Overview:

The goal of this program is to deepen our theoretical understanding of the phase structure of QCD—in particular the de-confinement transition and possible critical point—from present experimental programs, and to provide theoretical guidance to upcoming programs at several facilities worldwide.

Experiments at RHIC and the LHC have revealed several interesting and unexpected properties of the Quark Gluon Plasma (QGP), most prominently its near perfect fluidity. The QGP created at LHC and top RHIC energies consists of almost as much antimatter as matter characterized by the nearly vanishing baryon number chemical potential μB. Lattice calculations show that QCD predicts that under these condition the transition from the QGP to a hadron gas occurs smoothly as a function of decreasing temperature, with many thermodynamic properties changing dramatically but continuously within a narrow temperature range around the transition temperature Tc = 154 ± 9 MeV. This transition is referred to as the crossover region of the phase diagram of QCD.

In contrast, the quark-gluon plasma at large baryon number chemical potential may experience a sharp first order phase transition as it cools, with bubbles of quark-gluon plasma and bubbles of hadrons coexisting at a well-defined co-existence temperature, similar to bubbles of steam and liquid water coexisting in a boiling water. This co-existence region ends in a critical point, where the baryon number chemical potential is just large enough to instigate a first order phase transition. It is not yet known whether QCD has a critical point, nor where in its phase diagram it might lie. Model calculations typically predict the existence of a critical point, but do not precisely constrain its location. Model-independent lattice QCD calculations, meanwhile, become more difficult with increasing μB and thus do not yet provide definitive answers about the existence of a critical point.

The field has recognized the importance of probing the QCD phase diagram, and a major experimental program has begun that exploits the unique flexibility of RHIC to run at lower collision energies, which lead to systems with larger μB. First results from the RHIC Beam Energy Scan (BES) program are intriguing. There indeed seem to be several signals suggesting a softening of the equation of state, which may result from a first-order transition, as well as possible indications of a critical point. However, quantitative and even qualitative insight from these intriguing signals requires a comprehensive and rigorous theoretical treatment of the system. In the future, the second stage of the beam energy scan program at RHIC (BES-II), NA61 at CERN-SPS and new facilities like NICA and FAIR will investigate this whole region of the phase diagram in more detail.

Goals of the program:

The aim of the program is to evaluate what is already known about the QCD phase diagram at finite μB, and to use the recent progress in Lattice QCD and (fluid) dynamical descriptions of heavy ion collisions. We shall attempt to infer what the BES results imply about the phase diagram of nuclear matter and to provide guidance to future experimental programs. Additionally, we intend to address how present and future energy scan programs can best increase our understanding of the phases of strongly interacting matter. This includes a realistic assessment of what information is likely inaccessible to heavy ion experiments. Some of the key questions to consider are:

1. What features do dynamical models require in order to establish definitive links between BES observables and structures in the QCD phase diagram?

2. What advances in theory will be needed to take full advantage of the experimental data presently available?

3. Do we understand the initial conditions and which regions of the phase diagram we are probing?

4. Can we exclude certain regions of the phase diagram from having any structures of interest, such as a critical point or a first order transition?

5. What if anything is required of future experimental programs to further elucidate the QCD phase diagram?

As mentioned below, our field increasingly looks to its legacy in textbooks, where prospective students will be justifiably uninterested in the details of a given phenomenological model. Rather, the focus will be to drive towards definitive statements about the phases of QCD.

Outline of the program:

This will be a 4-week program with a larger workshop to stimulate discussions including some outside perspectives on the topic in the third week from October 3-7. The rough focus of the other 3 weeks is planned as

1. September 19-23: Theoretical foundations: What is known about the equation of state? How can we extend this knowledge and implement it in dynamical models? What sources of fluctuations need to be taken into account and how do we properly include them in a dynamical model?

2. September 26-30: How are conserved charges initially distributed and how well do we understand baryon stopping? Which charges need to be conserved during the evolution? How should anomalous currents be implemented in the transport or hydrodynamic models so that we can describe their evolution from initial state to final state? What quantities other than the EOS are needed for a realistic dynamical treatment, such as vorticity and transport coefficients, and how do they depend on temperature and net baryon density? How well does one need to know these quantities and what are the relevant observables to constrain them?

3. Workshop
   
   Talks will be scheduled to address the latest developments in theory and what has been done in energy scans to explore phase transitions. Input from related fields such as lower energy nuclear physics (e.g. liquid-gas transition) or even in laser-induced “micro-explosions” of plasmas in crystals will be discussed as well. Considering the preceding 2 weeks of the program, the goal of the workshop is to identify what may be considered firmly established on the QCD phase diagram.
   
   There will be a $50.00 USD registration fee to attend the 16-3 embedded workshop. The registration fee includes participation in the workshop, lectures, and coffee breaks.

4. During the final week which includes the wrap-up session, we will address strategy for theory development and how to draw firm conclusions from the data. Electromagnetic probes will also be discussed early in the week.
   
   October 10-11: What constraints do electromagnetic probes provide on the evolution? Are the traditional phase transition observables still relevant?
   
   October 12-14: Wrap up: What is the strategy for theory development, BESII measurements and extracting knowledge from the data? How can we either definitely show the existence of a critical point and a phase transition or rule it out in the region accessible by the RHIC BES?