Heavy quarks, like charm and beauty, are unique probes of the Quark-Gluon Plasma (QGP) created in Pb-Pb collisions at LHC energies. Their production occurs via high-Q2 processes, and therefore it is possible to test pQCD calculations for charm and beauty production at LHC energies. In addition, their large masses imply a short formation time, and therefore a longer exposure to the full system evolution. Measurements of D mesons, as well as electrons and muons from heavy-flavour hadron decays from ALICE during LHC Run 1 have proved strong quenching and collectivity of charm quarks in the QGP.
There are still some open key questions towards a more quantitative understanding of the processes behind energy loss and collective effects, in particular: are heavy quarks affected by initial state effects? What is the role of recombination of charm quarks with lighter quarks in the hadronization process? Are heavy quarks affected by the collective expansion of the medium, and how is this related to the bulk expansion? The recent ALICE results from LHC Run 2 on both p-Pb and Pb-Pb collisions improve the precision of Run 1 results, opening new avenues to characterize the QGP and initial state effects.
The p-Pb collisions at √sNN = 5.02 TeV recorded by ALICE in 2016 allowed studies of the nuclear modification factor of D mesons with high precision selecting the events on the basis of their centrality. To avoid the uncertainty on the pp reference, we performed the ratio (QCP) of the D0 pT distributions in central over peripheral p-Pb collisions, considering the different number of binary collisions. The D0 QCP, reported in Fig. 1, shows an enhancement from 1 in the pT range 3-8 GeV/c with a 1.7 level (considering statistical and systematic uncertainty including the one on normalization). The current precision of the measurement is still preventing us from drawing conclusions on the role of the different Cold Nuclear Matter effects and on possible presence of additional hot-medium effects, however it poses an interesting question on what are the mechanisms at play in small systems, and to what extent heavy quarks are influenced by them.
Figure 1: D0 central (0-10%)-to-peripheral (60-100%) nuclear modification factor (QCP) as a function of pT in p-Pb collisions at √sNN = 5.02 TeV (ALICE-PUBLIC-2017-008).
Moving to heavy-ion collisions, ALICE has measured the nuclear modification factor RAA in Pb-Pb at √sNN = 5.02 TeV. Figure 2 shows the RAA for the average of D0, D+, D*+ mesons as a function of pT, together with that of Ds+ for the 0-10% centrality class.
The larger statistics from Run 2 data allows for more precise measurements and higher pT reach. The central RAA values are higher for Ds+ mesons w.r.t. non-strange D mesons, but the two measurements are compatible within uncertainties. It is interesting to notice that the TAMU (Phys. Lett. B735 (2014) 445) and PHSD (PRC 93 (2016) 034906) models, which include recombination of charm quarks with the enhanced strange quarks in the QGP, predict a large increase of the Ds+ RAA.
Figure 2: Average of of D0, D+, D*+ (black) and Ds+ (orange) RAA as a function of pT for 0-10% Pb-Pb collisions at √sNN = 5.02 TeV (ALICE-PUBLIC-2017-003).
Figure 3: D0, D+ average v2 in 30–50% Pb–Pb collisions at √sNN = 5.02 TeV for events with largest q2 (blue), smallest q2 (red) and using the full sample (grey).
The observable used to characterize the azimuthal anisotropy of the produced particles is the elliptic flow v2, which quantifies, at low pT, the degree of collectivity of the system. The recent ALICE paper on the D-meson v2 in 30-50% Pb-Pb collisions at √sNN = 5.02 TeV (arXiv:1707.01005) shows a positive v2 at low-intermediate pT, suggesting that charm quarks interact with the medium constituents and are sensitive to the collective expansion of the medium. A step forward in this direction is to measure the D-meson v2 in events with different eccentricity, defined by the second-harmonic flow vector q2, obtained from the azimuthal angles of all the tracks in the event. The average of D0 and D+ v2 was measured for the 60% of events with the smallest q2 and for 20% of the events with the largest q2. The result, shown in Fig.3, shows a significant separation between D-meson v2 in events with large and small q2, suggesting that charm quarks are influenced by the light-hadron bulk collectivity and by event-by-event initial fluctuations. These event engineering techniques are becoming a new testing ground for models to understand the relation between heavy and light quark collectivity.
The LHC Run 2 has so far provided ALICE with large data samples in different collision systems to start setting constraints on models for heavy-flavour production, energy loss and collectivity. The remaining part of Run 2, together with Run 3 and Run 4, will allow for further improvements of the measurements and new differential observables to pose crucial constraints to theory models.