[Paper Review] Exploring the properties of the phases of QCD matter - research opportunities and priorities for the next decade

Ulrich Heinz, P. Sørensen+12 more|arXiv (Cornell University)|Jan 26, 2015

High-Energy Particle Collisions Research244 references11 citations

TL;DR

This paper proposes a comprehensive research agenda for the next decade in quantum chromodynamics (QCD), emphasizing the completion of the QCD phase diagram search and the use of jets as probes of the quark-gluon plasma (QGP). It advocates for upgrading RHIC with a new jet detector (sPHENIX), sustained U.S. participation in the LHC heavy-ion program, and the construction of a high-luminosity, polarized Electron-Ion Collider (EIC) to study gluon dynamics and hadron structure with unprecedented precision.

ABSTRACT

This document provides a summary of the discussions during the recent joint QCD Town Meeting at Temple University of the status of and future plans for the research program of the relativistic heavy-ion community. A list of compelling questions is formulated, and a number of recommendations outlining the greatest research opportunities and detailing the research priorities of the heavy-ion community, voted on and unanimously approved at the Town Meeting, are presented. They are supported by a broad discussion of the underlying physics and its relation to other subfields. Areas of overlapping interests with the "QCD and Hadron Structure" ("cold QCD") subcommunity, in particular the recommendation for the future construction of an Electron-Ion Collider, are emphasized. The agenda of activities of the "hot QCD" subcommunity at the Town Meeting is attached.

Motivation & Objective

• To define the next decade’s research priorities for the hot QCD community studying the quark-gluon plasma (QGP) and its phase structure.
• To address unresolved questions about the QCD phase diagram, particularly the location and nature of the critical point.
• To enable precision measurements of transport properties and collective behavior in QGP across multiple energy and density scales.
• To establish the Electron-Ion Collider (EIC) as a central facility for probing gluon-dominated matter and hadronization dynamics.
• To strengthen theoretical frameworks and experimental capabilities to fully understand the structure of QCD matter from both 'hot' and 'cold' QCD perspectives.

Proposed method

• Utilize upgraded RHIC capabilities, including the sPHENIX jet detector and luminosity enhancements, to study QGP at low energies and probe transport properties near the phase boundary.
• Maintain strong U.S. participation in the LHC heavy-ion program to extend measurements of QGP properties at high energy and high density.
• Implement a multi-scale experimental program using jets, bottomonia, and polarized p+A collisions to probe the QGP at different length and energy scales.
• Develop and advocate for the construction of a polarized, high-luminosity Electron-Ion Collider (EIC) using existing infrastructure at BNL (eRHIC) or JLab (MEIC) to access the gluon structure of hadrons and nuclei.
• Integrate theoretical advances in lattice QCD, effective field theories, and kinetic models to interpret experimental data and guide future experiments.
• Leverage the EIC’s unique capability to measure semi-inclusive deep-inelastic scattering (SIDIS) and diffractive processes to access the saturation of gluon fields and the quark mass dependence of parton interactions in nuclear matter.

Experimental results

Research questions

• RQ1 Where is the critical point in the QCD phase diagram, and what are its universal scaling properties?
• RQ2 How do transport properties such as shear viscosity and electrical conductivity vary across the QCD phase diagram, especially near the phase boundary?
• RQ3 What is the microscopic origin of the near-perfect fluidity of the quark-gluon plasma, and how can it be probed using jets and heavy quarks?
• RQ4 How do saturated gluon fields in high-energy nuclei influence parton energy loss and hadronization dynamics?
• RQ5 What is the quark mass dependence of parton interactions in nuclear matter, and how can it be measured in electron-nucleus collisions?

Key findings

• The quark-gluon plasma produced in heavy-ion collisions is the most perfect liquid known, with a shear viscosity-to-entropy density ratio close to the quantum lower bound.
• Lattice QCD calculations and experimental data from RHIC and the LHC now converge on a consistent equation of state for the QGP, supporting a smooth crossover transition at zero baryon chemical potential.
• The EIC is projected to achieve luminosities of 10^33–10^34 cm⁻²s⁻¹, enabling high-precision measurements of parton distributions and gluon saturation effects.
• The EIC will provide the first direct evidence for the formation of a color-singlet, saturated gluon matter state in high-energy electron-nucleus collisions through diffractive and semi-inclusive cross-section measurements.
• Measurements of pion and D⁰ meson production in e+p and e+Au collisions will reveal the quark mass dependence of parton energy loss and hadronization in nuclear matter.
• The EIC will enable the first direct imaging of the spatial and momentum distributions of gluons in nuclei, mapping the structure of the 'femtometer filter' that governs parton evolution in dense matter.