Abstract
The production of the prompt charmed mesons $D^0$, $D^+$ and $D^{*+}$ relative to the reaction plane was measured in Pb-Pb collisions at a centre-of-mass energy per nucleon-nucleon collision of $\sqrt{s_{\rm NN}} = 2.76$ TeV with the ALICE detector at the LHC. D mesons were reconstructed via their hadronic decays at central rapidity in the transverse momentum ($p_{\rm T}$) interval of 2-16 GeV/$c$. The azimuthal anisotropy is quantified in terms of the second coefficient $v_2$ in a Fourier expansion of the D meson azimuthal distribution, and in terms of the nuclear modification factor $R_{\rm AA}$, measured in the direction of the reaction plane and orthogonal to it. The $v_2$ coefficient was measured with three different methods and in three centrality classes in the interval 0-50%. A positive $v_2$ is observed in mid-central collisions (30-50% centrality class), with an mean value of $0.204_{-0.036}^{+0.099}$ (tot.unc.) in the interval $2 < p_{\rm T} < 6$ GeV/$c$, which decreases towards more central collisions (10-30% and 0-10% classes). The positive $v_2$ is also reflected in the nuclear modification factor, which shows a stronger suppression in the direction orthogonal to the reaction plane for mid-central collisions. The measurements are compared to theoretical calculations of charm quark transport and energy loss in high-density strongly-interacting matter at high temperature. The models that include substantial elastic interactions with an expanding medium provide a good description of the observed anisotropy. However, they are challenged to simultaneously describe the strong suppression of high-$p_{\rm T}$ yield of D mesons in central collisions and their azimuthal anisotropy in non-central collisions.
Highlights
Collisions of heavy nuclei at ultrarelativistic energies are expected to lead to the formation of a high-density colordeconfined state of strongly interacting matter
This modification is quantified by the nuclear modification factor RAA(pT) = dNAA/dpT TAA dσpp/dpT, where dNAA/dpT is the differential yield in nucleus-nucleus collisions in a given centrality class, dσpp/dpT is the cross section in pp collisions, and
An online selection based on the VZERO signal amplitude was used to enhance the sample of central and midcentral collisions through two separate trigger classes
Summary
Collisions of heavy nuclei at ultrarelativistic energies are expected to lead to the formation of a high-density colordeconfined state of strongly interacting matter. Heavy quarks (charm and beauty), with large masses mc ≈ 1.3 and mb ≈ 4.5 GeV/c2, are produced in pairs predominantly at the initial stage of the collision [5] in hard scattering processes characterized by time scales shorter than the medium formation time They traverse the medium and interact with its constituents via both inelastic (medium-induced gluon radiation, i.e., radiative energy loss) [6,7] and elastic (collisional) [8] QCD processes. The dependence of the partonic energy loss on the in-medium path length is expected to be different for each mechanism (linear for collisional processes [8] and close to quadratic for radiative processes [7]) It is an open question whether low-momentum heavy quarks participate, through interactions with the medium, in the collective expansion of the system and whether they can reach thermal equilibrium with the medium constituents [23,24].
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