Measurement of Elastic omega Photoproduction at HERA
  The reaction $\gamma p \rightarrow \omega p$ $(\omega \rightarrow \pi^+\pi^-\pi^0$ and $\pi^0\rightarrow\gamma\gamma)$ has been studied in $ep$ interactions using the ZEUS detector at photon-proton centre-of-mass energies between $70$ and $90 GeV$ and $|t| < 0.6 GeV^2$, where $t$ is the squared four momentum transferred at the proton vertex.The elastic \ome photoproduction cross section has been measured to be $\sigma_{\gamma p\rightarrow \omega p} = 1.21\pm 0.12\pm 0.23 \mu b$. The differential cross section $d\sigma_{\gamma p\rightarrow \omega p} /d|t|$ has an exponential shape $e^{-b |t|}$ with a slope $b = 10.0\pm 1.2\pm 1.3 GeV^{-2}$. The angular distributions of the decay pions are consistent with s-channel helicity conservation. When compared to low energy data, the features of $\omega$ photoproduction as measured at HERA energies are in agreement with those of a soft diffractive process. Previous measurements of the $\rho^0$ and $\phi$ photoproduction cross sections at HERA show a similar behaviour. 
ZEUS Collaboration, M. Derrick et al