D^{∗+}-meson production in proton-proton and proton-lead collisions in ALICE

Abstract

D^{*+} meson results in proton-proton collisions at \sqrt{s}=13 TeV and proton-lead collisions at \sqrt{s_{NN}}=5.02 TeV at the Large Hadron Collider collected with the ALICE detector are presented. Heavy quarks, such as the charm quark that is a constituent of the D^{*+} meson, are effective probes for the study of the properties of the Quark-Gluon Plasma formed in high energy Pb--Pb collisions, as heavy quarks are mainly produced in hard scattering processes in the initial stages of the collision. In p--Pb collisions the production of heavy flavour quarks can be influenced by Cold Nuclear Matter (CNM) effects. The nuclear modification, R_{AA} (R_{\rm pPb}) measures the plasma (CNM) effects in Pb--Pb (p--Pb) collisions on the transverse momentum(pt)-distribution of the final state particles compared to a particles distribution from minimum bias pp collisions. The comparison of heavy flavour production in proton-Pb and Pb--Pb collisions via the nuclear modification factor allows to distinguish between CNM effects and hot-nuclear matter effects present in Pb--Pb collisions. Measurements of the heavy flavour hadron cross-section in proton-proton collisions allows to test pertubative Quantum Chromo Dynamic calculations and forms a reference measurement for the R_{\rm{AA}} or R_{\rm pPb}. Ratio measurements of the cross-section in pp collisions between different collision energies make it possible to probe the gluonic properties of the theory with measurements at low D-meson pt. The proton-proton results presented in this thesis are within uncertainty consistent with FONLL predictions, though consistently above the central value. The presented preliminary results of the ratio of the cross-section at 13 TeV with respect to 5 TeV are fully consistent with predictions, and the uncertainty on the measurement and theory are similar. An increase in statistics could significantly improve these results and might make it possible to separate fluctuations from actual differences especially at low transverse momenta as well as further constrain the FONLL predictions. The presented proton-lead results are consistent with the results in proton-proton collisions. The nuclear modification factor for the average of D-mesons shows a flat trend for transverse momenta above 3GeV/c. These results are in agreement with various models that include CNM effects in the full pt range, disfavouring a calculation based on incoherent multiple scatterings at low transverse momenta. Models including also some form of Quark-Gluon Plasma show a different trend with respect to pt, but cannot fully be excluded based on the current measurements. Measurements done in multiplicity sets in proton-lead collisions show hints of a non-flat shape. These results also show a non-unity centrality ratio at 3sigma precision of which the shape is seen at all centralities, hinting towards some form of flow in p--Pb. Measurements of heavy flavour decay electrons find a non-zero $v_2$, possibly due to fluctuations in the initial state geometry, but it can also be explained by a very small QGP in proton-lead collisions. The ongoing upgrades on the ITS will allow the ALICE detector to do more precise measurements at lower pt. The extra precision will also allow us to correct for any B-decay D^{*+} mesons via measurement.