Exclusive diffractive photon bremsstrahlung at high energies

TL;DR

This paper analyzes high-energy exclusive diffractive photon bremsstrahlung in proton-proton collisions at the LHC, considering multiple mechanisms and identifying phase space regions where each dominates.

Contribution

It introduces the first comprehensive study of various diffractive bremsstrahlung mechanisms, including non point-like protons and multiple exchange processes, at high energies.

Findings

Cross section for diffractive bremsstrahlung is of the order of microbarns.

Classical bremsstrahlung dominates at large photon pseudorapidities.

Photon-pomeron mechanism dominates at midrapidities, but with small cross section.

Abstract

We discuss the $p p \to p p \gamma$ reaction at the LHC. We consider diffractive classical bremsstrahlung mechanisms including effects of non point-like nature of protons. In addition, we take into account (vector meson)-pomeron, photon-pion as well as photon-pomeron exchange processes for the first time in the literature. The integrated diffractive bremsstrahlung cross section ($E_{\gamma}>100$ GeV) is only of the order of $\mu$b. We try to identify regions of the phase space where one of the mechanisms dominates. The classical bremsstrahlung dominates at large forward/backward photon pseudorapidities, close to the pseudorapidities of scattered protons. In contrast, the photon-pomeron (pomeron-photon) mechanism dominates at midrapidities but the related cross section is rather small. In comparison the virtual-omega--rescattering mechanism contributes at smaller angles of photons (larger photon rapidities).