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

Abstract. Elastic and proton–dissociative photoproduction of $\rho^0$, $\phi$ and $J/\psi$ vector mesons ($\gamma p\rightarrow Vp$, $\gamma p\rightarrow VN$, 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 $\gamma p$ average center-of-mass energy of $\langle W\rangle$ = 94 GeV. Results for the differential cross sections, $\mbox{d}\sigma/\mbox{d}t$, 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 $\mbox{d}\sigma/\mbox{d}t$ for both $\rho^0$ and $\phi$ mesons.