Investigation of the Quark-Gluon Plasma With the ALICE Experiment

The quark-gluon plasma, or QGP, is a state of matter in which quarks and gluons, the elementary building blocks of ordinary baryonic matter (as protons and neutrons), are no longer confined into hadrons by the strong force. A phase transition from ordinary nuclear matter to a QGP is expected to occur in extreme conditions of high baryon density and temperature, as is thought to have characterized the Universe about 1–10 μs after the Big Bang or can be reached in the dense cores of neutron stars. In the laboratory, the conditions of high energy density necessary to form a QGP can be obtained by colliding heavy ions at velocities close to the speed of light. The experimental investigation of the QGP provides a test of Quantum Chromodynamics (QCD), the quantum field theory within the Standard Model of elementary particles describing the interaction among color charges. A QGP is an extended many-body system of color charges whose characteristics emerge from the fundamental properties of the strong interaction at high energy densities. Understanding the phenomenology of this state of matter is therefore an important step in the understanding of the strong interaction itself and of QCD. Following the first theoretical speculations about the existence of the QGP dating back to the 1970s, the field of experimental heavy-ion physics was established through the 1970s–1980s, first at the Bevalac at the Lawrence Livermore National Laboratory (USA), then at the Alternating Gradient Synchrotron at the Brookhaven National Laboratory (BNL, USA) and the Super Proton Synchrotron (SPS) at CERN, Switzerland. In the year 2000, the discovery of the QGP was announced at the CERN SPS. Since then, the major particle and nuclear physics laboratories all around the world have been running or planning a heavy-ion experimental program, covering a broad range of collision energies. The longest and most intensive heavy-ion studies have been pursued at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider (LHC), where they are still ongoing. In parallel, numerous advances on the theoretical side, partly also helped by the increase in computing power over time, have provided more and more sophisticated tools to describe the phenomenology of heavy-ion collisions and the properties of the QGP. Even if a QGP can be produced in the laboratory under appropriate conditions, its direct observation is not possible, because the matter created in the collision exists in a deconfined state for a time of the order of 10−23 s, after which it transitions to a system of hadrons. This represents a major challenge toward the characterization of a QGP: reliance on the measurement of final-state hadronic observables and select probes that are sensitive to the QGP properties of interest at different stages of its evolution is necessary. This requires the capability to disentangle the effects due to the presence of a QGP medium from many others, including those due to the presence of a nuclear environment in the target, or from reinteractions in the final hadronic stage. At the CERN LHC in Geneva, in an underground tunnel across the border between France and Switzerland, protons and fully ionized heavy ions are accelerated and collide at energies of a few TeV per nucleon pair, the largest ever reached in a particle accelerator, recreating the conditions present in the early Universe. A Large Ion Collider Experiment (ALICE) is the experiment specifically designed to study the QGP produced at the LHC. Operating at the energy frontier since 2009, ALICE has carried out a successful physics program that has enabled a quantitative assessment of the properties of the QGP produced in heavy-ion collisions at the LHC and led to some new discoveries. The results of ALICE and the other LHC experiments have also posed new questions related to the limits of QGP formation in different collision systems, thus prompting new advances in the theoretical field.