Measurements of Diffractive Vector-Meson Photoproduction at High   Momentum Transfer from the ZEUS Experiment at HERA
  We discuss recent preliminary results on the diffractive photoproduction of {\rhoz}, $\phi$, 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 $\gamma + p \to {\rm VM}+ Y$, 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 $\frac{\D\sigma}{\D t}$ 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 $\frac{\D\sigma}{\D t} \propto {(-t)}^{-n}$. Over the region in momentum transfer covered by the data, the power is found to be approximately $n\simeq 3$ for {\rhoz} and $\phi$ photoproduction, and approximately $n\simeq 2$ for {\jpsi} photoproduction. 
J.A. Crittenden (Physikalisches Institut der Universitaet Bonn)