Atomic nuclei occurring in nature tend to have the same number of neutrons and protons. For larger nuclear masses, this is attributed to the symmetry energy, a contribution to the nuclear energy associated with neutron-proton imbalance, that becomes larger the larger the imbalance. Nuclei with large imbalance can decay to more energetically favored products and, when that imbalance is excessive, the nuclei cannot even be formed. Understanding the symmetry energy is essential for extrapolating from the properties of existing nuclei to neutron stars kept stable by gravity while at the verge of collapse to black holes. In their efforts, the inverstigators will rely on their complementary expertise and will benefit from the expertise in nuclear structure of other members of the National Superconducting Cyclotron Laboratory (NSCL) Nuclear Theory Group and from the expertise in nuclear astrophysics of the members of the Joint Institute for Nuclear Astrophysics (JINA), housed partly at Michigan State University.
This project will advance our understanding of nuclear reactions, emphasizing aspects pertaining to the so-called nuclear symmetry energy. The symmetry energy governs average evolution of nuclear properties with neutron-proton imbalance that is now easier to vary experimentally given the modern existing and planned accelerators that can accelerate short-lived isotopes. The symmetry energy is important for extrapolating from the properties of the nuclei to those of neutron stars. The latter are macroscopic nuclear systems made stable, right at the verge of collapse into a black hole, by opposing forces of gravity and pressure tied to the dependence of symmetry energy on nuclear density. The symmetry energy at densities higher than normal for nuclei is studied using observables from central collisions of heavy nuclei, in particular charged pion ratios and collective flow. The investigators will improve on theoretical descriptions of those central collisions, by incorporating dynamic production of alpha particles into transport theory for the collisions. Because of large binding energy per nucleon and equal proton and neutron numbers, alpha particles are, on one hand, rather copiously produced and, on the other, they significantly change relative neutron-proton imbalance in their surroundings; hence their importance. The symmetry energy at normal and subnormal densities will be studied by simultaneously interpreting data from elastic scattering and charge exchange reactions, thus learning about differences in the neutron and proton distributions in those nuclei that have neutron-proton imbalance. Finally, the investigators will extract optical potentials of neutrons and protons from many-body theory relying on fundamental interactions, paying particular attention to the dependence of those potentials on neutron-proton imbalance. The fundamental extraction can help to reduce ambiguities in the interpretations of peripheral reaction experiments.
