by Andrea Dainese, Andreas Morsch and Marco van Leeuwen. Published: 13 March 2017

The 26th edition of the Quark Matter Conference was held from 5 to 11 February in Chicago and saw the participation of more than 700 attendees. The ALICE Collaboration had a strong presence with 28 contributed talks and over 60 posters.

Anthony Timmins presenting the ALICE Highlights in the opening plenary session.

Quark Matter conferences have been for long the most important venue for discussing new results in the field of high-energy nucleus-nucleus collisions. The 26th edition was held this February in Chicago, where more than 700 participants discussed the latest results, mainly from the CERN and BNL experiments, as well as a broad range of theory and phenomenology developments.

The ALICE Collaboration had a strong impact with 28 contributed talks and over 60 posters.(Direct links to all ALICE talks can be found at goo.gl/i1tcXd.) Anthony Timmins presented the ALICE Highlights in the opening plenary session and five ALICE collaborators (Helen Caines, Leticia Cunqueiro, Alice Ohlson, Jürgen Schukraft, Enrico Scomparin) were among the invited plenary speakers. The poster by Jihye Song, a graduate student from the Pusan National University, on S(1385) and X(1530) production in ALICE, was selected among the eight best posters out of almost 300 and she was awarded a flash talk in the closing plenary session. 

Most of the ALICE highlights were new results from the highest energy and luminosity samples of pp, p-Pb and Pb-Pb collisions of the LHC Run-2. The intense campaign of data calibration and reconstruction, as well as Monte Carlo simulations, that was steered by the Data Preparation and Offline groups, allowed the Physics Working Groups to produce several results based on the full sample of 2015 Pb-Pb data and first results from the p-Pb run of November 2016. For the first time, some of the highlight results were posted on Twitter (@ALICEexperiment) and Facebook (ALICE.EXPERIMENT) live during the conference .

New results of particle production over a large rapidity range and of the total transverse energy produced in 5.02 TeV collisions show that the produced matter in Pb-Pb collisions at 5.02 TeV is denser and hotter than in 2.76 TeV collisions. Other new results included measurements of transverse momentum distributions and flow coefficients of light flavour hadrons in Pb-Pb collisions at 5.02 TeV using up to four particle identification techniques, in addition to decay reconstruction. These measurements, illustrated in Fig. 1, show a clear mass-hierarchy: the mean transverse momentum of the particles increases with mass (pion, kaon, proton) and the elliptic flow shows the opposite behaviour. Both trends indicate a larger radial flow, and transverse expansion velocity, at 5.02 TeV than at 2.76 TeV. At higher transverse momentum, the elliptic flow v2 shows a characteristic difference between baryons and mesons, which however reduces again at high transverse momenta pT > 8 GeV, which is consistent with the idea that azimuthal anisotropy at high pT is determined by parton energy loss. These measurements will provide important input to the models that estimate the Quark-Gluon Plasma thermodynamic and hydrodynamic properties by performing simultaneous fits to large sets of experimental observables. Results of these analyses, which rely heavily on ALICE measurements in Pb-Pb collisions at 2.76 TeV, had a significant prominence at the conference.

Figure 1: (upper panels) Transverse momentum distributions for charged pions, kaons and protons in several centrality classes in Pb-Pb collisions at 5.02 TeV; (lower panels) Elliptic flow coefficient for charged pions, kaons, protons and φ mesons in two centrality classes in Pb-Pb collisions at 5.02 TeV.

 

Open heavy flavour production and elliptic flow measurements in Pb-Pb at 5.02 TeV were presented for the first time. Using the full data sample allowed us to achieve a significantly better precision than in Run-1 for the measurements based on muon triggers, in all centrality classes, and for those based on the minimum-bias trigger in semi-central collisions. The nuclear modification factors of heavy-flavour decay muons at forward rapidity and of D mesons at central rapidity were measured with almost a factor of two better precision than before. As observed also for light-flavour hadrons, the suppression with respect to binary scaling is found to be very similar at 2.76 and 5.02 TeV, indicating that the effect of a larger energy loss at higher energy is compensated by the less steep transverse momentum distribution. The elliptic flow coefficient of D mesons (see Fig. 2-left), including for the first time the Ds, was also measured with better precision. This allowed us to explore the event-shape engineering technique as well, addressing the correlation between the flow of charmed hadrons and of soft particles. 

