Photon from the annihilation process with CGC in the $pA$ collision

TL;DR

This paper investigates photon production in proton-nucleus collisions within the color glass condensate framework, focusing on annihilation processes and providing detailed calculations and numerical results.

Contribution

It introduces a detailed calculation of photon production via annihilation diagrams in the CGC framework, expanding in proton color source and providing numerical results.

Findings

Numerical photon production rates fitted with specific functional forms.

First-order annihilation diagrams contribute significantly to photon yield.

Methodology clarifies the calculation of annihilation processes in the CGC model.

Abstract

We discuss the photon production in the $pA$ collision in a framework of the color glass condensate (CGC) with expansion in terms of the proton color source $\rho_p$. We work in a regime where the color density $\rho_A$ of the nucleus is large enough to justify the CGC treatment, while soft gluons in the proton could be dominant over quark components but do not yet belong to the CGC regime, so that we can still expand the amplitude in powers of $\rho_p$. The zeroth-order contribution to the photon production is known to appear from the Bremsstrahlung process and the first-order corrections consist of the Bremsstrahlung diagrams with pair produced quarks and the annihilation diagrams of quarks involving a gluon sourced by $\rho_p$. Because the final states are different there is no interference between these two processes. In this work we elucidate calculation procedures in details focusing on the annihilation diagrams only. Using the McLerran-Venugopalan model for the color average we numerically calculate the photon production rate and discuss functional forms that fit the numerical results.