Hard Probes 2012 – the 5th International Conference on Hard and Electromagnetic Probes of Nuclear Collisions – took place in Cagliari on 27 May – 1 June. The most important topical meeting to focus on the study of hard processes in ultra-relativistic heavy-ion collisions, this was the first time that the LHC collaborations presented their results based on lead–lead data. The main focus was undoubtedly on the wealth of new high-quality results from ALICE, ATLAS and CMS, complemented with significant contributions from the PHENIX and STAR experiments at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven.
Quoting from the inspired opening talk given by Berndt Mueller of Duke University, the hard probes “manifesto” can be summarized as follows: hard probes are essential to resolve and study a medium of deconfined quarks and gluons at short spatial scales, and they have to be developed into as precise a tool as possible. This is accomplished by the study of the production and the propagation in the deconfined medium of heavy quarks, particles with high momentum transfer (pT), jets and quarkonia.
Jet quenching can be addressed by studying the suppression of leading hadrons in nuclear collisions with respect to the proton–proton case. The ALICE and CMS collaborations reported results on the production of open charm and beauty, and results were also presented from the STAR experiment. An important aspect of parton energy-loss in the medium is its mass dependence: the energy loss is expected to be strongest for light hadrons and smaller for heavy quarks. The LHC data shown at the conference are suggestive of such hierarchy, although more statistics are still needed to reach a firm conclusion.
In addition, the high-precision LHC data on light charged hadrons are significantly expanding the kinematic reach. This is fundamental to discriminating among theoretical models, which have been tuned at the lower energy of RHIC.
At the LHC, full reconstruction of high-energy jets has become possible for the first time, allowing ATLAS and CMS to present high-statistics results on jet–jet correlations. The emerging picture is consistent with one in which partons lose a large fraction of their energy while traversing the hot QCD medium – before fragmenting essentially in vacuum. First results on γ-jet correlations were also presented by the CMS and PHENIX collaborations; these allow the tagging of quark jets and give a better estimation of the initial parton energy. During the conference, an intense debate developed on how to exploit fully the information provided by full jet reconstruction.
Quarkonia suppression was another of the striking observables, for which results from LHC had been eagerly awaited. CMS presented the first exciting precision results on the suppression of the ϒ states. These reveal a clear indication of a much larger suppression for more weakly bound ϒ(2S) and ϒ(3S) with respect to the strongly bound ϒ(1S) states, in accordance with the predictions for the observation of colour screening. The ALICE collaboration presented new data on the rapidity and pT dependence of J/ψ suppression. The results show that, despite the higher initial temperatures reached at LHC, the size of the suppression remains significantly smaller than at RHIC. This is an intriguing hint that a regeneration mechanism from the large number of charm quarks present in the deconfined medium may take place at LHC energies.
Part of the conference was devoted to the study of initial-state phenomena. In particular, at high energy peculiar features related to the saturation of the gluon phase-space should emerge, leading to a state called “colour glass condensate”. A discussion took place on the how the existence of this state could be proved or disproved at LHC. The study of initial-state phenomena also came under debate because of its importance in disentangling the effects of cold nuclear matter from genuine final-state effects in hot matter.
With the advent of high-precision data, theory is being increasingly challenged, since the understanding of the bulk properties of the medium produced in heavy-ion collisions is rapidly advancing. As several speakers discussed, significant advances are being made both in the understanding of the parton energy-loss mechanism and in the quarkonia production, for which a quantitative picture is emerging.
Still, as CERN’s Jürgen Schukraft pointed out in his summary talk, there is a need for measurements of even higher precision, as well as a wish list for new measurements: for example, in the heavy-flavour sector, lowering the pT reach to measure the total charm cross-section; and reconstructing charmed and beauty baryons to gain further insight into thermalization of the medium.
On a shorter time scale, the next crucial step is the measurement of effects in cold nuclear matter, which will be possible in the forthcoming proton–nucleus run at the LHC. Based on the experience from the past lower energy measurements, new surprises might be just behind the corner.
The conference was preceded by introductory student lectures covering aspects of quarkonia production and jet quenching. About 40 students were supported by the organization, thanks to generous contributions by several international laboratories (CERN, EMMI, INFN) and, in particular, by the University of Cagliari and by the government of Sardinia. The conference was broadcast to a wider audience worldwide as a webcast.
• For more information, see the conference website www.ca.infn.it/hp12/.
