• Volume 84, Issue 5

May 2015,   pages  669-941

• Preface

• Thermalization and isotropization in heavy-ion collisions

Our current understanding of the processes driving the thermalization and isotropization of the quark gluon plasma (QGP) created in ultrarelativistic heavy-ion collisions (URHICs) is reviewed. Initially, the phenomenological evidence in favour of the creation of a thermal but momentum–space anisotropic QGP in URHICs is discussed. Further, the degree of isotropization using viscous (dissipative) hydrodynamics, weak-coupling approaches to QGP dynamics, and strong-coupling approaches to QGP dynamics are discussed. Finally, recent progress in the area of real-time non-Abelian gauge field simulations and non-Abelian Boltzmann–Vlasov-based hard-loop simulations are reported.

• The initial stages of heavy-ion collisions in the colour glass condensate framework

In this short review, we present the description of the early stages of a heavy-ion collision at high energy in the colour glass condensate (CGC) framework.

• Hydrodynamic modelling for relativistic heavy-ion collisions at RHIC and LHC

In this article, we briefly review recent progress on hydrodynamic modelling and its implementations to relativistic heavy-ion collisions at RHIC and LHC. The related topics include:

1. initial-state fluctuations, final-state correlations and event-by-event hydrodynamics,

2. extracting the QGP shear viscosity from flow data,

3. flow and hydrodynamics in p+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV.

• Transport models for relativistic heavy-ion collisions at Relativistic Heavy Ion Collider and Large Hadron Collider

We review the transport models that are widely used to study the properties of the quark-gluon plasma formed in relativistic heavy-ion collisions at RHIC and LHC. We show that transport model analysis of two important and complementary observables, the anisotropic flow of bulk hadrons and suppression of hadron yields at high transverse momentum, provide exciting new information on the properties of the plasma formed.

• Long-range correlations in high multiplicity 𝑝𝑝 and 𝑝𝐴 collisions

This review summarizes recent discoveries in high-energy proton+proton and proton+nucleus collisions, with particular attention on the observation of long-range azimuthal correlations in high multiplicity collisions. These correlations, which resemble those seen in ultrarelativistic nucleus–nucleus collisions, provide a unique window into the physics of the very early collision stage in high energy nuclear interactions. Here we present a compilation of the most important experimental results and briefly discuss successes and challenges for a selection of theoretical approaches.

• Freeze-out dynamics in heavy-ion collisions: Recent advances

We briefly review recent advances in the subject of hadron production in relativistic heavy-ion collisions. We focus on the issues of chemical freeze-out, chemical equilibration and the role of post-hadronization inelastic collisions. From the observations collected in elementary and heavy-ion collisions, a picture emerges in which hadrons are born in chemical equilibrium at hadronization, thereafter undergoing inelastic and elastic collisions whose impact on the primordial distribution depends on the system size.

• QCD critical point: The race is on

A critical point in the phase diagram of quantum chromodynamics (QCD), if established either theoretically or experimentally, would be as profound a discovery as the good-old gas–liquid critical point. Unlike the latter, however, first-principles-based approaches are being employed to locate it theoretically. Due to the short-lived nature of the concerned phases, novel experimental techniques are needed to search for it. The Relativistic Heavy Ion Collider (RHIC) in USA has an experimental programme to do so. This short review is an attempt to provide a glimpse of the race between the theorists and the experimentalists as well as the synergy between them.

• Experimental studies of the quantum chromodynamics phase diagram at the STAR experiment

We review the STAR experiment's results to date from the Beam Energy Scan (BES) at Brookhaven's Relativistic Heavy Ion Collider, and outline future plans and prospects in this area. BES Phase-I is based on Au+Au data taken in 2010 and 2011 at $\sqrt{s_{NN}} = 7.7$, 11.5, 19.6, 27 and 39 GeV, and when interpreted in conjunction with the large datasets available at 62.4 and 200 GeV, permits an initial exploration of the phase diagram of quantum chromodynamics (QCD) matter. The three goals of BES Phase-I are as follows:

1. a search for turn-off of the promising signatures of quark gluon plasma (QGP) already reported at the top RHIC energies;

2. a search for evidence of a possible first-order phase transition such as a signature of softening of the QCD equation of state (EoS);

3. a search for a critical end point as expected in a scenario where there is a cross-over transition from hadronic matter to QGP at the highest RHIC energies, but a first-order phase transition at lower energies with finite net-baryon density.

