Study of elastic ρ0 photoproduction at HERA using the ZEUS leading proton spectrometer

The differential cross section dσ/dt for elastic ρ0 photoproduction, γp→ρ0p(ρ0→π+π-), 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 ρ0 mesons has been investigated in the interval 0.073< vertical stroke t vertical stroke <0.40 GeV2, for photon virtualities q2<1 GeV2 and photon-proton centre-of-mass energies between 50 and 100 GeV. In the measured range, the t distribution exhibits an exponential shape with a slope parameter b=9.8±0.8(stat.)±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. (orig.)