Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at sqrt(S(NN))= 2.76 TeV with the ATLAS Detector at the LHC

TL;DR

This paper reports the first observation of a centrality-dependent increase in dijet asymmetry in lead-lead collisions at the LHC, indicating significant jet energy loss in a hot, dense medium.

Contribution

It presents the first measurement of centrality-dependent dijet asymmetry in heavy-ion collisions at 2.76 TeV with the ATLAS detector.

Findings

Dijet asymmetry increases with collision centrality.

Highly asymmetric dijets are more common in central collisions.

Results suggest strong jet quenching effects in the quark-gluon plasma.

Abstract

Using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally-segmented electromagnetic and hadronic calorimeters. The underlying event is measured and subtracted event-by-event, giving estimates of jet transverse energy above the ambient background. The transverse energies of dijets in opposite hemispheres is observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, and which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.