We summarize several analyses of BES data from 2010 and 2011 that are either published or submitted, as well as several more that have been reported at meetings in preliminary forms. The physics interpretation of BES Phase-I measurements is frequently limited by the increasing statistical error bars as the beam energy decreases, and the planned BES Phase-II will have much improved capabilities in this regard.

• Developments in lattice quantum chromodynamics for matter at high temperature and density

A brief overview of the QCD phase diagram at nonzero temperature and density is provided. It is explained why standard lattice QCD techniques are not immediately applicable for its determination, due to the sign problem. A selection of recent lattice approaches that attempt to evade the sign problem are then discussed and classified according to the underlying principle: constrained simulations (density of states, histograms), holomorphicity (complex Langevin, Lefschetz thimbles), partial summations (clusters, subsets, bags) and change in integration order (strong coupling, dual formulations).

• Jet-quenching and correlations

This article reviews recent advances in our understanding of the experimental aspects of jet-quenching and correlations in relativistic heavy-ion collisions at RHIC and LHC. Emphasis is put on correlation measurements, namely jet-like correlations with anisotropic flow subtraction in heavy-ion collisions and long-range pseudorapidity correlations in small systems. Future path on correlation studies is envisioned which may elucidate jet–medium interactions and the properties of the hot dense medium in QCD.

• Jet modification in the next decade: A pedestrian outlook

In this review, we recount the current status of the theory of jet modification in dense matter. We commence with an outline of the ‘traditional’ observables which may be calculated without recourse to event generators. These include single- and double-hadron suppression, nuclear modification factor vs. reaction plane etc. All these measurements are used to justify both the required underlying physical picture of jet modification as well as the final obtained values of jet transport coefficients. This is followed by a review of the more modern observables which have arisen with the ability to reconstruct full jets, and the challenges faced therein. This is followed by a preview of upcoming theoretical developments in the field and an outlook on how the interface between these developments, phenomenological improvements, and upcoming data will allow us to quantitatively determine properties of the medium which effect the modification of hard jets.

• Photon and dilepton production in high-energy heavy-ion collisions

The recent results on direct photons and dileptons in high-energy heavy-ion collisions, obtained particularly at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) are reviewed. The results are new not only in terms of the probes, but also in terms of the precision. We shall discuss the physics learned from the results.

• Electromagnetic probes of strongly interacting matter

The nuclear matter under extreme conditions of temperatures (𝑇) and baryonic densities (𝑛𝐵) undergoes a phase transition to quark gluon plasma (QGP). It is expected that such extreme conditions can be achieved by colliding nuclei at ultrarelativistic energies. In the present review, the suitability of photons and dileptons as diagnostic tools of QGP has been discussed. The photon and dilepton spectra originating from heavy-ion collisions at LHC energies have been explicitly displayed in this article. Results from SPS and RHIC have been discussed adequately with appropriate references. The role of single electron spectra originating from the decays of heavy flavoured mesons on QGP detection has also been discussed briefly.

• Quarkonia at finite temperature in relativistic heavy-ion collisions

The behaviour of quarkonia in relativistic heavy-ion collisions is reviewed. After a detailed discussion of the current theoretical understanding of quarkonia in a static equilibriated plasma, we discuss quarkonia yield from the fireball created in ultrarelativistic heavy-ion collision experiments. We end with a brief discussion of the experimental results and outlook.

• Anomalous transport effects and possible environmental symmetry 'violation' in heavy-ion collisions

The heavy-ion collision provides a unique many-body environment where local domains of strongly interacting chiral medium may occur and in a sense allow environmental symmetry 'violation' phenomena. For example, certain anomalous transport processes, forbidden in usual medium, become possible in such domains. We briefly review recent progress in both the theoretical understanding and experimental search of various anomalous transport effects (such as the chiral magnetic effect, chiral separation effect, chiral electric separation effect, chiral electric/magnetic waves, etc.) in the hot QCD fluid formed by such collisions.

• Neutron stars as probes of extreme energy density matter

Neutron stars have long been regarded as extraterrestrial laboratories from which we can learn about extreme energy density matter at low temperatures. In this article, some of the recent advances made in astrophysical observations and related theory are highlighted. Although the focus is on the much needed information on masses and radii of several individual neutron stars, the need for additional knowledge about the many facets of neutron stars is stressed. The extent to which quark matter can be present in neutron stars is summarized with emphasis on the requirement of non-perturbative treatments. Some longstanding and new questions, answers to which will advance our current status of knowledge, are posed.

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