Jet Quenching in Relativistic Heavy Ion Collisions at the LHC

TL;DR

This paper reports the first direct observation of jet quenching in relativistic heavy ion collisions at the LHC, demonstrating centrality-dependent modifications in dijet asymmetry and suppression of single jet production.

Contribution

It provides the first direct evidence of jet quenching at the LHC, using fully reconstructed jets and background subtraction techniques in Pb+Pb collisions.

Findings

Central collisions show increased dijet asymmetry.

Single jet spectra are suppressed by about a factor of two in central collisions.

Dijet angular correlations show little centrality dependence.

Abstract

Jet production in relativistic heavy ion collisions is studied using Pb+Pb collisions at a center of mass energy of 2.76 TeV per nucleon. The measurements reported here utilize data collected with the ATLAS detector at the LHC from the 2010 Pb ion run corresponding to a total integrated luminosity of 7 inverse microbarns. The results are obtained using fully reconstructed jets using the anti-kt algorithm with a per-event background subtraction procedure. A centrality-dependent modification of the dijet asymmetry distribution is observed, which indicates a higher rate of asymmetric dijet pairs in central collisions relative to peripheral and pp collisions. Simultaneously the dijet angular correlations show almost no centrality dependence. These results provide the first direct observation of jet quenching. Measurements of the single inclusive jet spectrum, measured with jet radius parameters R = 0.2,0.3,0.4 and 0.5, are also presented. The spectra are unfolded to correct for the finite energy resolution introduced by both detector effects and underlying event fluctuations. Single jet production, through the central-to-peripheral ratio RCP, is found to be suppressed in central collisions by approximately a factor of two, nearly independent of the jet pT. The RCP is found to have a small but significant increase with increasing R, which may relate directly to aspects of radiative energy loss.