Theory overview of Heavy Ion collisions

TL;DR

This paper reviews the theoretical understanding of high energy heavy ion collisions, emphasizing viscous hydrodynamics, initial conditions from gluon saturation, and recent findings in small system collisions.

Contribution

It provides an overview of the current theoretical framework and recent advances in modeling the spacetime evolution of quark-gluon plasma in heavy ion collisions.

Findings

Viscous hydrodynamics effectively describes the bulk evolution.

Gluon saturation models help understand initial conditions.

High multiplicity small system collisions show nucleus-nucleus-like features.

Abstract

This presentation discusses some recently active topics in the theoretical interpretation of high energy heavy ion collisions at the LHC and at RHIC. We argue that the standard paradigm for understanding the spacetime evolution of the bulk of the matter produced in the collision is provided by viscous relativistic hydrodynamics, which can be used to systematically extract properties of the QCD medium from experimental results. The initial conditions of this hydrodynamical evolution are increasingly well understood in terms of gluon saturation, and can be quantified using Classical Yang-Mills fields and QCD effective kinetic theory. Hard and electromagnetic probes of the plasma provide additional constraints. A particularly fascinating subject are high multiplicity proton-proton and proton-nucleus collisions, where some of the characteristics previously attributed to only nucleus-nucleus collisions have been observed.