Professor Joseph Kapusta

This group is working on the physics of strongly interacting matter at extreme temperatures and densities. The theory used to describe this matter is Quantum Chromodynamics (QCD), which is the theory of the strong interactions. The group is working on using QCD to understand the structure of neutron stars and theoretical interpretations of high-energy heavy-ion collision experiments.

It is of fundamental importance for physicists to understand matter and its phases. For nuclear physicists in particular, an important focus is to explore the QCD phase diagram which maps the thermodynamic states of quarks and gluons to the temperature and baryon chemical potential plane. For a complete description of the phase diagram, one needs to have an understanding of dense, strongly interacting hadronic matter and quark matter. A proper description of hadronic matter would require understanding QCD in its non-perturbative regime, which has not been completely understood so far. Theoretical efforts have been made to study the phase diagram in the zero chemical potential region by first principles lattice calculations, which exhibits a smooth crossover at a temperature of about 1.8 trillion Kelvin. However, due to the notorious sign problem, lattice calculations cannot be extended to the finite chemical potential regime. Meanwhile, experimental efforts have been made to probe the high temperature and low to intermediate baryon density regime of the phase diagram via heavy-ion collisions. There also has been a recent effort to reach the high baryon density region of the phase diagram in experiments. This was done systematically in the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York and is planned at the upcoming Facility for Antiproton and Ion Research (FAIR) in Germany. These allow researchers to search for the postulated first order phase transition line on the QCD phase diagram ending in a second order critical point.

This group is currently studying the first-order transition line and Critical End Point (CEP) of the QCD phase diagram. A critical point terminating the first-order phase transition line is a ubiquitous phenomenon that occurs in physically very different systems such as ferromagnets and liquids. It has been shown in different cases that, despite having completely different microscopic interactions, the collective effects near such critical points are remarkably universal. Based on this, the researchers embedded a critical point and first-order phase transition line in a smooth background equation of state to yield the critical exponents and critical amplitude ratios expected of a transition in the same universality class as the liquid-gas transition and the 3D Ising model. Furthermore, a different method based on the parametric scaling equation of state has been proposed by the group to study the critical behavior in the vicinity of the CEP and obtain the first-order phase transition line. However, it was noted that the critical curves obtained in these studies do not have the anticipated behavior when approaching low chemical potential as the pseudocritical lines do not intersect the temperature axis. Several lattice calculations have demonstrated that there should be a smooth crossover around 1.8 trillion Kelvin, which means that the pseudocritical line would meet with the temperature axis around this value. The researchers are currently working on solving this problem of determining the critical line by imposing different thermodynamic constraints. It has been shown that with the proper choice of constraints, one can obtain the critical line and pseudocritical line that have similar shapes to the contemporary QCD phase diagram. Nevertheless, to get reasonable thermodynamic behavior would require a very delicate calculation of the critical line and pseudocritical line. This has to be done using MSI resources.

Numerous model calculations show that if the up and down quark masses are set to zero, one should expect a second-order phase transition at zero chemical potential, which is significantly different from the case of non-zero quark masses. Furthermore, a tricritical point where the second-order line meets with the first-order line would appear in the phase diagram. The researchers plan to use the aforementioned models to study the thermodynamic properties of QCD matter by setting the quark masses to zero and comparing them with other works. This would require high numerical accuracy because the calculations are very sensitive to error, thus necessitating the use of MSI clusters. They would also need to produce accurate tables of the equation of state to be used in heavy-ion collision simulations which is a numerically expensive calculation requiring HPC resources.

Although physicists are able to reproduce super-hot states in laboratories, it has been impossible so far to attain super-dense states on Earth, the states needed to study the phase diagram along the baryon density axis. Neutron stars are natural candidates for studying phenomena under extremely high density conditions. Therefore, studying neutron stars has been an important topic for nuclear physicists. Recent years have seen many important developments in the observation of neutron stars and neutron star mergers (via gravitational waves), which have the potential to revolutionize our understanding of nuclear physics at high baryon density and low temperature. This group has shown that with reasonable parameters it may be possible to support neutron stars up to 2.2 solar masses. Other future work is to use new models to investigate the neutron stars’ properties such as their mass-radius relation in order to compare their model with experimental data directly. This requires intensive numerical calculations.

The researchers recently showed that the anomalous correlations between charged and neutral kaons observed in heavy-ion collisions at the large hadron collider could not be explained by known effects and point to exotic coherent phenomenon. They further proposed a novel mechanism of Disoriented Isospin Condensates (DIC) to explain the said correlations. This new mechanism suggests that the observed anomaly may be hinting at the first experimental observation of chiral symmetry breaking and restoration at finite temperature in QCD. This requires further investigation which would require integrating DIC in hydrodynamic models of heavy-ion collisions. This is a computationally expensive exercise.