Quark-Gluon Plasma 2

The following sections are included:

• CONTENTS

• PREFACE

Numerical simulations of quantum chromodynamics at nonzero temperature provide information from first principles about the physical properties of the quark gluon plasma. Because the lattice approximation can be refined indefinitely, results of lattice simulations now provide the most reliable basis for our understanding of the nonperturbative characteristics of the plasma and of the high temperature phase transition. Following a brief overview of the methodology of lattice gauge theory at nonzero temperature, recent results and insights from lattice simulations are discussed. These include our understanding of the phase diagram of QCD, the nature of the phase transition, and the structure of the plasma.

A recent development in finite temperature field theory, the so-called Braaten-Pisarski method, and its application to properties of a quark-gluon plasma, possibly formed in relativistic heavy ion collisions, are reviewed. In particular parton damping rates, the energy loss of energetic partons, thermalization times, viscosity, and production and damping rates of hard photons are discussed.

The following sections are included:

• Introduction

• Ultrarelativistic collisionless plasmas
  • Classical plasmas in the collisionless regime
  • Ultrarelativistic plasmas

• The equations of motion for collective excitations
  • Notations
  • The approximation scheme
  • Charge oscillations in the abelian plasma
  • Fermionic mean fields
  • Non abelian plasma
  • From kinetic equations to HTL's
  • Effective action for soft fields
  • Gauge structure
  • The energy-momentum tensor

• Abelian physics
  • Effective propagator for soft gauge fields
  • Screening properties
  • Energy transfer and Landau damping
  • Abelian plasma waves

• Nonabelian excitations
  • Static field configurations
  • Plane-wave solutions
  • Longitudinal plane waves
  • Transverse plane waves
  • Chaotic behaviour of the color waves

• Fermionic modes in ultrarelativistic plasmas
  • The spectrum of fermionic excitations
  • Explicit calculation of the states at T = 0

• Conclusions

• References

This is a brief review of three strongly related topics. In recent years, significant progress was reached in understanding of vacuum and hadronic structure: quantitative role of non-perturbative tunneling, described semiclassically by “instantons” was clarified. We review recent works on the point-to-point correlation functions, comparing those obtained from phenomenology, “instanton liquid” models and on the lattice. The second topic is physics of chiral symmetry restoration, which may lead to observable hadron modification and unusual event-per-event fluctuations. We discuss collective flow and phenomena related with a remarkable “softness” of the equation of state near the phase transition, as well as phenomena at small pt. We also describe recent ideas on the mechanism of chiral restoration, based on formation of polarized instanton-antiinstanton molecules. The third part, related to quark-gluon plasma, is related mainly with the issue of initial equilibration of QGP in high energy collisions. We discuss different “parton cascade” approaches and argue that multi-gluon processes dominate them. We discuss predictions for RHIC and LHC energies.

The space-time evolution of nuclear collisions at very high energies , is investigated in terms of the dissipative dynamics of short range (semi)hard quark and gluon interactions, by evolving the partons' phase-space distributions in real time according to a transport equation through multiple collisions, emission and absorption processes within perturbative QCD. The resulting parton cascade picture is applied to study multiparticle production, as well as issues of thermalization, chemical equilibration, and properties of plasma signatures in heavy ion collisions at the future RHIC and LHC experiments.

A pQCD-inspired model for multiple parton production and parton equilibration in ultrarelativistic heavy-ion collisions is reviewed. The model combines pQCD processes including initial and final state radiations together with string phenomenology for nonperturbative soft processes. Comparison with existing data is made and further tests of the model to constrain model parameters are proposed. With the obtained space-time history of the parton production, evolution of the minijet gas toward a fully equilibrated parton plasma is studied. Uncertainties and their consequences are also examined.

The following sections are included:

• Introduction
  • Preview
  • Probing Colour Deconfinement

• Quarkonium Production in Hadron-Hadron Collisions
  • Colour Evaporation
  • Quarkonium Production: Theory and Data

• Quarkonium Evolution and Hadron-Nucleus Collisions
  • Parton Fusion and Colour Evaporation in Nuclear Matter
  • Quantum-Mechanical Interference and Nuclear Shadowing
  • Momentum Loss of a Colour Charge in Nuclear Matter
  • Charmonium Interactions in Nuclear Matter
  • EMC Effect Modifications

• The Theory of Quarkonium-Hadron Interactions
  • The Short-Distance Analysis of Quarkonium-Hadron Interactions
  • Non-Perturbative Quarkonium Dissociation
  • The Experimental Study of Charmonium-Hadron Interactions

