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

Elastic and proton-dissociative photoproduction of $ρ^0$, $ϕ$ and $J/ψ$ vector mesons ($γp\to Vp$, $γp\to VN$, 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 GeV$^2$, and selects a $γp$ average center-of-mass energy of <W> = 94 GeV. Results for the differential cross sections, d$σ$/d$t$, for $ρ^0$, $ϕ$ 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 $ρ^0$ 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$σ$/d$t$ for both $ρ^0$ and $ϕ$ mesons.