Measurements of Diffractive Vector-Meson Photoproduction at High Momentum Transfer from the ZEUS Experiment at HERA

Abstract

We discuss recent preliminary results on the diffractive photoproduction of {\rhoz}, $\varphi$, and {\jpsi} mesons at high momentum transfer reported by the ZEUS collaboration at HERA. A special-purpose calorimeter served to tag the quasi-real photons ($\qsq<0.01 \gevsq$) in the process $\gamma +p\to \mathrm{VM}+Y$, where $Y$ represents a dissociated state of the proton. The resulting range in photon-proton center-of-mass energy extends from 80 to 120 \gev. The differential cross sections $\frac{\Dσ}{\D t}$ were obtained in the region ${\abst}>1.2 \gevsq$, where $t$ denotes the squared momentum transfer to the proton. The measurements provide good sensitivity to the observed power-law dependence $\frac{\Dσ}{\D t} \propto {(-t)}^{-n}$. Over the region in momentum transfer covered by the data, the power is found to be approximately $n\simeq 3$ for {\rhoz} and $\varphi$ photoproduction, and approximately $n\simeq 2$ for {\jpsi} photoproduction.

Contributed to the proceedings of DPF2000, the meeting of the Division of Particles and Fields of the American Physical Society, August 9-12, 2000, Columbus, Ohio, USA (3 pages including 2 figures) Replaced to remove postscript format error. Transparencies are available at http://www-zeus.physik.uni-bonn.de/~critten/dpf2000/dpft/index.html