Study of the Quark-Gluon Plasma with Hard Probes at the LHC

Abstract

Matter that surrounds us comes in a variety of phases that can be transformed into each other by a change of external conditions such as temperature, pressure, and composition. A good example is water that besides the liquid and gaseous phases, features a variety of solid phases [1]. Transitions from one phase to another are often accompanied by drastic changes in the physical properties of the matter, such as its electrical and thermal conductivity, elasticity, or transmittance. One may ask what happens when matter is under extreme conditions of high temperature and/or density. This question is of relevance for the early stage of the Universe as we go backwards in the cosmic evolution. It is also important in understanding of the properties of the inner core of neutron stars, the densest cosmic objects. Here the main players are no longer forces of electromagnetic origin but the strong interaction, which is responsible for the binding of protons and neutrons into nuclei and of quarks and gluons into the hadrons. The first realistic picture of the hadronic matter at high temperature was proposed by Hagedorn in the statistical bootstrap model of hadron production [2], well before the discovery of the Quantum Chromodynamics (QCD) [3]. In this model, hadrons are considered as composite particles (resonances of lighter hadrons), which results in the exponential increase in the density of mass states, r(mh) µ mmh-5=2 h emh=TH, where mh is the mass of a given hadronic state and TH is the Hagedorn temperature. This formula is well verified by summing up the measured hadronic states [4]. A fit to the data yields TH _ 170 MeV. An immediate consequence of the model is that the logarithm of the partition function of such hadron resonance gas and, thus, all thermodynamical quantities diverge at the limiting temperature T = TH. In 1973, Politzer [5], Gross and Wilczek [6] discovered that the QCD has properties of asymptotic freedom, i.e. the interaction between quarks and gluons weakens as they get closer to one another. It implies that at sufficiently high temperature and/or density, a new phase of deconfied quarks and gluons, referred to as quark-gluon plasma (QGP) [7–11], can be formed. The existence of a new phase was later confirmed in the calculations using the lattice formulation of QCD [12, 13]. Within this picture, the limiting temperature TH is close to the critical temperature for the phase transition between hadrons and quarks and gluons. Moreover, with point-like quarks and gluons the temperature can grow beyond any limits. These results inspired the community to explore the possibility to create and study the quark-gluon plasma by colliding heavy nuclei at high energy. In these collisions, the initial energy density and temperature should be sufficient to create the QGP for a short time. Experimental programmes started simultaneously in 1986 at the Alternating Gradient Synchrotron at the Brookhaven National Laboratory (BNL) and at the Super Proton Synchrotron (SPS) at CERN. Since 2000, the Relativistic Heavy Ion Collider (RHIC) at BNL has been colliding heavy-ions at psNN = 20–200 GeV. A new era of experimental search for the QGP started in 2009, when the Large Hadron Collided (LHC) at CERN became operational. The LHC was designed to collide heavy nuclei up to that of Pb at energy of psNN = 5:5 TeV, which is about 30 times larger than that at RHIC. Author joined A Large Ion Collider Experiment (ALICE) at the LHC in 2007. ALICE is a dedicated heavy-ion detector, which was designed to exploit the unique potential of nucleusnucleus interactions at the LHC energies. The main goal of ALICE is to study the physics of strongly interaction matter at extreme density and temperature, where the formation of the quark-gluon plasma is expected. Author has contributed to these studies using high momentum and/or high mass particles and jets (hard probes) to characterize the thermodynamic and transport properties of the QGP. Author is a key person in the ALICE Collaboration participating in measurements of transverse momentum spectra (pT) and nuclear modification factors of hard probes. This habilitation thesis is based on the results [14–28] obtained by the author in 2010-2018. It consists of 13 chapters, list of figures, list of tables and references. An introduction to the strongly interacting matter at extreme conditions is presented in Chap 1. The experimental apparatus, including ALICE and LHC, is described in Chap. 2. Production of hard probes in high energy nuclear collisions and their interaction with the hot and dense QCD matter in theoretical models is discussed in Chap. 3. The results on the production of hard probes at RHIC are shown in Chap. 4. Properties of QCD matter produced at the LHC determined using measurements of low energy (soft) particles are discussed in Chap. 5. The following chapters contain the results obtained by the author. The pT spectra of charged particles, charged pions, kaons and (anti)protons, D mesons, and charged jets, are presented in Chaps. 6, 7, 8, 9, respectively. The nuclear modification factors determined for charged particles, identified hadrons and jets, are discussed in Chaps. 10, 11 and 12, respectively. A summary is given in Chap. 13.