Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$ with the ATLAS Detector at the LHC

By 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 transverse energies of dijets in opposite hemispheres are 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, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

Figure 1

Event display of a highly asymmetric dijet event, with one jet with $E_T>100\mathrm{GeV}$ and no evident recoiling jet and with high-energy calorimeter cell deposits distributed over a wide azimuthal region. By selecting tracks with $p_T>2.6\mathrm{GeV}$ and applying cell thresholds in the calorimeters ($E_T>700\mathrm{MeV}$ in the electromagnetic calorimeter, and $E>1\mathrm{GeV}$ in the hadronic calorimeter), the recoil can be seen dispersed widely over the azimuth.

Figure 2

(Top) Distribution of uncorrected $\Sigma E_T$ in the 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 $\sqrt{s}=7\mathrm{TeV}$, analyzed with the same jet selection, are shown as open circles. (Bottom) Distribution of $\Delta \varphi$, the azimuthal angle between the two jets, for data and $\mathrm{HIJING}+\mathrm{PYTHIA}$, also as a function of centrality.