Diffraction Dissociation in Photoproduction at HERA

TL;DR

This paper investigates diffraction dissociation in photoproduction at HERA, analyzing the process gamma p --> XY with large rapidity gaps, measuring cross sections, and extracting the pomeron intercept to understand diffraction mechanisms.

Contribution

It provides the first measurements of proton dissociation at high energies and applies a triple-Regge model to describe the data across different energies and mass ranges.

Findings

The pomeron intercept is measured as 1.068 ± 0.016 (stat.) ± 0.022 (syst.) ± 0.041 (model).

Diffractive processes dominate at low masses, with a significant subleading contribution at higher masses.

The diffractive contribution to gamma p --> Xp is approximately 22% at W_gp = 187 GeV.

Abstract

A study is presented of the process gamma p -->XY, where there is a large rapidity gap between the systems X and Y. Measurements are made of the differential cross section as a function of the invariant mass mx of the system produced at the photon vertex. Results are presented at centre of mass energies of W_gp = 187 GeV and W_gp = 231 GeV, both where the proton dominantly remains intact and, for the first time, where it dissociates. Both the centre of mass energy and the mx^2 dependence of HERA data and those from a fixed target experiment may simultaneously be described in a triple-Regge model. The low mass photon dissociation process is found to be dominated by diffraction, though a sizable subleading contribution is present at larger masses. The pomeron intercept is extracted and found to be alpha_pom(0) = 1.068 \pm 0.016 (stat.) \pm 0.022 (syst.) \pm 0.041 (model), in good agreement with values obtained from total and elastic hadronic and photoproduction cross sections. The diffractive contribution to the process gamma p --> Xp with mx^2 / W_gp^2 < 0.05 is measured to be 22.2 \pm 0.6 (stat.) \pm 2.6 (syst.) \pm 1.7 (model) % of the total gamma p cross section at W_gp = 187 GeV.