Photoproduction with a leading proton at HERA

The total cross section for the photoproduction process with a leading proton in the final state has been measured at yp 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 less than or equal to 0.2 GeV and 0.68 less than or equal to z less than or equal to 0.88, where z = E'(p)/E-p is the scattered proton energy normalised to the beam energy. The cross section d sigma (gammap-->Xp')(W, z)/dz is observed to be independent of W and z within the measurement errors and amounts to (8.05 +/- 0.06 (stat) +/- 0.89 (syst)) Pb 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 alpha (i)(0) = 0.33 +/- 0.04 (stat) +/-0.04 (syst). The total cross section for the interaction of the photon with this mixture (gamma alpha (i) --> X) can be described by an effective trajectory of intercept alpha (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 yp 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 GeV2. The ratio of the cross section for leading proton production to the total cross section is found to rise with Q(2). (C) 2001 Elsevier Science B.V. All rights reserved.