New quarkonium production measurements were presented for several colliding systems, including pp at 5 and 13 TeV, p-Pb at 8.16 TeV, hadronic and ultra-peripheral Pb-Pb collisions at 5.02 TeV. Among the highlights is the evidence of positive elliptic flow of inclusive J/y at forward rapidity in Pb-Pb collisions at 5.02 TeV in the transverse momentum interval between 2 and 8 GeV/c (see Fig. 2-right). The most likely interpretation of this measurement is that a large fraction of J/y in this pT range is produced from recombination of a charm and anti-charm quark that participate in the collective expansion of the system. The recombination scenario is also supported by the new measurements of the nuclear modification factor of J/y at central and forward rapidity. New measurements were also presented for the 2S states of charmonium and  bottomonium: in both cases, a larger suppression than for the 1S states is observed. This is consistent with binding-energy dependent colour screening in the QGP, although in the case of charmonia the effects of recombination have to be taken into account in the interpretation of the results. The new J/y measurements in p-Pb at 8.16 TeV and in ultra-peripheral Pb-Pb collisions at 5.02 TeV provide insight in the nuclear modification of parton distribution functions, and other cold-nuclear matter effects in the case of p-Pb.

Figure 2: Elliptic flow coefficient of D mesons (left: average of D0 and D+ in black, Ds in orange) and of inclusive J/Ψ (right: forward rapidity in red, central rapidity in blue) in semi-central Pb-Pb collisions at 5.02 TeV.

 

One of the most striking observations in heavy ion collisions at the LHC is the suppression of jets indicating that the quarks and gluons which are at their origin loose energy traversing the QGP. New results in this area included measurements of jet-like two-particle correlations with a neutral pion trigger, which show a decrease of the suppression of yields at low pT, which is in line with the expected softening of the fragmentation due to parton energy loss. We presented new measurements of jet shape observables including jet mass and N-subjettiness. For the jet mass measurements, model calculations were presented by the theory group from the so-called ‘hybrid model’. These calculations show that there are two counteracting effects at work, from jet quenching and medium response.

One of the highly debated topics at the conference was the observation of the so called Chiral Magnetic Effect (CME), quantum currents induced by the strong magnetic fields prevailing at the moment when two nuclei collide. ALICE presented new results from multi-particle correlations which help to disentangle the more mundane flow effects and the CME signal.

Several measurements were reported for pp collisions, at both 7 and 13 TeV, as a function of inclusive particle multiplicity. The measurements of strange hadron and of J/y production in high-multiplicity collisions probe the dynamics of multiple partonic interactions at soft and hard scales providing stringent constraints to event generators. For example, it is observed that the production rate of J/y in events with multiplicity seven times larger than the average is about fifteen times larger than in an unbiased event sample. It is also observed that the increase of both strange particle and J/y production with increasing multiplicity is the same as 7 and 13 TeV. Several other measurements in high-multiplicity pp collisions were presented as well, including cumulant measurements to explore collective effects and two-particle correlations that look at (mini-)jet structure.

Finally, the progress towards the major ALICE detector upgrade in 2019-20 was reported in two talks on the Inner Tracking System and Time Projection Chamber upgrades, as well as in poster presentations on the Muon Forward Tracker, the Fast Interaction Trigger and the proposed Forward Calorimeter.

All in all, we can look back at a very lively conference, where we had an impressive array of new results to show, including many results using the Run-2 data. The next step for most analyses will be to prepare publications that report on the new results. At the same time, we are already looking forward to the summer conferences, including Strangeness in Quark Matter, the Workshop on Particle Correlations and Fluctuations and the EPS Conference on High-Energy Physics.

Closing talk (from left to right: Federico Antinori, Peter Jacobs, Ulrich Heinz, Russel Betts, Paolo Giubellino, Olga Evdokimov).

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