV. The measured cross sections apply to the kinematic range with the transverse momentum of the scattered proton restricted to p T ⩽0.2 GeV and 0.68⩽ z ⩽0.88, where z = E p ′/ E p is the scattered proton energy normalised to the beam energy. The cross section d σ γp → Xp ′ ( W , z )/d z is observed to be independent of  W and z within the measurement errors and amounts to (8.05±0.06 ( stat )±0.89 ( syst )) μb on average. The data are well described by a Triple Regge model in which the process is mediated by a mixture of exchanges with an effective Regge trajectory of intercept α i (0)=0.33±0.04 ( stat )±0.04 ( syst ) . The total cross section for the interaction of the photon with this mixture ( γα i → X ) can be described by an effective trajectory of intercept α k (0)=0.99±0.01 ( stat )±0.05 ( syst ) . Predictions based on previous triple Regge analyses of pp → pX data assuming vertex factorisation are broadly consistent with the γp data. The measured cross sections are compared with deep inelastic scattering leading proton data in the same region of  z and p T for photon virtuality Q 2 >2.5 GeV 2 . The ratio of the cross section for leading proton production to the total cross section is found to rise with  Q 2 .