Virtual Journal on QCD Matter

An eight-week program entitled “Quantifying the Properties of Hot QCD Matter” was held at the Institute for Nuclear Theory in Seattle during May-July 2010 (Program 10-2a). For details of the program, see the workshop website, including the agenda and online slides of all talks.

Each week was devoted to discussion in a specific area, as follows:

• New results from LHC and RHIC
• Initial conditions, small x
• Viscous hydrodynamics I
• Viscous hydrodynamics II
• Jet quenching
• Strong versus weak coupling
• Heavy flavor and quarkonium
• Electromagnetic probes

The linked document contains a list of questions and challenges assembled by the organizers of the program, compiled at the end of each week. No attempt has been made to unify the presentation of topics or to resolve conflicting statements, and only minor editing has been applied. This document should be regarded as a snapshot of the field of hot QCD matter research in relativistic heavy ion collisions in summer 2010, as seen through the biased lenses of the six organizers (Brian Cole, Ulrich Heinz, Peter Jacobs, Yuri Kovchegov, Berndt Müller, and James Nagle.

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Not to be outdone, the CMS Collaboration posted today its own circumstantial evidence of jet quenching in Pb+Pb collisions at the LHC. As in yesterday’s ATLAS publication, the evidence is based on the observation of strongly asymmetric di-jets. In one example, shown on the CMS web page, the trigger jet has a total transverse momentum of 205 GeV, while the back-jet has only 70 GeV momentum. Several other pairs with somewhat smaller, but still large transverse momentum asymmetry are also shown. The collaboration promises a comprehensive publication with these and similar results soon after the completion of the heavy ion run.

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The rapid fire succession of results from the Pb+Pb run at the LHC continues. This morning the ATLAS Collaboration submitted a letter on jet quenching entitled:

• Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at √s_NN = 2.76 TeV with the ATLAS Detector at the LHC

The manuscript reports a growing asymmetry between the triggered jet and the back-jet in dijet events as a function of increasing collision centrality. The trigger condition was a jet with total E_T > 100 GeV accompanied by a backward jet (relative azimuthal angle larger than 90 degrees) with E_T > 25 GeV. The average underlying event was subtracted in the determination of E_T. 1693 di-jet events satisfying this condition were recorded in a sample of Pb+Pb events with integrated luminosity of 1.7 μb^-1.

The di-jet asymmetry A_J is defined as the ratio of the difference between the E_T of the trigger jet and the back-jet and their sum. Given the threshold condition on the back-jet, the largest possible asymmetry for a 100 GeV trigger jet is 75 GeV / 125 GeV = 0.6. For p+p collisions and peripheral Pb+Pb collisions the asymmetry distribution peaks at or near zero; for the 10% most central events, however, the distribution peaks at A_J ≈ 0.4. This means that in the most likely case, the back-jet carries 57% less total transverse energy in a cone of R = 0.4 than the trigger jet!

The data also provide some hints about where the energy missing from the back-jet goes. First, a global missing energy analysis does indicates that the energy was not lost into particles twhich escape detection by the calorimeter. Rather, the energy lost from the back-jet seems to be redistributed over a wide azimuthal range as shown for one example in Figure 1 of the ATLAS Letter.

Although it is too early to draw a quantitative comparison of the strength of jet quenching at RHIC and the LHC from these data, it seems clear that back-jets in central Pb+Pb collisions are drastically modified by the presence of the medium. The message is that jet quenching as a probe for hot QCD matter is alive and well at the LHC. The numbers provided by the ATLAS analysis should make it possible to extract a value for the Jet R_AA for the trigger jet. Will the ATLAS Collaboration leave this analysis to theorists?

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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 large value of charged multiplicity  dN_ch/dη = 1584 observed by ALICE in the 5% most central collisions, and the large value of elliptic flow they report in minimum bias collisions both suggest that the demise of the strong coupling paradigm at the LHC have been greatly exaggerated. I attribute this in part to “the tyranny of asymptotic freedom”, that is, the beauty of the running coupling constant in QCD has led to some wishful thinking about how fast a logarithmic term can vary. There is also the enormous appeal and simplicity of the “classical QGP” described as a nearly free, massless Boltzmann gas. But the data from RHIC, and now these first ever so exciting results from LHC, have taught us that there is also a wealth of fundamental physics and new phenomena in the strongly-coupled regime that is so much more interesting than a non-interacting gas!

At the same time there has indeed been a demise, in fact several, in this case of the various predictions of (mostly) lower multiplicity densities than the value of ~1600 reported by ALICE. I was one of those who would have guessed the lower values of 1200-1300, but since that was only a guess, the surprise for me was a pleasant one. Ironically, earlier on the day this value was announced in Andrea Dainese’s talk at the LHCC , I had read with great interest this paper Hadron production at the LHC: Any indication of new phenomena by Levin and Rezaeian , describing in a very accessible fashion their predictions for LHC multiplicities based on saturation physics. There is also a nice discussion of the extensions incorporated in their approach over the KLN model, so it is more than a little puzzling that the simpler KLN model (at least in one of its various instantiations) better describes the Pb+Pb data at the LHC.

(See also Berndt Mueller’s post for a more quantitative discussion of what we learn from the first two ALICE papers on Pb+Pb collisions at the LHC.)

(*) The title of this post is drawn from one of the loveliest songs in the American jazz idiom.

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Today the ALICE Collaboration posted the first analyses of data from Pb+Pb collisions at the LHC:

• Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV
• Charged-particle multiplicity density at mid-rapidity in central Pb-Pb collisions at sqrt(sNN) = 2.76 TeV

(more…)

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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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