Revised production cross-section of gamma-rays in p-p collisions with LHC data for the study of TeV gamma-ray astronomy

TL;DR

This paper revises the gamma-ray production cross-section in proton-proton collisions using LHC data, improving models for gamma-ray spectra relevant to TeV gamma-ray astronomy across a wide energy range.

Contribution

The study provides a new, data-driven cross-section formula that aligns well with LHC measurements, especially in the forward region, enhancing gamma-ray modeling for astrophysical applications.

Findings

Revised cross-section matches LHC data from GeV to 30 PeV energies.

Gamma-ray spectra in the forward region are characterized for the first time with LHCf data.

Average transverse momentum of gamma-rays increases slowly with energy.

Abstract

We present the production cross-section of gamma-rays based on data of p-p collisions at LHC, revising the previous semi-empirical formula mainly for 1) the inelastic cross-section in p-p collisions, $\sigma_{pp}(E_0)$, and 2) the inclusive gamma-ray spectrum in the forward region, $\sigma_{pp \rightarrow \gamma}(E_0, E_\gamma)$. We find that the previous cross-section gives a significantly softer spectrum than found in the data of LHC. In this paper, we focus our interest mainly upon the LHC forward (LHCf) experiment, giving gamma-ray spectra in the very forward region with the pseudo-rapidity $\eta^*$ > 8.8 in the center of mass system (CMS), which have not been reported so far. We also give the pseudo-rapidity distribution of charged hadrons with -3 < $\eta^*$ < 3 obtained by ALICE and TOTEM experiments, both with LHC. We find that the revised cross-section reproduces quite well the accelerator data over the wide energy range from GeV to 30 PeV for projectile protons, corresponding approximately to 100 MeV to 3 PeV for secondary gamma-rays. The production cross-section of gamma-rays produced in the forward region is essential for the study of gamma-ray astronomy, while not important are those produced in the central region in CMS, and of much less importance in the backward. We discuss also the average transverse momentum of gamma-rays, $\bar{p}_{t}$, and the average inelasticity transferred to gamma-rays, $\bar{k}_\gamma^*$, obtaining that the former increases very slowly with $\bar{p}_{t}$ = 100 - 220 MeV/c for $E_0$ = 1 GeV - 26 PeV, and the latter is almost independent of $E_0$, with $\bar{k}_\gamma^* \approx 1/6$, while we can not exclude the possibility of a small increase of $\bar{k}_\gamma^*$.