D-meson production in p-Pb collisions at root S-NN=5.02 TeV and in pp collisions at root S=7 TeV

Background: In the context of the investigation of the quark gluon plasma produced in heavy-ion collisions, hadrons containing heavy (charm or beauty) quarks play a special role for the characterization of the hot and dense medium created in the interaction. The measurement of the production of charm and beauty hadrons in proton-proton collisions, besides providing the necessary reference for the studies in heavy-ion reactions, constitutes an important test of perturbative quantum chromodynamics (pQCD) calculations. Heavy-flavor production in proton-nucleus collisions is sensitive to the various effects related to the presence of nuclei in the colliding system, commonly denoted cold-nuclear-matter effects. Most of these effects are expected to modify open-charm production at low transverse momenta (p(T)) and, so far, no measurement of D-meson production down to zero transverse momentum was available at mid-rapidity at the energies attained at the CERN Large Hadron Collider (LHC). Purpose: The measurements of the production cross sections of promptly produced charmed mesons in p-Pb collisions at the LHC down to p(T) = 0 and the comparison to the results from pp interactions are aimed at the assessment of cold-nuclear-matter effects on open-charm production, which is crucial for the interpretation of the results from Pb-Pb collisions. The prompt charmed mesons D-0, D+, D*+, and D-s(+) were measured at mid-rapidity in p-Pb collisions at a center-of-mass energy per nucleon pair root S-NN = 5.02 TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D-0 -> K- pi(+), D+ -> K- pi(+) pi(+), D*+ -> D-0 pi(+), D-S(+) -> phi pi(+) -> K- K+ pi(+), and their charge conjugates, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. In addition, the prompt D 0 production cross section was measured in pp collisions at root S = 7 TeV and p-Pb collisions at root S-NN = 5.02 TeV down to p(T) = 0 using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D-0 decay vertex. Results: The production cross section in pp collisions is described within uncertainties by different implementations of pQCD calculations down to p(T) = 0. This allowed also a determination of the total c (c) over bar production cross section in pp collisions, which is more precise than previous ALICE measurements because it is not affected by uncertainties owing to the extrapolation to pT = 0. The nuclear modification factor R-pPb( p(T)), defined as the ratio of the p(T)-differential D meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium. Conclusions: These measurements add experimental evidence that the modification of the D-meson transverse momentum distributions observed in Pb-Pb collisions with respect to pp interactions is due to strong final-state effects induced by the interactions of the charm quarks with the hot and dense partonic medium created in ultrarelativistic heavy-ion collisions. The current precision of the measurement does not allow us to draw conclusions on the role of the different cold-nuclear-matter effects and on the possible presence of additional hot-medium effects in p-Pb collisions. However, the analysis technique without decay-vertex reconstruction, applied on future larger data samples, should provide access to the physics-rich range down to p(T) = 0.