Heavy Ion Collisions: The Big Picture and the Big Questions

Abstract

Heavy ion collisions quickly form a droplet of quark-gluon plasma (QGP) with a remarkably small viscosity. We give an accessible introduction to how to study this smallest and hottest droplet of liquid made on earth and why it is so interesting. The physics of heavy ions ranges from highly energetic quarks and gluons described by perturbative QCD to a bath of strongly interacting gluons at lower energy scales. These gluons quickly thermalize and form QGP, while the energetic partons traverse this plasma and end in a shower of particles called jets. Analyzing the final particles in a variety of different ways allows us to study the properties of QGP and the complex dynamics of multi-scale processes in QCD which govern its formation and evolution, providing what is perhaps the simplest form of complex quantum matter that we know of. Much remains to be understood, and throughout the review big open questions will be encountered. 2Busza, Rajagopal and van der Schee to be 1.65±0.1 TeV (4), meaning that the average energy density 1 fm/c after the collision is greater than 1.65 TeV/(π(7 fm) 2 (0.92 fm)) = 12 GeV/fm 3 , about twenty times the energy density of a hadron. The entropy produced in these collisions is also enormous; to get a sense of this note that before the collision the entropy of the two incident nuclei is essentially zero whereas the final state after the collision can contain as many as 30,000 particles, and hence has a very large entropy. We shall return to this later, and in particular we shall see that most of this entropy is produced quickly, in the initial moments after the collision. To get a further sense of the magnitude of the average energy density 1 fm/c after the collision, note that, as we shall see in Section 3, lattice calculations of QCD thermodynamics show that matter in thermal equilibrium at a temperature of 300 MeV has an energy density ≈ 12 T 4 = 12.7 GeV/fm 3 . Thus, the quarks and gluons produced in the collision cannot be described as a collection of distinct individual hadrons. Nevertheless, the quarks and gluons www.annualreviews.org • HIC: The Big Picture and Big Questions 3 4 Busza, Rajagopal and van der Schee 20 Busza, Rajagopal and van der Schee

Finite-density QCD transition in a magnetic background field

“…The quark-gluon plasma (QGP), which existed at certain stage of the evolution of the early Universe, may also be created in relativistic heavy-ion collisions. The QGP has been studied at Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, at the Large Hadron Collider (LHC) at CERN, and will also be subjected to further investigation at Nuclotron Ion Collider fAcility (NICA) at JINR in Dubna, and the Facility for Antiproton and Ion Research (FAIR) in Darmstadt [1].…”

“…The general form of the polynomial (1) is supported by the analyticity arguments at µ B = 0 along with the invariance of thermodynamic properties of the system under the charge reflection, µ B → −µ B : due to charge conjugation symmetry, the transition temperature of an equilibrium QGP is an even function of the baryon chemical potential µ B . Lattice simulations of the T -µ B phase diagram give the first-principles determination of the transition line (1), which may be confronted with the results of the heavy-ion experiments on the chemical freeze-out line. The freeze-out line corresponds to another curve in the T -µ B plane at which the hadron abundances, that encode the chemical composition of the expanding plasma, get stabilized and thus leave an imprint in the experimentally measured hadronic spectra.…”

Evolution of Quasiperiodic Structures in a Non-Ideal Hydrodynamic Description of Phase Transitions

“…The Standard Model of particle physics predicts that after the inflation the hot expanding Universe was filled with deconfined quarks, in the state of quark-gluon plasma [4]. This view on the early Universe is supported by simulations done in various cosmological and relativistic heavy ion collision models [5,6] and by the lattice calculations [7]. The quark-gluon plasma in baryon-poor matter persists down to a temperature T 160 MeV.…”

Glauber Monte Carlo predictions for ultrarelativistic collisions with O16

“…used in [44] as a possible probe of the α clusterization in 12 C+Au collisions. We begin the presentation of our Glauber model results for 16 O + 16 O collisions with the ellipticity and triangularity of the fireball obtained with two-and four-particle cumulants, where specifically These observables, of course, are not independent of the hydrodynamic response, yet it is worth to have a look at them, as they quantify the shape of the fireball and its fluctuations.…”