One of the main objectives in contemporary nuclear research is the discovery and quantitative characterization of new phases of strongly interacting matter at very high temperatures and densities, most notably the Quark-Gluon Plasma which is believed to have ruled the Universe in the first few microseconds of its evolution. A fascinating aspect of this field is the possibility to recreate this kind of matter, for a short moment, in the laboratory, by colliding heavy atomic nuclei at high energies. This project aims at advancing the theoretical understanding of the Quark-Gluon Plasma, and to develop phenomenological signatures of its creation and properties in nuclear collision experiments, as e.g. conducted with the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory. Electromagnetic radiation and particles containing heavy quarks are particularly suitable to extract information about the hot and dense matter formed in heavy-ion collisions. It is planned to relate this information to fundamental properties of the strong interactions (Quantum Chromodynamics, or QCD), to illuminate the generation of the visible mass in the Universe and the confinement of quarks and gluons into nucleons, the building blocks of atomic nuclei.
Basic research by graduate-student and postdoctoral researchers is integrated with educational activities aimed at undergraduate and regional high-school students, including previously established summer research and Saturday Morning Physics programs at Texas A&M University.
It is believed that during the first ~10 microseconds of its existence, the Universe consisted of an extremely hot plasma of quarks and gluons, which subsequently condensed into bound states ("hadrons") making up today's atomic nuclei. It is truly fascinating that this primordial form of matter can be recreated, for short moment, in the laboratory by accelerating and colliding atomic nuclei at high energies, as done at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Lab and the Large Hadron Collider (LHC) at CERN. To investigate the properties of this transient medium, suitable probes are required. In this project, the PI has developed two types of promising probes to extract key properties of the quark-gluon plasma and its transition into hadrons. First, he computed the diffusion properties of heavy quarks in the hot medium to charactarize its transport properties. Employing both quark- and hadron-based approaches, he found the diffusion coefficient to exhibit maximal strength at temperatures where the transition from quarks to hadrons is expected to occur. Applying these results to heavy-ion collisons, good agreement with experimental data at RHIC was found; the diffusion coefficient thus indicates a very strongly coupled medium closely related to the quark-hadron phase transition - the most perfect liquid known to date. He also developed the framework to compute the properties of heavy-quark bound states. Based on his calculations of their dissociation and regeneration in heavy-ion collisions at RHIC, new data at the LHC could be correctly predicted. These findings further corroborate the strongly coupled nature of the medium. Second, he computed the properties of electromagnetic radiation from the hot medium. This radiation traverses the fireball in heavy-ion collisions undistorted and thus enables a direct glimpse at its interior. In particular, electromagnetic decays of the rho meson inside the fireball provide clues about microscopic properties of the medium. The PI's calculations of medium effects on the rho meson have now established a consistent description of electromagnetic spectra in heavy-ion collisions, including new data from RHIC spanning a large range in collision energies. The underlying strong broadening of the rho-meson's spectral shape indicates a melting of hadron into quark degrees of freedom at fireball temperatures around the expected transition regime. These findings also suggest that the masses of hadrons (which are responsible for the masses of all objects around us) dissolve in a "hadron-melting" process as predicted by the PI. The present project has fostered in-depth training in solving complex problems in nuclear-theory research as described above. Two graduate students have completed their degrees, one (PhD) moving on to a postdoctoral (academic) position and the other one (MS) to a high-skill technology/industrial position. Two postdotoral trainees moved on to software and academic jobs, while one postdoc and one graduate student continue in the PI's research group. The PI has regularly supervised a summer student within the Research Experience for Undergraduate (REU) program at the TAMU Cyclotron Institute. He has thus established a thriving scholarly environment of under-/graduate and postodoctoral researchers, which keeps attracting capable young students into his group. The PI has further developed his outreach initiatives through the Saturday Morning Physics program which he introduced at Texas A&M in 2006. The recent 7-event series each spring brought faculty lectures and demo experiments to an audience of well above 100 high-school students each event (well above 200 in total), raising awareness and excitement of fundamental research with the future generation in Southeast-central Texas.
