Study of Elastic rho0 photoproduction at HERA using the ZEUS Leading Proton Spectrometer

TL;DR

This study measures the differential cross section of elastic rho0 photoproduction at HERA, utilizing the ZEUS Leading Proton Spectrometer to directly measure the scattered proton's momentum, providing precise data on the t distribution and eliminating diffractive dissociation contamination.

Contribution

The paper introduces the use of the ZEUS Leading Proton Spectrometer for direct proton momentum measurement in rho0 photoproduction, improving the accuracy of t distribution analysis.

Findings

t distribution follows an exponential shape with slope b = 9.8 ± 0.8 (stat.) ± 1.1 (syst.) GeV^{-2}

LPS effectively removes events with proton dissociation, enhancing measurement purity.

Photon virtualities are below 1 GeV^2, with W between 50 and 100 GeV.

Abstract

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.