Signatures of the QCD Phase Diagram
Final Report Abstract
Within this research program we studied various aspects of the phase structure of strongly interacting matter, described by quantum chromodynamics (QCD). This is of fundamental importance as it is directly related to the question how the matter around us was formed in the early universe. This formation process is, to some extend, recreated by colliding heavy ions at ultra-relativistic energies. A major challenge is to find signatures of a phase transition in the matter that is produced by such collisions. In order to understand possible signatures, one needs to account for the conditions imposed on the system by the experimental setup. One of these, which follows from the initial conditions of the collision, is that the net content of strange quarks at the time where the produced particles cease to interact has to be zero. We have shown that this strangeness-neutrality condition has a sizable effect on the thermodynamic properties of strongly interacting matter at finite density and is therefore indispensable for an accurate description of heavy-ion collisions. The phase transition from the quark-gluon plasma to ordinary hadronic matter manifests itself through long-range correlations between particles. We found that correlations between baryons and particles containing a net amount of strange quarks are most drastically affected by strangeness neutrality. These correlations can be measured directly and are particularly sensitive to the nature of strongly interacting matter at finite density. This allowed us to show that there is an exact relation between the phase structure of QCD and the chemical potentials required to enforce net particle number conservation. The QCD phase diagram itself is known only at vanishing density since conventional, and very powerful, methods to compute it from first principles cannot be applied in this regime. We took a first major step towards the computation of the phase diagram at finite density with functional renormalization group methods. Our key progress is based on the efficient description of the fundamental dynamics of the building blocks of strongly interacting matter - quarks and gluons - at finite temperature and density. The results are in excellent agreement with available first-principles results at vanishing density. Most notably, we find a critical endpoint at a temperature of 107 MeV and a baryon chemical potential of 635 MeV. Surprisingly, we discovered strong indications for the existence of a large spatially inhomogeneous regime around the location of the endpoint. This hints at the existence of crystal-like phases in this region of the phase diagram, which is well within reach of future heavy-ion experiments. However, to make definite statements and predictions, this needs to be studied in more detail. Further fundamental properties of strongly interacting matter follow from the existence of a quantum anomaly, which is generated by topological gauge field configurations. These have typically been studied based on field configurations with unit topological charge. We have shown that fluctuations of configurations with higher topological charge lead to novel physical effects. They give rise to higherorder anomalous correlations of quarks with various phenomenological implications. Examples are anomalous modifications of the mass-spectrum and the interactions of hadrons and other bound states of quarks. Our results have opened up new avenues to study the relation between topology and particle phenomenology, which can be relevant not only for hadron physics and the phase structure of strongly interacting matter, but also for dark matter cosmology.
Publications
- Strangeness Neutrality and Baryon-Strangeness Correlations, Phys. Rev. D 100, 111501(R) (2019)
W.-J. Fu, J. M. Pawlowski, F. Rennecke
(See online at https://doi.org/10.1103/PhysRevD.100.111501) - Strangeness Neutrality and QCD Thermodynamics, SciPost Phys. Core 2, 002 (2020)
W.-J. Fu, J. M. Pawlowski, F. Rennecke
(See online at https://doi.org/10.21468/SciPostPhysCore.2.1.002)
