The Hot QCD White Paper: Exploring the Phases of QCD at RHIC and the LHC

TL;DR

This white paper reviews recent experimental and theoretical progress in understanding the quark-gluon plasma produced in heavy ion collisions at RHIC and LHC, highlighting its properties, initial state, and phase diagram.

Contribution

It provides a comprehensive overview of advances in hot QCD research and outlines future scientific opportunities for resolving key open questions.

Findings

QGP is a strongly coupled liquid with minimal viscosity

Recent data have deepened understanding of QGP properties

Outstanding issues include initial state and microscopic interactions

Abstract

The past decade has seen huge advances in experimental measurements made in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and more recently at the Large Hadron Collider (LHC). These new data, in combination with theoretical advances from calculations made in a variety of frameworks, have led to a broad and deep knowledge of the properties of thermal QCD matter. Increasingly quantitative descriptions of the quark-gluon plasma (QGP) created in these collisions have established that the QGP is a strongly coupled liquid with the lowest value of specific viscosity ever measured. However, much remains to be learned about the precise nature of the initial state from which this liquid forms, how its properties vary across its phase diagram and how, at a microscopic level, the collective properties of this liquid emerge from the interactions among the individual quarks and gluons that must be visible if the liquid is probed with sufficiently high resolution. This white paper, prepared by the Hot QCD Writing Group as part of the U.S. Long Range Plan for Nuclear Physics, reviews the recent progress in the field of hot QCD and outlines the scientific opportunities in the next decade for resolving the outstanding issues in the field.