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

ATLAS analyzes Pb+Pb collisions at sqrt(s_NN)=2.76 TeV (integrated luminosity ~1.7 μb^-1) to study dijet energy balance using AJ = $(E_T1 - E_T2)/(E_T1 + E_T2)$ for oppositely directed jets (Δφ > π/2) with E_T1 > 100 GeV and E_T2 > 25 GeV after subtracting the event-by-event underlying energy. Jets are reconstructed with the anti-kT algorithm (R=0.4) using calorimeter towers, with layer-by-layer underlying-event subtraction. The AJ distributions broaden and shift toward larger values with increasing centrality, while Δφ remains largely back-to-back, indicating stronger jet quenching in more central collisions. Compared to pp data and HIJING+PYTHIA simulations, central Pb+Pb events show a pronounced enhancement of highly asymmetric dijets, consistent with energy loss of partons in a hot, dense medium (quark-gluon plasma); extensive detector- and background-control studies show no alternative explanation. These results provide direct evidence for jet quenching at LHC energies and motivate further quantitative jet-medium interaction studies.

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.

Figures (3)

• Figure 1: Event display of a highly asymmetric dijet event, with one jet with $E_T > 100$ GeV and no evident recoiling jet, and with high energy calorimeter cell deposits distributed over a wide azimuthal region. By selecting tracks with $p_T>2.6$ GeV and applying cell thresholds in the calorimeters ($E_T>700$ MeV in the electromagnetic calorimeter, and $E>1$ GeV in the hadronic calorimeter) the recoil can be seen dispersed widely over azimuth.
• Figure 2: (top) Distribution of uncorrected $\Sigma E_T$ in the Forward Calorimeter (FCal). Bins in event activity or "centrality" are indicated by the alternating bands (see text for details) and labeled according to increasing fraction of lead-lead total cross section starting from the largest measured $\Sigma E_T$. (bottom) Correlation of uncorrected $\Sigma E_T$ in $|\eta|<3.2$ with that measured in the FCal ($3.2<|\eta|<4.9$).
• Figure 3: (top) Dijet asymmetry distributions for data (points) and unquenched HIJING with superimposed PYTHIA dijets (solid yellow histograms), as a function of collision centrality (left to right from peripheral to central events). Proton-proton data from $\sqrt{s}=7$ TeV, analyzed with the same jet selection, is shown as open circles. (bottom) Distribution of $\Delta\phi$, the azimuthal angle between the two jets, for data and HIJING+PYTHIA, also as a function of centrality.