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

We discuss central-exclusive production (CEP) of photons via different fusion processes in the reaction pp-+ 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, IyI < 2.5, and at relatively low transverse momentum, 0.1 GeV < k perpendicular to < 1 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 y, k perpendicular to, 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 where the basic pp-+ pp reaction is due to strong interaction diffraction. This is remarkable as the CEP cross section is of order alpha 3em whereas the bremsstrahlung one is only of order alpha em. On the other hand, the soft-photon bremsstrahlung is more important than CEP in the forward rapidity range, IyI > 4, and/or at very low k perpendicular to. 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.

Automated summary

In this study, we discuss the central-exclusive production (CEP) of photons in proton-proton collisions at high energies using the tensor-Pomeron model. We consider different fusion processes, including photon-Pomeron and photon-Reggeon fusion reactions, as well as Reggeon-Pomeron and Odderon-Pomeron fusion reactions. Our predictions show that photoproduction is an important process for midrapidity photons with low transverse momentum. This has implications for the ALICE detector at the LHC. The measurement of outgoing protons is not necessary to validate our results.