The ZEUS Leading Proton Spectrometer and its use in the measurement of elastic ρ⁰ photoproduction at HERA

Abstract

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 < ¦t¦< 0.40 GeV², for photon virtualities Q² < 1 GeV² 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 ± 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.