• Quarkonium Dissociation in Confined and Deconfined Media
  • The Parton Structure of Confined vs. Deconfined Matter
  • Quarkonium Dissociation by Deconfined Gluons
  • Charmonium Survival in an Expanding Medium

• Pre-Equilibrium Deconfinement
  • Shadowing and J/ψ Suppression in Nucleus-Nucleus Collisions
  • Deconfined Gluons and QGP

• Conclusions

• Acknowledgements

• References

The influence of the Landau-Pomeranchuk-Migdal effect on the production of dileptons and soft real photons in a hadronic gas and in a quark-gluon plasma is considered. In the low invariant mass region this effect is shown to significantly reduce the rate of the lepton-pair production via virtual bremsstrahlung. For high invariant masses its influence is negligible. This behaviour is of importance for the theoretical analysis of low-mass lepton pairs produced in relativistic heavy ion collisions.

In QCD with two flavours of massless quarks, the chiral phase transition is plausibly in the same universality class as the classical four component Heisenberg magnet. Therefore, renormalization group techniques developed in the study of phase transitions can be applied to calculate the critical exponents which characterize the scaling behaviour of universal quantities near the critical point. As a consequence of this observation, a quantitative description of the universal physics of the chiral phase transition in circumstances where the plasma is close to thermal equilibrium as it passes through the critical temperature can be obtained. This approach to the QCD phase transition has implications both for lattice gauge theory and for heavy ion collisions. Future lattice simulations with longer correlation lengths will provide quantitative measurements of the various exponents and the equation of state for the order parameter as a function of temperature and quark masses. Present lattice simulations already allow many qualitative tests. In a heavy ion collision, the consequence of a long correlation length would be large fluctuations in the number ratio of neutral to charged pions. Unfortunately, we will see that because of the explicit chiral symmetry breaking introduced by the masses of the up and down quarks, no equilibrium correlation length gets long enough for this phenomenon to occur. In the third chapter of this article, I discuss attempts to model the dynamics of the chiral order parameter in a far from equilibrium QCD phase transition by considering quenching in the O(4) linear sigma model. I argue, and present numerical evidence, that in the period immediately following the quench long wavelength modes of the pion field are amplified. This could have dramatic phenomenological consequences in heavy ion collisions. I review various recent theoretical advances — attempts to include expansion and relax the quench assumption; attempts to include quantum mechanical effects. Long wavelength pion oscillations have now been seen in a number of simulations in which different assumptions are made. However, all of the theoretical treatments involve idealizations and assumptions, and should only be regarded as qualitative guides to what can happen. It is up to experimentalists to determine whether or not such phenomena occur; detection in a heavy ion collision would imply an out of equilibrium chiral transition.

We review the progress in understanding the strange particle yields in nuclear collisions and their role in signalling quark-gluon plasma formation. We report on new insights into the formation mechanisms of strange particles during ultrarelativistic heavy-ion collisions and discuss interesting new details of the strangeness phase diagram. In the main part of the review we show how the measured (multi-)strange particle abundances can be used as a testing ground for chemical equilibration in nuclear collisions, and how the results of such an analysis lead to important constraints on the collision dynamics and space-time evolution of high energy heavy-ion reactions.

Relativistic heavy ion collisions offer the possibility to produce exotic metastable or even absolutely stable states of nuclear matter containing (roughly) equal number of strangeness compared to the baryon number: Strangelets, small pieces of strange quark matter, were proposed as a signal of quark gluon plasma formation. As their hadronic counterpart, also small pieces of strange hadronic matter may show up with rather similar properties. The reasoning of both their stability and existence, the possible separation of strangeness necessary for their formation and the chances for their detection are reviewed.

The theory and phenomenology of two-particle and many-particle measurements is presented. Theoretical formulations are presented at an introductory level along with more detailed answers to commonly asked questions. Experimental signals of a first-order phase transition, phase separation and coherent pion production are discussed. Experimental results from CERN and the AGS are briefly reviewed.

The following sections are included:

• Introduction

• Scaling Behaviors in the Ginzburg-Landau Theory
  • Factorial Moments
  • Second-Order Phase Transition
  • Experimental Verification of ν
  • Inhomogeneity
  • First-Order Phase Transition
  • Remarks

• Cluster Production in the Mixed Phase
  • The Problem
  • Cluster Growth in Heavy-Ion Collisions
  • Self-Organized Criticality
  • Cellular Automaton
  • Scaling Behavior
  • Modified CA
  • Phenomenology

• General Comments

• Acknowledgments

• REFERENCES