Inclusive prompt photon production in electron-nucleus scattering at small x

TL;DR

This paper calculates the inclusive prompt photon production cross-section in electron-nucleus deep inelastic scattering at small x using the Color Glass Condensate framework, incorporating leading and next-to-leading order corrections.

Contribution

It provides the first leading order CGC-based calculation of prompt photon production in DIS, including all-order resummation and a discussion of NLO corrections in this framework.

Findings

LO result proportional to dipole and quadrupole Wilson line correlators

Soft photon limit recovers Low-Burnett-Kroll theorem results

Leading twist approximation aligns with dominant NLO perturbative QCD contribution

Abstract

We compute the differential cross-section for inclusive prompt photon production in deeply inelastic scattering (DIS) of electrons on nuclei at small $x$ in the framework of the Color Glass Condensate (CGC) effective theory. The leading order (LO) computation in this framework resums leading logarithms in $x$ as well as power corrections to all orders in $Q_{s,A}^2/Q^2$, where $Q_{s,A}(x)$ is the nuclear saturation scale. This LO result is proportional to universal dipole and quadrupole Wilson line correlators in the nucleus. In the soft photon limit, the Low-Burnett-Kroll theorem allows us to recover existing results on inclusive DIS dijet production. The $k_{\perp}$ and collinearly factorized expressions for prompt photon production in DIS are also recovered in a leading twist approximation to our result. In the latter case, our result corresponds to the dominant next-to-leading order (NLO) perturbative QCD contribution at small $x$. We next discuss the computation of the NLO corrections to inclusive prompt photon production in the CGC framework. In particular, we emphasize the advantages for higher order computations in inclusive photon production, and for fully inclusive DIS, arising from the simple momentum space structure of the dressed quark and gluon "shock wave" propagators in the "wrong" light cone gauge $A^-=0$ for a nucleus moving with $P^{+}_{N} \rightarrow \infty$.