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Measurement of Elastic $ϕ$ Photoproduction at HERA

The production of $ϕ$ mesons in the reaction $e^{+}p \rightarrow e^{+} ϕp$ ($ϕ\rightarrow K^{+}K^{-}$) at a median $Q^{2}$ of $10^{-4} \ \rm{GeV^2}$ has been studied with the ZEUS detector at HERA. The differential $ϕ$ photoproduction cross section $dσ/dt$ has an exponential shape and has been determined in the kinematic range $0.1<|t|<0.5 \ \rm{GeV^2}$ and $60 < W < 80 \ \rm{GeV}$. An integrated cross section of $σ_{γp \rightarrow ϕp} = 0.96 \pm 0.19^{+0.21}_{-0.18}$ $\rm{μb}$ has been obtained by extrapolating to {\it t} = 0. When compared to lower energy data, the results show a weak energy dependence of both $σ_{γp \rightarrow ϕp}$ and the slope of the $t$ distribution. The $ϕ$ decay angular distributions are consistent with $s$-channel helicity conservation. From lower energies to HERA energies, the features of $ϕ$ photoproduction are compatible with those of a soft diffractive process.