Measurement of diffractive photoproduction of vector mesons at large momentum transfer at HERA

Elastic and proton-dissociative photoproduction of rho(0), phi and J/psi vector mesons (gamma p --> Vp, gamma p --> VN, respectively) have been measured in e(+)p interactions at HERA up to -t = 3 GeV(2), where t is the four-momentum transfer squared at the photon-vector-meson vertex. The analysis is based on a data sample in which photoproduction reactions were tagged by detection of the scattered positron in a special-purpose calorimeter. This limits the photon virtuality, Q(2), to values less than 0.01 GeV2, and selects a gamma p average center-of-mass energy of (W) = 94 GeV. Results for the differential cross sections, d sigma/dt, for rho(0), phi and J/psi mesons are presented and compared to the results of recent QCD calculations. Results are also presented for the t-dependence of the pion-pair invariant-mass distribution in the rho(0) mass region and of the spin-density matrix elements determined from the decay-angle distributions. The Pomeron trajectory has been derived from measurements of the W dependence of the elastic differential cross sections d sigma/dt for both rho(0) and phi mesons.