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.
Categories: hep-ex