${\cal O}(\alpha_s^3)$ analysis of inclusive jet and di-jet production in heavy ion reactions at the Large Hadron Collider

TL;DR

This paper presents a perturbative QCD analysis up to third order of inclusive jet and di-jet production in heavy ion collisions at the LHC, providing predictions for jet suppression and asymmetry that inform understanding of quark-gluon plasma effects.

Contribution

It offers the first ${ m O}(\alpha_s^3)$ calculations for jet observables in heavy ion reactions, including medium effects and non-perturbative corrections, enhancing precision in jet quenching studies.

Findings

Predicted jet suppression consistent with LHC data

Quantified di-jet asymmetry enhancement in Pb+Pb collisions

Identified sources of jet-medium interaction ambiguity

Abstract

Jets physics in heavy ion reactions is an important new area of active research at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) that paves the way for novel tests of QCD multi-parton dynamics in dense nuclear matter. At present, perturbative QCD calculations of hard probes in elementary nucleon-nucleon reactions can be consistently combined with the effects of the nuclear medium up to $ {\cal O}(\alpha_s^3) $. While such accuracy is desirable but not necessary for leading particle tomography, it is absolutely essential for the new jet observables. With this motivation, we present first results and predictions to $ {\cal O}(\alpha_s^3) $ for the recent LHC lead-lead (Pb+Pb) run at a center-of-mass energy of 2.76 TeV per nucleon-nucleon pair. Specifically, we focus on the suppression of the single and double inclusive jet cross sections. Our analysis includes not only final-state inelastic parton interactions in the QGP, but also initial-state cold nuclear matter effects and an estimate of the non-perturbative hadronization corrections. We demonstrate how an enhanced di-jet asymmetry in central Pb+Pb reactions at the LHC, recently measured by the ATLAS and CMS experiments, can be derived from these results. We show quantitatively that a fraction of this enhancement may be related to the ambiguity in the separation between the jet and the soft background medium and/or the diffusion of the parton shower energy away from the jet axis through collisional processes. We point to a suite of measurements that can help build a consistent picture of parton shower modification in heavy ion collisions at the LHC.