Measurement of the ${\rm D}^0$ meson production in Pb-Pb and p-Pb collisions with the ALICE experiment at the LHC

This thesis presents the measurement of the charmed ${\rm D}^0$ meson production relative to the reaction plane in Pb-Pb collisions at the centre-of-mass energy per nucleon-nucleon collision of $\sqrt{s_{\rm NN}}=2.76$ TeV, and the measurement of the ${\rm D}^0$ production in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV with the ALICE detector at the CERN Large Hadron Collider. The ${\rm D}^0$ azimuthal anisotropy with respect to the reaction plane is sensitive to the interaction of the charm quarks with the high-density strongly-interacting medium formed in ultra-relativistic heavy-ion collisions and, thus, to the properties of this state of matter. In particular, this observable allows to establish whether low-momentum charm quarks participate in the collective expansion of the system and whether they can reach thermal equilibrium with the medium constituents. The azimuthal anisotropy is quantified in terms of the second coefficient $v_2$ in a Fourier expansion of the ${\rm D}^0$ 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 measurement of the ${\rm D}^0$ production in p-Pb collisions is crucial to disentangle the effects induced by cold nuclear matter from the final state effects induced by the hot medium formed in Pb-Pb collisions. The ${\rm D}^0$ production is measured in both systems by reconstructing the two-prong hadronic decay ${\rm D}^0\rightarrow K^{-}+\pi^{+}$ in the central rapidity region, exploiting the separation of the decay vertex from the primary vertex. The raw signal is obtained with an invariant mass analysis, and corrected for selection and reconstruction efficiency. A positive elliptic flow $v_2$ is observed in Pb-Pb collisions in the centrality class 30-50%, with a mean value of $0.204^{+0.099}_{-0.036}$ in the interval $2<p_{\rm T}<6$ GeV/$c$, which decreases towards more central collisions. Consequently, the nuclear modification factor shows a stronger suppression in the direction orthogonal to the reaction plane. The $v_2$ and the $R_{\rm AA}$ measured in two azimuthal regions with respect to the reaction plane are compared to theoretical calculations of charm quark transport and energy loss in high-density strongly-interacting matter. The models that include substantial elastic interactions with an expanding medium provide a good description of the observed anisotropy. The ${\rm D}^0$ nuclear modification factor $R_{\rm pPb}$ in p-Pb collisions is compatible with unity within uncertainties. The measured $R_{\rm pPb}$ is compared to theoretical models including initial state effects, as well as to the nuclear modification factor measured in central Pb-Pb collisions. The ${\rm D}^0$ $R_{\rm pPb}$ results are consistent with the modification of the nucleon parton distribution functions induced by the nuclear environment, and provide experimental evidence that the modification of the D meson momentum spectrum observed in Pb-Pb with respect to pp collisions is due to strong final state effects induced by the hot medium.