In the nuclear theory program, the main focus is in the area of strongly interacting matter, including transport models for heavy-ion collisions at low and high energies (Ko), hadrons in medium (Rapp), perturbative QCD (Fries) and RHIC phenomenology (Ko, Rapp, Fries).

Prof. Rainer Fries is interested in collisions of hadrons and nuclei at high energies in order to investigate the properties of the underlying Strong Interaction (QCD). One direction of his research focuses on the dynamics of relativistic heavy ion collisions, beginning from the Color Glass Condensate in nuclei, to the formation of a Quark-Gluon Plasma (QGP) phase in the nuclear collisions. Quark-Gluon Plasma is a primordial form of matter which existed a few microseconds after the big bang at temperatures larger than 1,000,000,000,000 Kelvin. He is also conducting research to effectively use QCD jets and weakly interacting particles (photons, leptons) as tomographic probes for both cold nuclei and Quark-Gluon Plasma. A related research goal is a consistent treatment of multiple scattering in perturbative QCD computations. His recent research accomplishments include a phenomenologically very successful recombination model describing how quarks in a cooling Quark-Gluon Plasma form hadrons, and fundamental work on properties of the proton wave function. Prof. Fries's theoretical work is connected to the large experimental programs at the Relativistic Heavy Ion Collider (RHIC), the upcoming Large Hadron Collider (LHC) at CERN and a future Electron Ion Collider (EIC) in the US.

Prof. Che-Ming Ko's recent research includes theoretical work in both isospin physics and RHIC physics. Using an isospin-dependent transport model that includes different proton and neutron mean-field potentials and scattering cross sections, he and his collaborators have found many interesting phenomena in collisions of radioactive nuclei that are sensitive to the properties of nuclear symmetry energy. In particular, they have extracted from the experimental isospin diffusion data valuable information on the density dependence of the nuclear symmetry energy at subnormal densities, which has further allowed them to put stringent constraints on the parameters in nuclear effective interactions. For relativistic heavy ion collisions, they have developed a multi-phase transport model that includes interactions in both initial partonic and final hadronic matters as well as the transition between these two phases of matters. Using this model, they have obtained useful information on the properties of the quark-gluon plasma produced at RHIC from studying the abundance, collective dynamics, and correlations of produced hadrons of both light and heavy flavors. They are currently extending these studies to make predictions for heavy ion collisions at future Facilities for Rare Isotope Beams and Large Hadron Collider.

Prof. Ralf Rapp and his group conduct systematic theoretical studies of spectral properties of hadrons in hadronic matter and in the Quark-Gluon Plasma (QGP), their relations to QCD phase transitions and observable signatures in heavy-ion collisions. Most of the visible mass in the universe is believed to be generated by the spontaneous breaking of chiral symmetry in QCD, which, however, will be restored in the QGP. Employing hadronic many-body theory, Rapp and his co-workers are evaluating spectral functions of the rho meson (and its chiral partner, the a1 in hot and dense matter. The calculations predict a strong broadening (even "melting") of the rho resonance in hot and dense matter. Recent measurements of dilepton invariant-mass spectra in heavy-ion collisions at the CERN-SPS have confirmed these calculations. Employing heavy-quark potentials from finite-temperature lattice QCD computations, Rapp's group is evaluating the properties of heavy-quark bound states in the QGP. The resulting quarkonium spectral functions are checked against Euclidean correlation functions computed in lattice-QCD and applied to charmonium and bottomonium observables at RHIC and LHC. Lattice-QCD potentials are furthermore implemented to compute heavy-quark transport properties of the QGP. Large friction and small diffusion coefficients support the notion of a strongly coupled QGP as suggested by experimental results from RHIC. Rapp and co-workers are also interested in color-superconducting cold dense quark matter and associated signatures in observables from compact ("neutron") stars, including mechanisms for gamma ray bursters.