Heavy Ion Collisions: Achievements and Challenges

TL;DR

This paper reviews the progress in heavy ion collision research over the past decade, highlighting discoveries of the quark-gluon plasma, recent experimental insights, and ongoing challenges in understanding the transition, equilibration, and properties of the sQGP.

Contribution

It consolidates recent experimental and theoretical developments, emphasizing new phenomena like sound waves in the plasma and identifying key open questions for future research.

Findings

Discovery of strongly-coupled Quark-Gluon Plasma with low viscosity

Identification of hydrodynamical perturbations as sound waves

Exploration of novel sound-related reactions in heavy ion collisions

Abstract

A decade ago brief summary of the field could be formulated as a discovery of strongly-coupled Quark-Gluon-Plasma, sQGP, making a very good liquid with surprisingly small viscosity. Since 2010 we have LHC program, which added a lot to our understanding, and now there seems to be a need to consolidate what we learned and formulate a list of issues to be studied next. Hydrodynamical perturbations, leading to higher harmonics of angular correlations, are identified as long-lived sound waves. Recently studied reactions involving sounds include phonon decays into two ("loop viscosity"), phonon+magnetic field into photons/dileptons (sono-magneto-luminescence), and two phonons into a gravity wave, a penetrating probe of the Big Bang. The mainstream issues in the field now include a quest to study transition between $pp,pA$ and heavy ion $AA$ collisions, with an aim to locate "the smallest drops" of the sQGP displaying collective/hydrodynamics behavior. The issues related to out-of-equilibrium stage of the collisions, and mechanisms of the equilibration, in weak and strong coupling, are also hotly debated.