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

Elastic and proton–dissociative photoproduction of ρ, φ and J/ψ vector mesons (γp → V p, γp→ V N , respectively) have been measured in ep interactions at HERA up to −t = 3 GeV, 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, to values less than 0.01 GeV, and selects a γp average center-of-mass energy of 〈W 〉 = 94 GeV. Results for the differential cross sections, dσ/dt, for ρ, φ and J/ψ 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 ρ mass region and of the spindensity matrix elements determined from the decay-angle distributions. The Pomeron trajectory has been derived from measurements of theW dependence of the elastic differential cross sections dσ/dt for both ρ and φ mesons. DESY 99-160 October 1999