How To Find Charm in Nuclear Collisions at RHIC and LHC

TL;DR

This paper calculates the hard scattering contributions to dilepton production in nuclear collisions at RHIC and LHC using next-to-leading order QCD, showing charm production dominates and can be used to probe gluon distributions.

Contribution

It provides detailed NLO QCD calculations of dilepton backgrounds from charm and Drell-Yan processes in nuclear collisions, aiding thermal signal extraction.

Findings

Charm production dominates dilepton background.

Experimental cuts can reduce charm background.

Results enable probing of gluon shadowing.

Abstract

Measurements of dilepton production from charm decay and Drell-Yan processes respectively probe the gluon and sea quark distributions in hadronic collisions. In nucleus-nucleus collisions, these hard scattering processes constitute a `background' to thermal contributions from the hot matter produced by the collision. To determine the magnitude and behavior of this background, we calculate the hard scattering contribution to dilepton production in nuclear collisions at RHIC and LHC at next to leading order in perturbative QCD. Invariant mass, rapidity and transverse momentum distributions are presented. We compare these results to optimistic hydrodynamic estimates of the thermal dilepton production. We find that charm production from hard scattering is by far the dominant contribution. Experiments therefore can measure the gluon distribution in the nuclear target and projectile and, consequently, can provide new information on gluon shadowing. We then illustrate how experimental cuts on the rapidity gap between the leptons can aid in reducing the charm background, thereby enhancing thermal information.