Diffractive dissociation in photoproduction at HERA
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A new measurement of the differential cross section d sigma /d M_X^2 for the process gamma p -> X Y, 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  effective pomeron trajectory intercept alpha_{eff}(0) obtained from the W dependence of the data is presented as a function of M_X^2. A combined Triple Regge fit is performed to these rapidity gap data, in combination with H1 data at larger $M_x$ and fixed target data. The pomeron intercept alpha_{pom}(0)$ is extracted from this fit.