by Panos Charitos & Andrea Rossi. Published: 29 July 2015

The 7th edition of the Hard Probes International Conference series was hosted by McGill University in Montréal, Québec, Canada, in the Summer of 2015 (June 29 - July 3). McGill University also hosted the summer school for the graduate students and postdoctoral fellows just before the Conference.

During the last two decades, high-energy nuclear physics has seen a tremendous progress in both experiment and theory. With the combined efforts of CERN SPS, BNL RHIC and the LHC, we now have an ample amount of evidence that the long sought-after Quark-Gluon Plasma (QGP) is indeed being created in relativistic heavy ion collisions. Experimental and theoretical studies of this matter under extreme conditions have already given us the first glimpse of what our Universe was like right after the Big-Bang - temperatures well above 2 trillion Kelvin and the energy density well above that at the center of neutron stars, yet flowing more freely than liquid helium. However, much about this fascinating state of matter remains yet to be explored.

In this regard, the International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions has made a critical contribution in understanding the pivotal role of jets and other high energy probes in unraveling the fascinating complexity of the hottest and densest state of matter ever created. The conference brought together experts from all over the world for an intense five days of fruitful presentations and discussions. Amongst the vibrant cultural scenes in Montréal the world-famous Montréal International Jazz Festival took place during the same time.

With 13 contributions between talks and posters, ALICE presented a wealth of scientific results from hard probes and their implications in assessing the properties of the QGP. Equipped with a central barrel with high-resolution tracking systems, several detectors for identifying particles from low to high momentum, electromagnetic calorimeters, and with a muon spectrometer at forward rapidity, the ALICE detector is ideally suited for studies involving hard probes.

Heavy-flavour, quarkonia, and jets are fundamental probes of the microscopic processes underlying the interaction of high-energy quarks and gluons with the QGP constituents. ALICE results indicate that partons interact strongly with the medium quarks and gluons, losing energy, as evidenced by the suppression of the yield of high momentum charged particles, p0, heavy-flavour hadrons, and jets over a broad momentum range in central Pb-Pb collisions with respect to the yields measured in proton-proton and proton-Pb collisions. Contrarily to single particles, jets are extended object: the suppression of their production implies that the energy lost is dissipated outside the jet area. Despite the significant interaction of partons with the QGP, ALICE measurements suggest that the emerging jets do not appear as medium-modified objects and have properties similar to those measured for jets in proton-proton collisions, thus coming from parton fragmentation “in vacuum”. This is supported by the study of the angular correlation between a high momentum particle and jets: no evidence, within uncertainties, was found neither for a medium-induced increased angular de-correlation between the trigger particle and the jet nor for a pronounced modification of the “recoiling” jet intra-structure, regardless of the measured suppression of the recoiling jet spectrum, indicated also by the analysis of azimuthal correlations of p0 with charged particles. The additional suppression of recoiling jets is usually interpreted as due to a geometrical bias, with a longer distance covered by the recoiling parton in the medium when a high-momentum trigger particle is required. The absence of a modification of the relative yields of high momentum pion, proton, and kaons suggests that the jet hadrochemical composition is also unaltered. This is further supported by the fact that no evidence, within uncertainties, was found for an enhancement of the relative yield of L baryons with respect to K0s mesons among jet constituents, contrasting the dramatic increase characterizing the “bulk” of medium particles, effect that is usually ascribed to the hydrodynamic expansion of the medium. Run 2 data, that will allow us to carry out more energy-differential studies and with reduced experimental uncertainties, will be fundamental to shed further light on possible medium modifications to jet intra-structure and hadrochemistry.

The final results of the measurement of D meson suppression at high momentum as a function of the collision centrality, recently submitted for publication by ALICE, were also presented. The comparison with the preliminary results of the suppression of J/y from   B-meson decay, measured by CMS, provides an indication that in-medium energy loss is larger for charm than for the more massive beauty quark. The suppression of the production of high-momentum D mesons is compatible and similar to that of pions, which mainly come from the fragmentation of gluons and light quarks. This similar suppression of the momentum spectra of two hadron species is reproduced by models in which, at parton level, the energy loss is actually larger for gluons than light quarks and reduces with increasing quark mass, though a small effect is expected from charm mass.

One of the most important results for heavy-ion physics obtained at the LHC is the observation of a smaller suppression of J/ψ production in central heavy-ion collisions at LHC than RHIC energies. Proposed as a possible signature of the phase transition to QGP with deconfined colour charges, the “melting” of the J/ψ meson was predicted as due to a Debye-like screening of the attractive potential binding charm and anti-charm quarks into charmonium states in a medium with free colour charges. A stronger screening is expected in the denser medium formed at LHC energies. However, charm pairs are abundantly produced at the LHC, more than a hundred per event in central collisions, and charm quarks from different hard scatterings can combine into bound states. The fact that this new production mechanism could be effective for J/ψ was indicated already from the first ALICE measurements and supported by later observations. At Hard Probes, ALICE presented further studies for characterizing the suppression trend as a function of the collision centrality for J/ψ different momenta. The results confirm that the reduced suppression concerns mainly low momentum J/ψ, in agreement with what is expected by models including charmonia formation via recombination. This investigation brought also the unexpected observation of an excess (about a factor 7!) of the J/ψ yield at transverse momenta smaller than 0.3 GeV/c in peripheral collisions with respect to what is expected by scaling the pp result according to the number of binary nucleon-nucleon collisions. Further studies are ongoing to understand the nature of this excess, which shows a momentum spectrum resembling that of J/ψ photo-produced in ultra-peripheral collisions at impact parameters larger than twice the nuclear radius.

ALICE also presented several important new results from the study of p-Pb collisions, which are fundamental for measuring “cold” nuclear matter effects that can modify particle production spectra with respect to those of proton-proton collisions even in the absence of a hot deconfined medium. ALICE made a complete diagnosis of the J/ψ production by investigating its dependence on momentum, rapidity, and event multiplicity, setting stringent constraints to theoretical predictions. ALICE observed also a suppression of ψ(2s) production resembling what seen also by PHENIX in d-Au collisions at RHIC. The suppression is described by a model including interactions of quarkonia with comoving particles in the system. This “final state” effect could describe the observation that the suppression increases with the event multiplicity.

Since the first tantalizing observation of a double-ridge structure in the angular correlation distribution of particles produced in p-Pb events with high multiplicity, resembling the elliptic flow (v2) correlation typical of peripheral Pb-Pb collisions, an effort  was done by all the experiments for investigating its nature. ALICE presented the result of a new analysis in which muons reconstructed in the dedicated forward spectrometer are correlated with particles reconstructed in the central barrel, thus separated by a large rapidity gap. Positive v2 values were measured also for muons with transverse momentum larger than 2 GeV/c, which predominantly come from decays of heavy-flavour particles. Slightly larger v2 values were measured in the case in which the muon follows the direction of the Pb nucleus than in the case in which it goes in the direction of the proton nucleus. These results are important to set constraints to models that explains the observed structure in terms of initial or final state effects.

While finalizing the analysis of Run 1 data, ALICE now looks forward to Run 2 data. Pb nuclei will be collided at an energy larger by a factor of 2 with respect to Run 1, which translate in a significantly higher production cross-section at high momentum for hard probes. Therefore new exciting results are expected. Stay tuned!