CERN Accelerating science

Within the Standard Model, the strong interaction between quarks is described by the theory of quantum chromodynamics (QCD). Due to the presence of color confinement, the isolated quarks and gluons cannot be seen in normal conditions. On the other hand, one of the features of QCD, the asymptotic freedom, tells us that the free quarks and gluons can be produced in an environment of extreme high temperatures and/or high densities, which results in a new phase of matter, the quark–gluon plasma (QGP). Nowadays, ultra-relativistic heavy-ion collisions are regarded as an effective way to produce the QGP and study its properties. ALICE (A Large Ion Collider Experiment) is one of the four experiments oper- ating at the largest particle collider in the world, the Large Hadron Collider (LHC). It is the only LHC experiment which is dedicated to the study of heavy-ion physics. In past decades, many important probes to study the properties of the QGP were measured by the ALICE collaboration. Among them, the hard probes, such as heavy flavours, high transverse momentum (pT) particles, jets, are of particular interest since they are believed to be produced in hard scattering processes during the early stage of the collision and subsequently undergo the whole evolution of the hot and dense medium. The suppression observed in the production of open heavy flavours in heavy-ion collisions relative to that in binary-scaled proton-proton collisions can be characterized in terms of the nuclear modification factor (RAA), which is a crucial observable to understand the mechanism of in-medium energy loss of heavy quarks. The measurement of the anisotropic flow of heavy flavours provides additional in- sights on the possible thermalization of heavy quarks in the medium at low pT and the path-length dependent in-medium energy loss at high pT. On the other hand, in small collision systems, the QGP is thought not to exist due the absence of sup- pression observed in the measurement of the nuclear modification factor. However, recent measurements of anisotropic flow indicate a significant collective behaviour in small collision systems, especially for high-pT particles. It challenges our under- standing about the small collision systems, which need to be further studied via more specific and precise measurements. This thesis presents three measurements with hard probes at both midrapidity (|y| <0.8) and forward rapidity (2.5< y <4) in p–Pb collisions at √sNN = 5.02 and 8.16 TeV, and Pb–Pb collisions at √sNN = 2.76 and 5.02 TeV, with the ALICE detector at the LHC. The first chapter of the thesis is dedicated to an introduc- tion about the theoretical background of quantum chromodynamics and heavy-ion collisions, and the latest experimental findings in both large and small collision systems. Chapter 2 presents the current anisotropic flow analysis methods, includ- ing the event-plane, scalar-product, cumulants and pair-wise azimuthal correlation methods. Chapter 3 gives an overview of the ALICE experiment with a particular emphasis on the description of the detectors relevant for the analyses discussed in this thesis. Data processing and reconstruction based on the online and offline sys- tems are also introduced. Chapter 4 presents the measurement of the RAA of muons from heavy-flavour hadron decays in Pb–Pb collisions at √sNN = 2.76 and 5.02 TeV. The strategy to obtain the muon distribution normalized to minimum bias events and the subsequent subtraction of background muons is introduced. The results concerning the measurement of the RAA as a function of pT in various centrality classes are discussed and compared to the model calculations. Chapter 5 addresses the measurement of the azimuthal anisotropy of inclusive muons in p–Pb collisions at √sNN = 8.16 TeV. Several non-flow subtraction strategies are also discussed. Chapter 6 shows the measurement of the azimuthal anisotropy of particles associ- ated with jets down to low pT in p–Pb and Pb–Pb collisions at √sNN = 5.02 TeV. A novel three-particle correlation method is developed to separate the contributions from hard and soft components in flow measurements for the first time. A summary of the three analysis is given in Chapter 7. In addition, a brief introduction about the ALICE rivetization service work is given in appendix.