Professor Joseph Kapusta

CSENG Physics & Astronomy
College of Science & Engineering
Twin Cities
Project Title: 
Calculating Equations of State and Hydrodynamical Evolution of Hot Nuclear Matter in High Energy Nuclear Collisions

This group focuses on quantum chromodynamics (QCD is the fundamental theory behind nuclear physics) at extremely high temperatures and densities. These conditions are reached in ultra-relativistic heavy-ion collisions in experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab and at the Large Hadron Collider (LHC) at CERN. By colliding heavy ions, researchers are recreating and studying the kind of hot and dense matter formed in the first microseconds after the Big Bang. This group is leading a robust research program which is improving the state of the art in modeling and understanding the early, middle, and late stages of heavy-ion collisions.

When heavy ions collide at near the speed of light in accelerators, a variety of new states of matter are formed that are fundamentally different from that found in the incoming ions, which are made of protons and neutrons. In the first moments of the collision, strong color gluon fields develop between quarks, generating a state of matter called a color glass condensate. These researchers have developed new techniques to calculate the compression and excitation energy of the colliding nuclei immediately after the collision. These provide the initial conditions for the subsequent hydrodynamic expansion of the hot and dense matter. The initial density can be more than twenty times that of normal atomic nuclei and replicate the conditions in the interiors of neutron stars, supernovae explosions, and neutron star mergers as measured by detection of gravitational waves.

After a short period of time, the produced matter evolves into another novel state of matter known as the quark-gluon plasma. Tremendous pressures in the plasma cause it to expand extremely rapidly. As it does so, it cools into yet another phase of matter called a hadronic gas. Past work of the group focused on constructing a crossover equation of state which interpolates between the quark-gluon plasma state at high temperatures and densities and the hadronic gas phase at low temperatures and densities. This equation of state was applied to the observations of skewness and kurtosis in the distribution of net proton yields at RHIC, suggesting new parameters for describing the evolution of this gas.

Currently, the crossover equation of state is being extended to include the possible existence of a critical point in the QCD phase diagram, where the line of phase separation is expected to switch from a rapid crossover to a first-order phase transition. Such a critical point has been conjectured on a variety of theoretical bases, but has yet to be experimentally confirmed, making a generalized equation of state which includes a possible critical point an essential component of the search for such a feature. This equation of state will be used in relativistic hydrodynamic simulations of heavy ion collisions and compared to experimental measurements to be taken at RHIC in the next few years. The location of a purported critical point in the phase diagram and the latent heat are controlled by parameters which can be adjusted to obtain a best fit to the data.

The researchers have developed a new theoretical framework for computing the viscosities and thermal conductivity of the fluid. They will be using this framework to numerically compute these transport coefficients which involves solving multi-dimensional integral equations. In addition, they showed how these transport coefficients can not only be used in the relativistic fluid equations but also to compute the departure from equilibrium of the single-particle distribution functions as the system transitions from a continuous fluid to the individual particles observed in the experiments.

A related line of research investigates the role of thermal fluctuations in heavy-ion collisions. This group, in collaboration with researchers from the University of Illinois and Duke University, developed the theory of hydrodynamic fluctuations for use in relativistic heavy-ion collisions. The group made the necessary steps to calculate the effect of this noise in hydrodynamic simulations of heavy-ion collisions. They continued to explore the effects of these hydrodynamic fluctuations on femtoscopic radii and on charge balance functions in heavy-ion collisions. Subsequent collaborations with Michigan State University studied charge correlations via particle diffusion on top of a background of hydrodynamic evolution of heavy-ion collisions and compared the results to experimental data. The researchers have continued to explore the effects of fluctuations as well as other mechanisms on the shape of the correlation function used in femtoscopic studies. They have demonstrated that even a correlation with a fundamentally Gaussian shape can be distorted in various ways due to the effects of fluctuations and related mechanisms.

Finally, inflation is a phenomenological model of the early universe that was developed for explaining issues like the flatness and causality problems. The theory tells us that the early universe experienced an accelerated phase of expansion. The researchers have studied some inflation models as sources of primordial black hole formation. A sufficient abundance of primordial black holes helps address various cosmological problems like dark matter and the presence of supermassive black holes. Stochastic methods that are used to study fluctuations in heavy ion collisions can also be used to characterize quantum back-reactions on inflation potentials. It has been proposed that quantum back-reactions might induce an extra abundance of primordial black holes. 

Project Investigators

Aritra De
Professor Joseph Kapusta
Rafid Mahbub
Christopher Plumberg
Thomas Welle
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