Virtual Journal on QCD Matter

The ATLAS Collaboration released its first result from Pb+Pb collisions at the LHC, and it’s a blockbuster. Their new paper, titled Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at √s_NN = 2.76 TeV with the ATLAS Detector at the LHC, has been fast-tracked for acceptance in Physical Review Letters. In this paper ATLAS reports the observation of strong quenching in highly energetic jets (> 100 GeV) produced in Pb+Pb collisions at the LHC. The quenching is sufficiently strong that the trigger jet may properly be referred to as a monojet.

The effect is clearly seen in Figure 1 of their paper, reproduced above. The high  p_T towers that dominate the Lego plot and the event display are unbalanced by a partner jet on the away side in azimuth. Instead, their appears to be enhanced low p emission over nearly the entire away side azimuthal distribution, mostly clearly seen in the towers of the beams-eye view event display.

A more quantitative analysis is of course provided in the paper, in which the evolution of the quenching is presented as a function of centrality, and also compared to HIJING events supplemented with unquenched jets from PYTHIA. For peripheral events, both the the energy asymmetry of the away-side jet and the distribution in azimuthal angle is in good agreement with those seen in p+p events and in the HIJING+PYTHIA Monte Carlo events. This is in sharp distinction to the most central Pb+Pb events, where the jet events have a pronounced asymmetry in energy and a significantly broadened angular distribution.

Of course, the disappearance of the away-side “jet” was discovered by the STAR collaboration in data collected during the first full energy RHIC run. But several aspects of the ATLAS result are even more striking: The observed quenching is for true jets, fully reconstructed in heavy ion events with complete and essentially hermetic hadronic calorimetry; the quenching remains to jet energies of (at least) 100 GeV; and the modification of the jet azimuthal energy distribution is clearly observed in the away-size distribution from the trigger jet. These are all firsts in heavy ion physics, and to see them in a Physical Review Letter a few days after the start of heavy ion physics running at the LHC is indeed remarkable.

The ATLAS results is featured in a CERN press release, dated today (26-Nov-10), where it is stated a similar result from CMS will follow shortly. Also posted today is a Symmetry Magazine article on the ATLAS result containing comments by Brian Cole and Peter Steinberg.

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The long-awaited day has arrived, in spectacular fashion. The LHC has declared “Stable beams with ions”, and ALICE, ATLAS and CMS are all seeing events, and sharing their beautiful event displays with the world.

It is enormously satisfying to see the field of relativistic heavy ion physics take this tremendous (large) step on the energy frontier. Congratulations and kudos to all who have worked for decades to make this happen. Surely extraordinary discoveries await.

Update: There is now a press release from CERN with useful links to various event displays.

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On September 21st 2010 the CMS Collaboration gave a seminar at CERN detailing the observation of long-range near-side angular correlations in p+p collisions at the LHC. A preprint was submitted to arXiv the same day:

Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions at the LHC

The significance of these two particle correlations in azimuthal angle and pseudo-rapidity space is that they are thought to be the result of a collective, hydrodynamic, response of a medium to fluctuations of a multi-particle initial state. The medium in question of course most likely being a Quark-Gluon-Plasma. (more…)

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