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 √sNN = 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  pT 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.

Indications of QGP formation in p+p collisions at the LHC?

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…)

Quark Matter 2009 Highlights

The 21st International Conference on Ultrarelativistic Nucleus-Nucleus Collisions, colloquially referred to as Quark Matter 2009, took place in Knoxville, Tennessee from March 30th to April 4th, 2009. (more…)

A Conservative Approach

A wide variety of thermodynamic and/or statistical techniques have been applied to describe particle spectra and yields in relativistic heavy ion collisions. Not having done  a detailed, ‘statistical’ analysis of the hundreds of such papers, it is still safe to say the vast majority of such calculations do not start from the microcanonical ensemble, that is, they do not incorporate the effects of global energy-momentum conservation.  In Conservation Laws and the Multiplicity Evolution of Spectra at the Relativistic Heavy Ion Collider Chajecki and Lisa study the role of Energy and Momentum Conservation-Induced Constraints (EMCIC’s) on single particle spectra at RHIC.  (In a nice piece of acronym-overloading, this work builds on their previous studies of Energy and Momentum Conservation-Induced Correlations in femtoscopic measurements.) Their studies suggest that the effects of EMCIC’s can lead to surprisingly large shifts in the momentum distributions between low and high multiplicity states. 

The key plot in this well-written paper is Figure 3, which shows the ratio of the yields in  p+p collisions to central Au+Au collisions as a function of pT, compared to curves calculated on the basis of energy-momentum conservation alone. Even light particles such as pions show 50% effects in the low pT region 0.2 to 0.7 GeV/c; the effect is much larger (a factor of ~5) for protons in the same transverse momentum regime.   The structure is consistent with one’s naive expectations- the presence of a larger ‘reservoir’ in the Au+Au case makes it easier to access high transverse momentum than in p+p collisions. 

There is additional information in this figure that the authors may wish to extract. This low momentum regime is where one expects participant scaling to hold, and this seems to be (roughly) the case for the lowest momentum pions, but is badly violated for the protons. Protons with  pT < 0.7 GeV/c are suppressed in Au+Au collisions relative to participant scaling; protons with momenta greater than this value are enhanced. The conventional explanation for this is radial flow, which has a larger effect on higher mass particles, and which will lead to a depletion of the low  pT region. This paper suggests that identical trends can result from EMCIC’s, leading the authors to state “Extracting physics messages from the changing spectra, while ignoring kinematic effects of the same order as the observed changes themselves, seems unjustified.”


In reading this paper, I was reminded of a very clever analysis by  T.T. Chou, C.N. Yang and E. Yen: Single Particle Momentum Distribution At High-Energies And Concept Of Partition Temperature . These authors noted that energy-momentum conserving delta-function in the microcanonical ensemble ‘inevitably’ leads to an exponential distribution of single-particle energies, with an exponential slope they labeled the ‘partition temperature’ (and took pains to note was not necessarily a real temperature). They applied this idea to the analysis of the rapidity distributions measured by UA5 in proton-antiproton collisions at center-of-mass energy 540 GeV. After introducing another, independent, ansatz, i.e., that confinement leads to  an exponential distribution in transverse momentum, they obtained a striking good description of how “phase space” considerations (aka EMCIC’s) described the systematic variation of the pseudo-rapidity distributions with collision multiplicity.     


Over the past few months a new initiative, called Theory-Experiment Collaboration for Hot QCD Matter (TECHQM), has taken shape, which potentially will have great impact on how our community arrives at scientific conclusions on the nature and properties of hot and dense QCD matter created at the RHIC and LHC facilities. (more…)

A New Angle on the Ridge

The discovery of the “ridge” by the STAR Collaboration has proven stubbornly resistant to quantitative theoretical analysis. (See Berndt Mueller’s “Theorists Confront the Ridge” for a description of two recent attempts.) The discovery turns out to be nearly as resistant to historical analysis, at least for this outside observer. (more…)

What is “reco” scaling?

Over the past 1-2 years some confusion has entered the field as to what exactly constitutes the elliptic flow scaling law predicted by the parton recombination models. (more…)

Scaling and Non-Scaling of Elliptic Flow

The detailed dependence of the elliptic flow parameter v2 on the particle masses and transverse momentum has provided strong evidence for the applicability of hydrodynamics for describing the bulk motion of final-state particles at RHIC. (For a partial list of references, see the Hydro Primer elsewhere in this journal.) Equally intriguing have been various scaling patterns observed in the flow data: The separate curves v2(pT,m) for different mass particles are substantially more alike when studied versus “transvese kinetic energy” KET = mT -m. Plotting v2 as as a function of KET collapses the data into one curve for KET < 1 GeV, while showing two distinct branches for mesons and baryons for larger values of KET. These two branches collapse into one curve when both v2 and KET are divided by the quark content nq (2 for mesons, 3 for baryons). (more…)

The One That Started It All

The one that started it all was the first experimental paper from RHIC. (more…)