Photon production in high energy proton-nucleus collisions

TL;DR

The paper investigates photon production in proton–nucleus collisions within the Color Glass Condensate framework, treating the nucleus as a classical color field and the proton with standard PDFs. It shows that bremsstrahlung diagrams, enhanced by the strong nuclear field, dominate over direct photon emission and resum to all orders in the background field. The inclusive photon cross-section is expressed through a universal correlator C(l⊥) that encodes saturation effects, making photon–jet correlations a direct probe of gluon saturation. The analysis also finds that diffractive photon production vanishes under reasonable kinematic assumptions, and highlights forward-region amplifications and potential extensions to dileptons, di-photons, and gluon radiation.

Abstract

We calculate the photon production cross-section in $pA$ collisions under the assumption that the nucleus has reached the saturation regime, while the proton can be described by the standard parton distribution functions. We show that due to the strong classical field $O(1/g)$ of the nucleus, bremsstrahlung diagrams become dominant over the direct photon diagrams. In particular, we show that $γ-$jet transverse momentum spectrum and correlations are very sensitive to gluon saturation effects in the nucleus.

Figures (2)

• Figure 1: The physical processes contributing to the bremsstrahlung of a quark in a colored glass condensate. The black dot denotes the interaction of a quark to all orders with the background field, i.e. the scattering matrix ${\cal T}_{_{F}}$ defined in Eq. (\ref{['eq:taures']}).
• Figure 2: Behavior of the correlator $C({\fam\imbf k}_\perp)$ as a function of ${\fam\imbf k}_\perp$. In this plot, the value of $Q_s$ is such that $Q_s/\Lambda_{_{QCD}}=10$. Circles: computed value of $C({\fam\imbf k}_\perp)$. Solid line: the "perturbative" value $C({\fam\imbf k}_\perp)\approx 2Q_s/k_\perp^4$, valid if $k_\perp\gg Q_s$.