Measurement of $\rm D^0$ production in Pb-Pb collisions at 5.02 TeV with ALICE

The Quark-Gluon Plasma (QGP), a state of strongly-interacting matter in which quarks and gluons are deconfined, is thought to have existed a few micro-seconds after the Big Bang. Ultra-relativistic heavy-ion collisions provide a unique opportunity to study the properties of such deconfined medium. These collisions are realized at the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC). Jet quenching, the phenomenon in which high transverse momentum partons undergo energy loss by collisions with medium constituents and medium-induced gluon radiation, is widely considered as one of the evidences for QGP formation. Jet quenching can be observed via the measurements of di-hadron angular correlations, the nuclear modification factor ($R_{\rm AA}$), di-jet energy imbalance and via the modification of the jet fragmentation function. Heavy quarks are a powerful probe of the QGP, as they are produced in hard scattering processes on a timescale shorter than the QGP formation time and experience the whole system evolution. This thesis presents the measurements of jet-like correlations with neutral pion triggers in pp and central Pb--Pb collisions at 2.76 TeV, the measurements of prompt $\rm D^0$ production in Pb--Pb collisions at ${\sqrt{s}_{\rm NN}}$ = 5.02 TeV, and the on-going measurements of non-prompt $\rm D^0$ production in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The measurements using two-particle angular correlations between trigger particles and associated particles have been extensively used to search for remnants of the radiated energy and the medium response to the high-$p_{\rm T}$ parton. By varying the transverse momentum for trigger and associated particles one can probe different momentum scales to study the interplay of soft and hard processes. Besides providing access to medium properties, measurements of $\pi^0$-hadron correlations determine the most important background contribution of direct photon-hadron correlation measurements. For this analysis, the neutral pions used as triggers are identified in the di-photon decay channel, with energy deposits are reconstructed with the Electromagnetic Calorimeter (EMCal), avoiding admixtures from changing particle composition of the trigger particle, hence should simplify comparisons with calculations. Associated hadrons are reconstructed with the Inner Tracking System (ITS) and Time Projection Chamber (TPC) down to 0.5 GeV/$c$, and significantly extend the previous measurement that only used charged hadrons above 3 GeV/$c$. After subtracting the dominant background, including the anisotropic flow harmonics $v_{2}$ to $v_{5}$, the per-trigger yields are extracted for Near side with $|\Delta \varphi| < 0.7$ and Away side with $|\Delta\varphi-\pi| < 1.1$. The per-trigger yield modification factor, $I_{\rm AA}$, defined as the ratio of per-trigger yields in Pb--Pb to those in pp collisions, is measured on the near and away side for the 0-10\% most central Pb--Pb collisions. On the Away side, the $I_{\rm AA}$ is strongly suppressed to around 0.6 for $p_{\rm T}$ > 3 GeV/$c$, and increase as the momenta decrease, reaching about $5.2$ at lowest $p_{\rm T}$. On the near side, an enhancement of $I_{\rm AA}$ from $1.2$ to $1.8$ at lowest $p_{\rm T}$ is observed. The data provides a good constrain to the theoretical models which aim to fully describe jet-medium interactions. The measurements of prompt $\rm D^0$ nuclear modification factor ($R_{\rm AA}$) can provide important information about the microscopic interactions of heavy quarks with the medium constituents, in particular on the colour-charge and parton-mass dependence of heavy-quark energy loss. The prompt $\rm D^0$ are reconstructed at mid-rapidity via the hadronic decay channel $\rm D^0\to K^-\pi^+$. Selections on the decay topology and particle identification are applied in order to reduce the combinatorial background. The signal is extracted via an invariant-mass analysis. The feed-down from beauty-hadron decays is subtracted according to expectations based on FONLL calculations and assumptions of feed-down nuclear modification factor. The prompt $\rm D^0$ nuclear modification factor ($R_{\rm AA}$) has been measured in Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, in the centrality class 0-10\%, 30-50\% and 60-80\%. The results are compatible with those measured at $\sqrt{s_{\rm NN}}$ = 2.76 TeV, with better precision and extended $p_{\rm T}$ coverage. The data set stringent constraints to theoretical models with different implementations of in-medium energy loss. The cross section of non-prompt $\rm D^0$ has been measured in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, with the goal to further understanding of beauty hadronisation. A multivariate technique (Boosted Decision Trees) is utilised to improve the separation between non-prompt and prompt $\rm D^0$ by optimally combining discriminating variables related to the $\rm D^0$ decay topology. This method can significantly improve the statistical precision with which the measurement of non-prompt $\rm D^0$ can be made in Pb--Pb collisions with ALICE in the future. At the same time, this technique can be utilised for extracting other physics signal with low signal-to-background ratio. In the first Chapter, the physics of QCD theory and Heavy Ion Collisions will be introduced. The second Chapter describes the role of heavy flavour observables to investigate the QGP. A brief introduction of the ALICE apparatus will be given in Chapter 3. The fourth and fifth Chapters present the main work of the thesis, about $\pi^0$-hadron correlations and prompt $\rm D^0$ $R_{\rm AA}$ measurements. These results have already been published. The sixth chapter describes an on-going work about non-prompt $\rm D^0$ analysis, which aims to prepare a new measurement in Pb--Pb collisions in the future. Conclusions and Outlook will be drawn in the end. \vspace{0.5cm} {\large\textbf{Keywords:}} LHC, ALICE experiment, Pb--Pb collisions, ultra-relativistic heavy-ion collisions, D mesons, heavy flavour production, nuclear modification factor, Quark-Gluon Plasma (QGP), two-particle correlations