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

TL;DR

This paper reports measurements of elastic and proton-dissociative photoproduction of vector mesons at high momentum transfer at HERA, comparing results with QCD predictions and analyzing the Pomeron trajectory.

Contribution

It provides new differential cross section data for $ ho^0$, $phi$, and $J/eta$ mesons at large -t, and extracts the Pomeron trajectory from the data.

Findings

Differential cross sections dσ/dt measured up to -t=3 GeV^2.

Pomeron trajectory parameters derived from W dependence.

Spin-density matrix elements analyzed from decay angles.

Abstract

Elastic and proton-dissociative photoproduction of $\rho^0$, $\phi$ and $J/\psi$ vector mesons ($\gamma p\to Vp$, $\gamma p\to VN$, 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 $\gamma p$ average center-of-mass energy of <W> = 94 GeV. Results for the differential cross sections, d$\sigma$/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 d$\sigma$/d$t$ for both $\rho^0$ and $\phi$ mesons.