Michael W. Guidry

Fermion dynamical symmetries and the nuclear shell model

Abstract A fermion dynamical symmetry model is presented which can account for a multitude of nuclear collective phenomena.

A Comparison of Boltzmann and Multigroup Flux-limited Diffusion Neutrino Transport during the Postbounce Shock Reheating Phase in Core-Collapse Supernovae

We compare Newtonian three-flavor multigroup Boltzmann (MGBT) and (Bruenns) multigroup flux-limited diffusion (MGFLD) neutrino transport in postbounce core-collapse supernova environments. We focus our study on quantities central to the postbounce neutrino heating mechanism for reviving the stalled shock. Stationary-state three-flavor neutrino distributions are developed in thermally and hydrodynamically frozen time slices obtained from core collapse and bounce simulations that implement Lagrangian hydrodynamics and MGFLD neutrino transport. We obtain distributions for time slices at 106 and 233 ms after core bounce for the core of a 15 M☉ progenitor, and at 156 ms after core bounce for a 25 M☉ progenitor. For both transport methods, the electron neutrino and antineutrino luminosities, rms energies, and mean inverse flux factors, all of which enter the neutrino heating rates, are computed as functions of radius and compared. The net neutrino heating rates are also computed as functions of radius and compared. Notably, we find significant differences in neutrino luminosities and mean inverse flux factors between the two transport methods for both precollapse models and for all three time slices. In each case, the luminosities for each transport method begin to diverge above the neutrinospheres, where the MGBT luminosities become larger than their MGFLD counterparts, finally settling to a constant difference maintained to the edge of the core. We find that the mean inverse flux factors, which describe the degree of forward peaking in the neutrino radiation field, also differ significantly between the two transport methods, with MGBT providing more isotropic radiation fields in the gain region. Most important, for a region above the gain radius we find net heating rates for MGBT that are as much as ~2 times the corresponding MGFLD rates, and we find net cooling rates below the gain radius that are typically ~0.8 times the MGFLD rates. These differences stem from differences in the neutrino luminosities and mean inverse flux factors, which can be as much as 11% and 24%, respectively. They are greatest at earlier postbounce times for a given progenitor mass and, for a given postbounce time, greater for greater progenitor mass. We discuss the ramifications that these new results have for the supernova mechanism.

A fermion dynamical symmetry model for high-spin physics

Abstract A fermion dynamical symmetry model is shown to yield the particle-rotor model and the basic features of high-spin physics.

Constraints imposed by symmetry on pairing operators for the iron pnictides

Considering model Hamiltonians that respect the symmetry properties of the pnictides, it is argued that pairing interactions that couple electrons in different orbitals with an orbital-dependent pairing strength inevitably lead to interband pairing matrix elements, at least in some regions of the Brillouin zone. Such interband pairing has not been considered of relevance in multiorbital systems in previous investigations. It is also observed that if, instead, a purely intraband pairing interaction is postulated, this requires that the pairing operator has the form k= fkdk,, d k,, , where labels the orbitals considered in the model and fk arises from the spatial location of the coupled electrons or holes. This means that the gaps at two different Fermi surfaces involving momenta kF and kF can only differ by the ratio fkF / fkF and that electrons in different orbitals must be subject to the same pairing attraction, thus, requiring fine tuning. These results suggest that previously neglected interband pairing tendencies could actually be of relevance in a microscopic description of the pairing mechanism in the pnictides.

Performance analysis and acceleration of explicit integration for large kinetic networks using batched GPU computations

We demonstrate the systematic implementation of recently-developed fast explicit kinetic integration algorithms that solve efficiently N coupled ordinary differential equations (subject to initial conditions) on modern GPUs. We take representative test cases (Type Ia supernova explosions) and demonstrate two or more orders of magnitude increase in efficiency for solving such systems (of realistic thermonuclear networks coupled to fluid dynamics). This implies that important coupled, multiphysics problems in various scientific and technical disciplines that were intractable, or could be simulated only with highly schematic kinetic networks, are now computationally feasible. As examples of such applications we present the computational techniques developed for our ongoing deployment of these new methods on modern GPU accelerators. We show that similarly to many other scientific applications, ranging from national security to medical advances, the computation can be split into many independent computational tasks, each of relatively small-size. As the size of each individual task does not provide sufficient parallelism for the underlying hardware, especially for accelerators, these tasks must be computed concurrently as a single routine, that we call batched routine, in order to saturate the hardware with enough work.

Computational Infrastructure for Nuclear Astrophysics

A Computational Infrastructure for Nuclear Astrophysics has been developed to streamline the inclusion of the latest nuclear physics data in astrophysics simulations. The infrastructure consists of a platform‐independent suite of computer codes that is freely available online at nucastrodata.org. Features of, and future plans for, this software suite are given.