The 5th International Conference on Hard and Electromagnetic Probes of Nuclear Collisions (Hard Probes 2012) was held in Cagliari (Italy) between May 27 and June 1, 2012. This is the most important topical meeting focused on the study of hard processes in ultra-relativistic heavy-ion collisions. Hard Probes 2012 was the first edition where LHC experiments presented their results based on lead-lead data.
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 labs (CERN, EMMI, INFN) and, in particular, by the University of Cagliari and by the government of Sardinia.
The talks and discussions were enjoyed not only by the participants in Cagliari. The conference was broadcasted world-wide and an enlarged audience had the possibility to follow the conference webcast.
The scientific program included 32 plenary and 85 parallel talks. The main focus was undoubtedly on the wealth of new high-quality results from ALICE, ATLAS and CMS, complemented with significant contributions from the RHIC experiments (PHENIX and STAR).
Quoting from the inspired opening talk given by B. Mueller (Duke University), the hard probes “manifesto” can be summarized as follows: in order to resolve and study a medium of deconfined quarks and gluons at short spatial scales, hard probes are essential and have to be developed into as precise tools as possible. This is accomplished by the study of the production and the propagation of heavy quarks, high pT particles, jets and quarkonia in the deconfined medium.
Jet quenching can be addressed by studying the suppression of leading hadrons in nuclear collisions with respect to proton-proton. ALICE and CMS reported results on the production of open charm and beauty. Results were presented also by STAR. An important aspect of parton energy loss in the medium is its mass dependence: it is expected that the energy loss is strongest for lights hadrons, and smaller for heavy quarks. The LHC data shown at the conference is suggestive of such hierarchy although more statistics is still needed to reach a firm conclusion.
In addition, the high precision LHC data on light charged hadrons allow expanding significantly the kinematic reach. This is fundamental to discriminate among theoretical models which were tuned at the lower RHIC energy.
At the LHC, full reconstruction of high energy jets became possible for the first time. ATLAS and CMS also presented high statistics results on jet-jet correlations. The emerging picture is consistent with partons losing a large fraction of their energy while traversing the hot QCD medium, and then fragmenting essentially in vacuum. First results from CMS and PHENIX were shown also on ?jet correlations which allow tagging quark jets and give a better estimation of the initial parton energy. During the conference, an intense debate developed on how to fully exploit the information provided by full jet reconstruction.
Quarkonia suppression was also among the striking observables, for which results from LHC were eagerly awaited. CMS presented the first exciting precision results on the suppression of the Y states. A clear indication of a much larger suppression for the less bound Y(2S) and Y(3S) with respect to the strongly bound ?(1S) states was seen, in accordance with the predictions for the observation of color screening. ALICE presented new data on the rapidity and pT dependence of J/psi suppression. The results show that, in spite of 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 re-generation 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 color glass condensate. A discussion took place on the how this state could be proved or disproved at LHC. The study of initial state phenomena was also debated because of its importance to disentangle cold nuclear matter effects from genuine final state effects in hot matter.
With the advent of high precision data, theory is more and more challenged since the understanding of the medium bulk properties is rapidly advancing. As discussed by several talks at the conference, 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 pointed out by J. Schukraft (CERN) in his summary talk, there is a need for measurements of even higher precision and a wishlist for new measurements. Among them, in the heavy flavor sector: lowering the pT reach in order to measure the total charm cross-section and reconstructing charmed and beauty baryons to gain further insight on the medium thermalization.
On a shorter time scale, the next crucial step is the measurement of the cold nuclear matter effects thanks to 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!