Theoretical Nuclear & Particle Physics

The Universe we know today has evolved through an intriguing interplay of strong, weak, electromagnetic, and gravitational interactions. Marked by distinct epochs of evolution, quarks, the fundamental constituents of matter, have combined through strong interactions to become neutrons and protons which in turn have fused to form light nuclei, such as, the deuteron and the alpha particle among others. In aggregates of such nuclei formed due to the gravitational attraction of matter, nuclei up to Iron have been synthesized in stars, the timescales for their formation depending crucially on the various nuclear reaction rates which are determined by the strong, weak and electromagnetic interactions. Stars in a select range of masses have collapsed due to gravitational attraction and subsequently exploded as supernovae with neutrino and photon emissions surpassing those emitted from their host galaxies as a whole. In their aftermath, gravitational core collapse explosions have given birth to neutron stars in which the ultimate energy density of observable matter resides. In many instances, such explosions result in black holes from which light can only emerge as a result of quantum fluctuations. In short, and over a long period of time, it has all been very breathtaking.

Now, in addition to understanding how all of the above marvels happened, we are awaiting the direct detections of Einstein′s gravity waves from supernova environments and binary neutron star mergers in which elements more massive than Iron are thought to be synthesized. Spurred by theoretical predictions that nuclei with neutron to proton ratios well in excess of those found in terrestrial laboratories exist in astrophysical settings, laboratory experiments have succeeded in producing many neutron-rich and proton-rich nuclei, and hope to produce many more with upcoming facilities.

Theoretical research at Ohio University, conducted under the auspicious of the Institute of Nuclear and Particle Physics (INPP), contributes in important ways to provide answers to several "milestones" identified in the 2007 Nuclear Physics Long-Range Plan, namely (a) "What are the phases of strongly interacting matter, and what roles do they play in the cosmos?", (b) "What does Quantum Chromodynamics (QCD) predict for the properties of strongly interacting matter?", (c) "What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes?", (d) "What is the nature of neutron stars and dense nuclear matter?". In addition, the INPP enjoys a healthy collaboration with the Astrophysical Institute (API) so that significant leaps can be taken in unravelling the connections between microphysics and macrophysics.

Broadly speaking, theoretical investigations at the INPP explore manifestations of strong-interaction dynamics in terrestrial experiments and astrophysical phenomena. Some examples are: (a) calculations that address data on the chiral structure of the nucleon, electron scattering from light nuclei, and high-energy three-body scattering, (b) predictions for reaction rates involving nuclei near the neutron- and proton-drip lines, (c) delineation of the roles of neutron star structure and composition on emissions of multi-wavelength photons, neutrinos and gravitational waves, and (d) calculations of the transport properties of the hadronic matter produced in heavy-ion collisions.

Charlotte Elster conducts research on several topics involving the strong interaction between nucleons and nuclei. Her current work is concerned with reactions of systems comprising of few nucleons and employs ab-initio methods for a quantitative description. Because of the short range of the nuclear interaction, analyses of collision experiments carried out at energies high enough to probe the short-distance characteristics require special techniques. Utilizing a novel computational approach pioneered with her collaborators, Elster has highlighted the role of relativistic effects in high energy scattering reactions involving three-nucleon systems.

Elster is a part of the Department of Energy funded Topical Collaboration TORUS (Theory of Reactions for Unstable Isotopes) for the development of new methods to advance nuclear reaction theory for unstable isotopes. Use of new three-body techniques to improve direct-reaction calculations, and the development of a new partial-fusion theory to integrate descriptions of direct and compound-nucleus reactions are major goals of this collaboration. Results of this research are directly relevant to investigate properties of nuclei far from stability. In addition to developing a comprehensive nuclear reaction theory based on first principles, this project aims to provide essential inputs for studies of nuclear reactions in astrophysical settings.

Daniel Phillips leads the research on the dynamics of few-body strongly interacting systems. The goal here is to understand how properties of these systems emerge from the underlying interactions. A key tool is effective field theory (EFT), in which the interaction between the relevant degrees of freedom is written in a very general way. Phillips is especially interested in EFT for electromagnetic properties of few-body systems, and has applied the technique to understand electron scattering from deuterium, and Compton scattering from the proton, deuteron, and Helium-3. He is now working to use the same methods to understand the E1 strength in "halo" nuclei (e.g. Be-11, C-19, He-6, Li-11) in which some neutrons and protons live at large distances from a "core".

In all cases, Phillips′s major goal is to learn about the underlying interactions between the constituents by studying the electromagnetic response of the system. In the case of light nuclei, this means that electron and photon scattering data can educate us about the nucleon-nucleon interaction. For halo nuclei, which are not well-described by the standard nuclear shell model because they are at the limits of nuclear stability, EFT provides a controlled way to interpret experiments, such as Coulomb-dissociation measurements, thereby helping to reveal the novel dynamics occurring in such systems.

Gabriela Popa′s research elucidates how nuclear interactions cause nuclei to become deformed from spherical shapes. Shapes of nuclei (most nuclei are deformed), their bulk properties, and excitation spectra have been studied extensively using different models, but the fundamental cause for the variety of properties observed still remains elusive, particularly in heavy nuclei. Popa′s research aims to shed light on this longstanding mystery through studies of symmetries in nuclear structure using algebraic models. From studies of electromagnetic properties of deformed nuclei, Popa′s research offers much insight into the properties of strong interactions in heavy deformed nuclei.

The major areas of Prakash′s research are: (1) the interface between nuclear theory and nuclear, neutrino and gravitational astrophysics, and (2) the extreme energy density physics encountered in the collisions of highly energetic nuclei as at the relativistic heavy-ion colliders. Questions addressed in connection with astrophysical phenomena include: (i) How do new theoretical calculations of the equation of state and neutrino processes in dense matter impact numerical simulations of supernovae, proto-neutron stars and cooling neutron stars? (ii) How do astronomical observations of supernovae, neutron stars (including pulsars) and black holes delineate dense matter properties, such as its symmetry energy, specific heat and compressibility? Do exotic phases that contain hyperons, Bose condensates or deconfined quark matter exist in observable dense matter? Do they have distinct signatures? (iii) How do nuclear experiments, such as those involving rare isotope accelerators, heavy ion collisions, and parity-violating electron scattering reactions, restrict the parameters of nuclear equation of state models? (iv) How do gravitational wave detections elucidate the properties of dense baryonic matter? In the area of relativistic heavy-ion collisions (RHIC and LHC), Prakash addresses issues related with (i) How do fundamental interactions between quarks and hadrons determine their thermal and transport properties? How are these properties manisfested in observables of heavy-ion collisions?

Prakash works in concert with the Joint Institute for Nuclear Astrophysics -- Center for the Evolution of the Elements, or JINA-CEE, of which the INPP is an affiliate member. His role in this collaboration is to provide microphysical inputs (to large-scale computer simulations of astrophysical phenomena) on the equation of state of and neutrino reaction rates in the dense matter encountered in core collapse supernovae, young and old neutron stars, and, binary mergers involving neutron stars and black holes.

Associated Faculty: Elster, Phillips, Popa, Prakash,

Emeritus Faculty: Onley, Wright