The ALICE collaboration has shown at the Quark Matter 2012 conference results on heavy-flavour particle production measured in lead–lead collisions at the LHC. In head-on (central) collisions, a large suppression of the heavy-flavour particle yield is observed with respect to expectations at large transverse momentum. In non-central collisions, heavy-flavour particles have non-isotropic azimuthal emission with respect to the reaction plane. Both observations are understood as resulting from the interaction of heavy quarks with the partonic medium formed in the collisions.
Heavy quarks play a special role in heavy-ion physics because they constitute a tagged (identified) probe (from production to observation), which enables a unique access to their interactions in the Quark-Gluon Plasma, the highly excited system (medium) formed in high-energy nucleus–nucleus collisions. This allows us to gain microscopic insights into the transport properties of the medium. Heavy-flavour particles may be thought of as “Brownian motion'' markers, the kinematical distributions of which (especially in momentum and azimuthal angle) reflect their re-interaction history.
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More in detail, heavy quarks are expected to be sensitive to the medium energy density, through the mechanism of in-medium energy loss. The nuclear modification factor RAA – the ratio of the yield measured in nucleus–nucleus collisions to that expected from proton–proton collisions – is well established as a sensitive observable for the study of the interaction of hard partons with the medium. Because of the QCD nature of parton energy-loss, quarks are predicted to lose less energy than gluons (which have a higher colour charge); in addition, the so-called "dead-cone" effect and other mechanisms are expected to reduce the energy loss of heavy partons with respect to light ones.
Therefore, a pattern of gradually decreasing RAA suppression should emerge when going from the mostly gluon-originated light-flavour hadrons (e.g. pions) to the heavier D and B mesons: RAA(?) < RAA(D) < RAA(B) . The measurement and comparison of these different probes provides, therefore, a unique test of the colour-charge and mass dependence of parton energy-loss. Further information can be obtained from the measurement of the azimuthal anisotropy of heavy-flavour particles with respect to the reaction plane in non-central collisions. This results from the conversion of the spatial anisotropy in the collision zone into a momentum anisotropy of the produced particles. The magnitude of the asymmetry can be quantified using a Fourier decomposition. The second Fourier coefficient v2, which is commonly called elliptic flow, is expected to be sensitive, in the low momentum region, to the degree of thermalization of heavy quarks within the partonic medium. At high momentum, the anisotropy brings information on the path length dependence of heavy-flavour energy loss. In other words, a non-zero heavy-flavour particle v2 is an indication of the interaction of heavy quarks with the medium in the early stage.
In Pb-Pb collisions, heavy-flavour production has been measured in ALICE in the charm hadronic decay channels (including so far D0→ Kπ , D+ → Kππ , D*+→ D0π and D+s → KKπ in |y|<0.5), in the semi-electronic decay channel (inclusive D,B → e+X in |y|<0.8), and in the semi-muonic decay channel (inclusive D,B → μ+X in 2.5 < y< 4). Practically all ALICE detectors are used for these analyses.
Figure 1: RAA of various heavy-flavour particle species as a function of transverse momentum in central Pb-Pb collisions at √sNN = 2.76 TeV: average D0, D+ and D*+ mesons (purple), D+s mesons (orange) electrons from heavy-flavour decays (blue) and muons from heavy-flavour decays (green). Vertical black lines indicate the statistical uncertainties, empty boxes the systematic uncertainties
First ALICE measurements of heavy-flavour particle production in Pb-Pb collisions were already presented at the Quark Matter conference last year. These have been completed and extended using the data collected in fall 2011. The new results presented at the Quark Matter conference in 2012 include:
• RAA of muons from heavy-flavour decays
• RAA of electrons from heavy-flavour decays over a broad pt range
• RAA of D0, D+ and D*+ over a broad pt range, and as a function of the reaction plane for D0.
• RAA of Ds+ (thanks to the first measurement of Ds+ in heavy-ion collisions)
• azimuthal anisotropy of electrons from heavy-flavour decays
• azimuthal anisotropy of charm hadrons (D0, D+ and D*+)
As an illustration of the progress achieved, Figure 1 shows the average RAA of D0, D+ and D*+ compared to the RAA of Ds+ and to those of electrons and of muons from heavy-flavour decays. A suppression of a factor 4-5 is observed for pt > 5 GeV/c. This implies a large in-medium energy loss for heavy quarks. The magnitude of the suppression is similar for all particle species considering the semi-leptonic decay kinematics.
Moreover, the light-charged particle RAA (not shown in the Figure) is similar to the average RAA of D0, D+ and D*+. Thus, a different behavior of light and heavy-flavour particles cannot be concluded at present. Results from the elliptic flow analysis evidence a non-zero v2 for electrons from heavy-flavour decays and for D0, D+ and D*+ (Figure 2). This confirms that heavy-quarks strongly interact with the medium. These data have been compared to model predictions. A reasonable description of the D meson RAA, together with those of leptons from heavy-flavour decays is achieved when in-medium parton energy loss is taken into account in the calculations. However, the simultaneous description of RAA and v2 appears to be a challenge for models (as shown in Figure 2 for v2 of D mesons and electrons).
In addition to Pb-Pb results, also a number of new measurements in pp collisions have been presented, like for example final results on beauty-decay electron and Ds+ production.
In summary, the ALICE results on heavy-flavour particle production in Pb-Pb collisions presented at the Quark Matter conference this year indicate a strong interaction of heavy quarks with the medium. The precision of the measurements will be improved in the future by increasing the statistics and with the planned upgrades of the detectors. Moreover, initial-state effects, which may play an important role at low pt, will be investigated with p-Pb collisions early next year.
The ALICE heavy-flavour results had a large visibility at the QM2012 conference, with one plenary and five parallel talks (Z. Conesa del Valle, D. Caffarri, A. Grelli, G.M. Innocenti, S. Sakai, X. Zhang), and nine posters. Overall, eleven out of these fifteen presentations were given by students or first-year postdocs.
Figure 2: v2 of D mesons (top) and heavy-flavour decay electrons (bottom) as a function of transverse momentum in Pb-Pb collisions at √sNN = 2.76 TeV. The data are compared to various theoretical calculations.