ATLAS Experiment CERN

ATLAS Observes Eye-Catching Imbalance of Jet Energies

Geneva, 26 November 2010.

After less than three weeks of heavy-ion running, the ATLAS Experiment has reported a dramatic effect that causes an unexpectedly large imbalance in the energy of jets of particles created in lead-ion collisions at CERN's Large Hadron Collider. This evidence may bring new insight to the primordial universe where a hot, dense medium of quarks and gluons may have prevailed.

Concentrated jets (clusters) of particles are formed in the violent collisions of lead-ions at the LHC. As the jets materialize, they traverse the hot, dense medium. Unavoidably, the jets will interact with the medium, but what will be the nature of the interactions? Will the jets interact faintly with the nucleons that comprise the colliding ions, or will they interact strongly with the constituent quarks and gluons of the nuclei? ATLAS observes jets that lose a surprising amount of energy, signalling interactions with the medium more intense than ever seen before. This phenomenon is referred to as jet quenching.

A highly asymmetric collision event, with one jet with ET > 100 GeV and no evident recoiling jet, and with high energy calorimeter cell deposits distributed over a wide azimuthal region. Only tracks with pT > 2.6 GeV are shown, and only calorimeter energy deposits with cell energy ET > 700 MeV in the electromagnetic calorimeter, and E > 1 GeV in the hadronic calorimeter. The two "lego" plots (on the right) show the detector unrolled with "towers" showing energies. From Observation of a centrality-dependent dijet asymmetry in lead-lead collisions at sqrt(sNN) = 2.76 TeV with the ATLAS detector at the LHC.

"ATLAS is the first experiment to report direct observation of jet quenching ," said ATLAS Spokesperson Fabiola Gianotti. "It's a very exciting result of which the Collaboration is proud, obtained in a very short time thanks in particular to the dedication and enthusiasm of young scientists."

The excellent capabilities of ATLAS to determine jet energy, coupled with the high energies of LHC lead-ion beams, enabled ATLAS to observe a striking imbalance in energies of pairs of jets, where one jet is almost completely absorbed by the medium, and the other jet (presumably closer to the edge) is mostly unabsorbed. This result is reported in a paper from the ATLAS Collaboration accepted for publication in the scientific journal Physical Review Letters, and it will be presented at a seminar on Thursday 2 December at CERN. Data taking with ions continues to 6 December.

"Based on earlier work at RHIC, said Prof. Brian Cole of Columbia University, "we think the observed degree of quenching can only be easily understood in the context of creation of a very hot quark gluon plasma with which the jets interact intensely."

The ALICE and CMS experiments, which make use of detector technologies different and complementary to ATLAS, also have interesting results with their lead-ion data. The observations of the three experiments mark the beginning of a very broad and exciting physics program with heavy ions at the LHC, where comparisons can also be made with high-energy proton-proton collisions. Studying the quark-gluon plasma will bring important insights into the evolution of the early Universe, and into the nature of the strong force that binds quarks and gluons together into protons, neutrons, and ultimately all the nuclei of the periodic table of elements. Future jet quenching and other measurements from ATLAS, ALICE, and CMS will provide powerful insight into the properties of the primordial plasma and the interactions among its quarks and gluons.

Sample event display:
Heavy-ion event animation:
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