Prompt photon - jet angular correlations at central rapidities in p+A collisions

TL;DR

This paper investigates photon-jet angular correlations in proton-nucleus collisions to probe the gluon distribution in nuclei, deriving analytic expressions and identifying dominant processes at small momentum fractions.

Contribution

It introduces a detailed calculation of photon-jet correlations in p+A collisions, highlighting the dominance of a specific process and providing analytic formulas for angular moments.

Findings

Photon-jet correlations are sensitive to the gluon distribution in nuclei.

The process $g+A o q ar{q} \gamma$ dominates over $q+A o q \gamma$ in certain regimes.

Analytic expressions for angular moments are derived and their transverse momentum dependence estimated.

Abstract

Photon-jet azimuthal correlations in proton-nucleus collisions are a promising tool for gaining information on the gluon distribution of the nucleus in the regime of non-linear color fields. We compute such correlations from the process $g+A\to q \bar{q} \gamma$ in the rapidity regime where both the projectile and target light-cone momentum fractions are small. By integrating over the phase space of the quark which emits the photon, subject to the restriction that the photon picks up most of the transverse momentum (to pass an isolation cut), we effectively obtain a $g+A\to q \gamma$ process. For nearly back-to-back photon-jet configurations we find that it dominates over the leading order process $q+A\to q \gamma$ by two less powers of $Q_\perp / Q_S$, where $\boldsymbol{Q}_\perp$ and $Q_S$ denote the net photon-jet pair momentum and the saturation scale of the nucleus, respectively. We determine the transverse momentum dependent gluon distributions involved in $g+A\to q \gamma$ and the scale where they are evaluated. Finally, we provide analytic expressions for $\langle\cos n\phi\rangle$ moments, where $\phi$ is the angle between $\boldsymbol{Q}_\perp$ and the average photon-jet transverse momentum $\tilde{\boldsymbol{P}}_\perp$, and first qualitative estimates of their transverse momentum dependence.