The strongly coupled quark-gluon plasma created at RHIC

TL;DR

RHIC experiments have created a hot, dense, strongly coupled quark-gluon plasma that behaves like an almost perfect fluid with very low viscosity, challenging prior expectations of a weakly interacting gas.

Contribution

This paper reviews the experimental evidence for the strongly coupled quark-gluon plasma and discusses its implications for relativistic heavy-ion physics and related fields.

Findings

Evidence of thermally equilibrated quark-gluon plasma at high temperatures.

Discovery that the plasma behaves as an almost perfect fluid with low viscosity.

Support for the strongly coupled nature of the QGP from elliptic flow measurements.

Abstract

The Relativistic Heavy Ion Collider (RHIC) was built to re-create and study in the laboratory the extremely hot and dense matter that filled our entire universe during its first few microseconds. Its operation since June 2000 has been extremely successful, and the four large RHIC experiments have produced an impressive body of data which indeed provide compelling evidence for the formation of thermally equilibrated matter at unprecedented temperatures and energy densities -- a "quark-gluon plasma (QGP)". A surprise has been the discovery that this plasma behaves like an almost perfect fluid, with extremely low viscosity. Theorists had expected a weakly interacting gas of quarks and gluons, but instead we seem to have created a strongly coupled plasma liquid. The experimental evidence strongly relies on a feature called "elliptic flow" in off-central collisions, with additional support from other observations. This article explains how we probe the strongly coupled QGP, describes the ideas and measurements which led to the conclusion that the QGP is an almost perfect liquid, and shows how they tie relativistic heavy-ion physics into other burgeoning fields of modern physics, such as strongly coupled Coulomb plasmas, ultracold systems of trapped atoms, and superstring theory.