### Measurement of the $t$-dependence in Exclusive Photoproduction of $\Upsilon (1S)$ Mesons at HERA.

Jacek Ciborowski
The exclusive photoproduction reaction $\gamma p \rightarrow \Upsilon (1S) p$ has been studied with the ZEUS detector in $ep$ collisions at HERA using an integrated luminosity of 468 pb$^{-1}$. The measurement covers the kinematic range $60 < W < 220\:\GeV$ and $Q^2 < 1\:\GeV^2$, where $W$ is the photon-proton c.m.s. energy and $Q^2$ is the photon virtuality. The exponential slope, $b$, of the $t$ dependence of the cross section, where $t$ is the squared four-momentum...