Study of Elastic rho0 photoproduction at HERA using the ZEUS Leading   Proton Spectrometer
  The differential cross section $d\sigma/dt$ for elastic $\rho^0$ photoproduction, $\gamma p \rightarrow \rho^0 p (\rho^0 \rightarrow \pi^+ \pi^-)$, has been measured in $ep$ interactions at HERA. The squared four-momentum exchanged at the proton vertex, $t$, has been determined directly by measuring the momentum of the scattered proton using the ZEUS Leading Proton Spectrometer (LPS), a large scale system of silicon micro-strip detectors operating close to the HERA proton beam. The LPS allows the measurement of the momentum of high energy protons scattered at small angles with accuracies of 0.4% for the longitudinal momentum and 5 MeV for the transverse momentum. Photoproduction of $\rho^0$ mesons has been investigated in the interval $0.073< |t| <0.40 GeV^2$, for photon virtualities $Q^2<1 GeV^2$ and photon-proton centre-of-mass energies $W$ between 50 and 100 GeV. In the measured range, the $t$ distribution exhibits an exponential shape with a slope parameter $b = 9.8 \pm 0.8 (stat.) \pm 1.1 (syst.) GeV^{-2}$. The use of the LPS eliminates the contamination from events with diffractive dissociation of the proton into low mass states. 
ZEUS Collaboration