Inclusive diffractive dissociation in photoproduction at HERA

A new measurement of the differential cross section M-X(2) dsigma/dM(2)(X) for the process gammap --> XY, with a large rapidity gap between the systems X and Y and with Y a proton or a low mass proton excitation, is presented at three centre-of-mass energies (W) = 91, 187 and 231 GeV. The slope parameter alpha(eff) (0) obtained from a single regge type parameterization with a single effective trajectory is presented as a function of M-X(2). A combined X triple regge fit is performed over these data together with leading proton data from the same experiment and lower energy fixed target data. The pomeron intercept alpha(P)(0) is extracted from, this fit and was found to be alpha(P)(0)= 1. 127 +/- 0.004(stat) +/- 0.025 (syst) +/- 0.046 (mod).