Correlations in particle production in proton-lead and lead-lead collisions at the LHC

In high-energy heavy-ion collisions at the Large Hadron Collider (LHC), a hot and dense state of matter called the Quark-Gluon Plasma (QGP) is formed. The initial collision geometry and the subsequent expansion during the QGP stage result in the correlations of produced particles, through which the properties of the QGP can be investigated. Two analyses based on the geometrical correlations of produced particles, one in proton-lead (p–Pb) collisions and the other in lead-lead (Pb–Pb) collisions, are presented in this thesis. The data analyzed in this thesis were collected with the ALICE detector at the LHC in p– Pb collisions at a nucleon–nucleon center-of-mass energy of 5.02 TeV, and Pb–Pb collisions at a nucleon–nucleon center-of-mass energy of 2.76 TeV. In the forward-central two-particle correlation analysis in p–Pb collisions, two-particle an- gular correlations between trigger particles in the forward pseudorapidity range (2.5 < |η| < 4.0) and associated particles in the central range (|η| < 1.0) are studied. The trigger parti- cles are muon tracks reconstructed in the Forward Muon Spectrometer, and the associated particles are charged tracks reconstructed in the central barrel tracking detectors. In high- multiplicity events, the double-ridge structure, previously observed in two-particle angular correlations at midrapidity (|η| < 1.2), is also found in the pseudorapidity ranges studied in this analysis. The azimuthal distribution is quantified using the Fourier decomposition, and the second-order Fourier coefficients for muons in the forward pseudorapidity range in high-multiplicity events are extracted after the contributions from jet-like correlations are removed. The coefficients are measured as a function of transverse momentum (pT) in the p-going direction and in the Pb-going direction, separately. Similar pT dependence of the coefficients in both directions are observed, with the Pb-going coefficients larger by 16±6% independent of pT. These observations further characterize the collective features in a small collision system (p–Pb). The results are compared with calculations using the AMPT model, which produces qualitatively the different pT and η dependence of the observables. In the analysis of the azimuthal collectivity of longitudinal structures in Pb–Pb collisions, the newly developed method is applied to investigate correlations among the longitudinal structures of produced particles in different azimuthal regions. In addition to the expan- sion of the QGP in the transverse direction, commonly quantified using Fourier coefficients, the initial geometry and resulting longitudinal expansion as a function of azimuthal angle enable us to better understand the full 3-dimensional evolution of heavy-ion collisions. The azimuthal angle is divided into regions in-plane and out-of-plane, and coefficients (an) of Legendre polynomials from a decomposition of the longitudinal structure at midrapidity (|η| < 0.8) on an event-by-event basis are estimated in each region for different central- ities. Correlations among the longitudinal structures in different azimuthal regions are studied via the correlations among coefficients from the decomposition. The results of conditional an measurements indicate collective features of longitudinal structure in the azimuthal direction in particular with the first and second order coefficients, which repre- sent the forward-backward asymmetry and mid-peripheral asymmetry, respectively. The results are compared with various heavy-ion collision models, including the Ncoll PYTHIA, HIJING and AMPT models. While the AMPT model shows similar centrality dependence in the conditional a1, the distinctive feature in conditional a2 observed in data is absent in all models considered in this thesis.