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 center-of-mass energy per nucleon-nucleon collision of $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$ with the ALICE detector at the CERN Large Hadron Collider. $D$ mesons were reconstructed via their hadronic decays at central rapidity in the transverse-momentum ($p_{\mathrm{T}}$) interval 2–16 $\mathrm{GeV}/c$. 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 $R_{\mathrm{AA}}$, 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 midcentral collisions (30%–50% centrality class), with a mean value of $0.{204}_{-0.036}^{+0.099}$ (tot. unc.) in the interval $2<p_{\mathrm{T}}<6\mathrm{GeV}/c$, 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 midcentral 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-$p_{\mathrm{T}}$ yield of $D$ mesons in central collisions and their azimuthal anisotropy in noncentral collisions.

• Received 9 May 2014

DOI:https://doi.org/10.1103/PhysRevC.90.034904

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©2014 CERN, for the ALICE Collaboration