Measurement of proton-dissociative diffractive photoproduction of vector mesons at large momentum transfer at HERA

Abstract

Diffractive photoproduction of vector mesons, $\gamma p \to V Y$ , where Y is a proton-dissociative system, has been measured in e+p interactions with the ZEUS detector at HERA using an integrated luminosity of 25 $\pbi$ . The differential cross section, $d\sigma/d t$ , is presented for $1.2 < -t<12{\rm GeV}^2$ , where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, $d\sigma/d t \propto (-t)^{-n}$ . The slope of the effective Pomeron trajectory, measured from the W dependence of the $\rho^0$ and $\phi$ cross sections in bins of t, is consistent with zero. The ratios $d\sigma_{\gamma p \to \phi Y}/d t$ to $d\sigma_{\gamma p \to \rho^0 Y}/d t$ and $d\sigma_{\gamma p \to J/\psi Y}/d t$ to $d\sigma_{\gamma p \to \rho^0 Y}/d t$ increase with increasing -t. Decay-angle analyses for $\rho^0$ , $\phi$ and $J/\psi$ mesons have been carried out. For the $\rho^0$ and $\phi$ mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models.