Probing gluon saturation with next-to-leading order photon production at central rapidities in proton-nucleus collisions

TL;DR

This paper calculates the photon production cross section in proton-nucleus collisions using the CGC effective field theory, highlighting its sensitivity to gluon saturation effects and multi-parton correlations.

Contribution

It provides a next-to-leading order computation of photon production within the CGC framework, emphasizing its sensitivity to gluon saturation and multi-parton correlations.

Findings

Photon cross section proportional to all-twist Wilson line correlators

Collinear and $k_ot$ factorization are leading twist approximations

Photon production is insensitive to hadronization, enhancing sensitivity to gluon saturation

Abstract

We compute the cross section for photons emitted from sea quarks in proton-nucleus collisions at collider energies. The computation is performed within the dilute-dense kinematics of the Color Glass Condensate (CGC) effective field theory. Albeit the result obtained is formally at next-to-leading order in the CGC power counting, it provides the dominant contribution for central rapidities. We observe that the inclusive photon cross section is proportional to all-twist Wilson line correlators in the nucleus. These correlators also appear in quark-pair production; unlike the latter, photon production is insensitive to hadronization uncertainties and therefore more sensitive to multi-parton correlations in the gluon saturation regime of QCD. We demonstrate that $k_\perp$ and collinear factorized expressions for inclusive photon production are obtained as leading twist approximations to our result. In particular, the collinearly factorized expression is directly sensitive to the nuclear gluon distribution at small $x$. Other results of interest include the realization of the Low-Burnett-Kroll soft photon theorem in the CGC framework and a comparative study of how the photon amplitude is obtained in Lorenz and light-cone gauges.