Central exclusive diffractive production of a single photon in high-energy proton-proton collisions within the tensor-Pomeron approach

TL;DR

This paper models the central exclusive production of photons in high-energy proton-proton collisions using the tensor-Pomeron approach, predicting measurable distributions and highlighting the significance of photoproduction processes at the LHC.

Contribution

It introduces a tensor-Pomeron model for CEP of photons, providing detailed predictions and emphasizing the importance of photoproduction over diffractive bremsstrahlung in specific kinematic regions.

Findings

Photoproduction dominates in the specified midrapidity and low transverse momentum range.

CEP cross section is of order α_em^3, larger than bremsstrahlung which is of order α_em.

Soft-photon bremsstrahlung is more relevant at very forward rapidities and very low transverse momentum.

Abstract

We discuss central-exclusive production (CEP) of photons via different fusion processes in the reaction $pp \to pp \gamma$ at high energies, available at RHIC and LHC, within the tensor-pomeron model. We consider two types of processes, the photoproduction contribution via the photon-pomeron and photon-reggeon fusion reactions, and the purely diffractive contribution via the reggeon-pomeron and odderon-pomeron fusion reactions. We present predictions for the measurements of photons at midrapidity, $|{\rm y}| < 2.5$, and at relatively low transverse momentum, $0.1~{\rm GeV} < k_{\perp} < 1~{\rm GeV}$. To check the main results of our study the measurement of the outgoing protons is not necessary. This is of relevance, e.g., for the present version of the ALICE detector at the LHC. Several differential distributions, for instance, in ${\rm y}$, $k_{\perp}$ and $\omega$, the rapidity, the absolute value of the transverse momentum, and the energy of the photon, respectively, are presented. We show that the photoproduction is an important process in the kinematic region specified above. There it gives a much larger cross section than diffractive bremsstrahlung. This is remarkable as the CEP cross section is of order $\alpha_{\rm em}^{3}$ whereas the bremsstrahlung one is only of order $\alpha_{\rm em}$. On the other hand, the soft-photon bremsstrahlung where the basic $pp \to pp$ reaction is due to strong interaction diffraction is more important than CEP in the forward rapidity range, $|{\rm y}| > 4$, and/or at very low $k_{\perp}$. We leave it as a challenge for the planned ALICE 3 experiment at the LHC to study these two contributions to soft photon production in $pp$ collisions. This could shed new light on the so called ``soft photon puzzle'' in hadron-hadron collisions.