New State of Nuclear Matter: Nearly Perfect Fluid of Quarks and Gluons in Heavy Ion Collisions at RHIC Energies

TL;DR

This paper reviews experimental evidence from RHIC and LHC demonstrating the creation of a nearly perfect fluid of quarks and gluons, known as the quark-gluon plasma, in high-energy heavy-ion collisions, highlighting its unique properties and ongoing mysteries.

Contribution

It summarizes key experimental findings and theoretical models supporting the existence of a strongly-interacting, nearly perfect liquid quark-gluon plasma in heavy-ion collisions at RHIC and LHC.

Findings

Creation of a dense, strongly-interacting medium in Au+Au collisions

Reproduction of flow results by IP-Glasma and hydrodynamic models

Evidence of a nearly perfect liquid with low shear viscosity to entropy ratio

Abstract

This article reviews several important results from RHIC experiments and discusses their implications. They were obtained in a unique environment for studying QCD matter at temperatures and densities that exceed the limits wherein hadrons can exist as individual entities and raises to prominence the quark-gluon degrees of freedom. These findings are supported by major experimental observations via measuring of the bulk properties of particle production, particle ratios and chemical freeze-out conditions, and elliptic flow; followed by hard probe measurements. The results reveal that a dense, strongly-interacting medium is created in central Au+Au collisions at 200 GeV at RHIC. This revelation of a new state of nuclear matter has also been observed in measurements at the LHC. Further, the IP-Glasma model coupled with viscous hydrodynamic models, which assumes the formation of a QGP, reproduces well the experimental flow results from Au+Au at 200 GeV. This implies that the fluctuations in the initial geometry state are important and the created medium behaves as a nearly perfect liquid of nuclear matter because it has an extraordinarily low ratio of shear viscosity to entropy density. However, these discoveries are far from being fully understood. Furthermore, recent experimental results from RHIC and LHC in small p+A, d+Au and He+A collision systems provide brand new insight into the role of initial and final state effects. These have proven to be interesting and more surprising than originally anticipated; and could conceivably shed new light in our understanding of collective behavior in heavy-ion physics. Accordingly, the focus of the experiments at both facilities RHIC and the LHC is on detailed exploration of the properties of this new state of nuclear matter, the QGP.