Measuring gluon shadowing with prompt photons at RHIC and LHC

TL;DR

This paper explores how high-p_T prompt photon production at RHIC and LHC can be used to observe gluon shadowing effects in nuclei by analyzing nuclear modification ratios and their sensitivity to nuclear parton densities.

Contribution

It demonstrates that isolated prompt photon production is a promising method to reliably extract gluon and structure function ratios in nuclei, providing a new approach to study gluon shadowing.

Findings

Isolated prompt photons effectively reveal gluon shadowing effects.

Production ratios at different rapidities estimate gluon to quark structure function ratios.

The method reduces dependence on proton-proton reference data.

Abstract

The possibility to observe the nuclear modification of the gluon distribution at small-x (gluon shadowing) using high-p_T prompt photon production at RHIC and at LHC is discussed. The per-nucleon ratio, sigma(p+A -> gamma+X) / A sigma(p+p -> gamma+X), is computed for both inclusive and isolated prompt photons in perturbative QCD at NLO using different parametrizations of nuclear parton densities, in order to assess the visibility of the shadowing signal. The production of isolated photons turns out to be a promising channel which allows for a reliable extraction of the gluon density, G^A/G^p, and the structure function, F_2^A/F_2^p, in a nucleus over that in a proton. Moreover, the production ratio of prompt photons at forward-over-backward rapidity in p-A collisions provides an estimate of G^A/G^p (at small x) over F_2^A/F_2^p (at large x), without the need of p-p reference data at the same energy.