Charm production at the LHC via $D^{0} -> K^{-} \pi^{+}$ reconstruction in ALICE:cross section in pp collisions and first flow measurement in Pb-Pb collisions

ALICE (A Large Hadron Collider Experiment) is one of the CERN-LHC (Large Hadron Collider) experiments. The main goal of ALICE is investigating the properties of the state of matter formed in ultra-relativistic heavy-ion collisions, the strongly-interacting quark-gluon plasma (QGP). The extremely high-energy-density and high-temperature matter produced in Pb--Pb collisions at centre-of-mass-energy of 2.76 TeV at the LHC is similar to the state of the matter that constituted the Universe few microseconds after the Big Bang. Nucleus-nucleus collisions have been carried out for the last 20 years by the Super Proton Synchrotron at CERN and by the Relativistic Heavy Ion Collider at Brookhaven to study this state of matter in a laboratory. The experimental results confirm the production of a deconfined system that reaches a thermal equilibrium and behaves like an almost perfect fluid, following the equations of hydrodynamics, until it cools down and hadrons are produced again. The properties of such a medium can be studied via probes coming from the interaction itself, namely the final-state detected particles. Heavy quarks are particularly effective in probing the medium. The charm quark is the subject of this thesis, which presents the measurement of the production of $\rm D^0$ mesons, containing a charm quark, at the LHC. The $\rm D^0$ meson production cross section measurement in pp collisions and the first measurement of the $\rm D^0$ elliptic flow in Pb--Pb collisions will be described. The first result serves as a test for pQCD calculations at unprecedented high centre-of-mass energy, 7 TeV for pp and 2.76 TeV for Pb--Pb collisions, and as a reference for measurements made in a heavy-ion environment, like the measurement of the $\rm D^0$ yield suppression due to energy loss in the medium. The second result is a starting point to investigate the degree of thermalization of the quark-gluon plasma. The topics treated in each chapter of this thesis are outlined in the following. In Chapter 1 a general introduction about the physics goals in heavy-ion and proton-proton collisions will be drawn. In the context of the Standard Model, the strong interaction will be introduced. Then, the evolution of a nucleus-nucleus collision will be described and the observables useful for probing the properties of the quark-gluon plasma will be presented. A large part of the chapter will be finally devoted to the physics of heavy quarks in pp and heavy-ion collisions. Chapter 2 will present the ALICE detector, describing mainly the sub-detectors used for heavy-flavour analyses. The tools developed for simulation, reconstruction and analyses will be summarised as well. The performance achieved by the detectors involved in heavy-flavour analyses, both in pp and Pb--Pb collisions, during the first year of data taking will be presented. The analysis tools implemented during this thesis will be described in Chapter 3. Chapter 4 will be devoted to the description of the measurement of the $\rm D^0$ meson cross section in pp collisions, in the decay channel $\rm D^0 \rightarrow K^- \pi^+$. In particular, I participated in the study of the variables used for the selection of the signal and in the yield extraction through an invariant mass analysis. The displaced decay vertices ($c\tau (\rm D^0) = 122.9~ \mu $m) are reconstructed, then pairs of unlike-sign tracks are combined and their species identified. Finally the topological selections are applied and a fit to the invariant mass spectrum is performed to determine the $\rm D^0$ yield. To compute the final cross section, the extracted yield must be corrected and properly normalized. The measured $\rm D^0$ production cross section extends to the region of transverse momentum from 1 to 16~GeV$/c$ and was compared to pQCD calculations, which are found to be in agreement with data. Chapter 5 will present the first measurement of the $\rm D^0$ meson elliptic flow in the transverse momentum region from 2 to 12 GeV$/c$. A sample of 3 million events from 2010 data, in the centrality class 30-50\% was used and several equivalent methods were applied, giving compatible results. I was responsible for the tuning of the selection of the $\rm D^0$ candidates, for the signal extraction and for the determination of the elliptic flow with the event plane method, which consists in extracting the signal in two intervals of angle between the $\rm D^0$ and the event plane azimuths. The preliminary result was approved by the ALICE Collaboration and I presented it for the first time in an International conference. Finally, Chapter 6 will contain a summary of the motivation for the work carried out during this thesis and the conclusions that can be drawn from the results