Theory Talk is an informal setting for fellow NC State physicists to see and hear work currently being pursued within the department. Speakers are encouraged to talk about work in progress and exchange ideas with the audience. Faculty, postdocs, and especially graduate students are encouraged to attend and present. Interested visitors and researchers from the Triangle area are also welcome.

SPRING 2008 SCHEDULE

Suzanne Parete-Koon (University of Tennessee and ORNL)
The QSE-reduced Nuclear Reaction Network for Silicon Burning
Wed., Jan. 23, 4:00-5:00, Riddick 400P Conference Room

The nuclei of the "iron peak" are formed in massive stars shortly before core collapse and during their supernova outbursts as well as during thermonuclear supernovae. Complete and incomplete silicon burning during these events are responsible for the production of a wide range of nuclei with atomic mass numbers from 28 to 64 and sets the stage for the r-process. Because of the large number of nuclei involved, accurate modeling of silicon burning is computationally expensive. However, examination of the physics of silicon burning has revealed that the nuclear evolution is dominated by large groups of nuclei in mutual equilibrium. I will present a hybrid equilibrium-network scheme which takes advantage of this quasi-equilibrium in order to reduce the number of independent variables calculated. With 3 QSE groups of fixed membership, we have reduced the computational cost by 5-10x relative to the full network without significant loss of accuracy. Because the size and membership of these groups vary as the temperature, density and electron faction change, achieving maximal efficiency requires dynamic adjustment of group number and membership. Toward this end, I have implemented a scheme beginning with 2 QSE groups at appropriately high temperature, then progressing through 3 group stages (with successively more independent abundances) as temperature declines during a simple adiabatic expansion that has been used to approximate the thermodynamics of an exploding star. This combination allows accurate prediction of the nuclear abundance evolution, deleptonization and energy generation at a further reduced computational cost (20-30x) when compared to a conventional nuclear reaction network without significant loss of accuracy. These reductions in computational cost and the number of species evolved make QSE-reduced networks well suited for inclusion within hydrodynamic simulations.

Thomas Schaefer
The holographic life of the eta prime meson
Thurs., Jan. 31, 4:00-5:00, Riddick 400P Conference Room

Dean Lee
Spherical cubes: Quantum scattering on a lattice
Thurs., Feb. 21, 4:00-5:00, Riddick 400P Conference Room
For a few idealized cases it is possible to construct quantum many-body systems from scratch. You start with the quantum interactions of pairs of particles and compute properties of the many-body system using efficient lattice methods and stochastic algorithms developed for this task. But suppose you wish to study more complicated naturally-occurring systems such as a gas of neutrons at densities expected in the crust of a neutron star. These interactions are a good deal more complicated and include effects at nonzero orbital angular momentum and spin-orbit coupling. Can any of this be treated on the lattice? This talk attempts to answer that question.

Laura Mersini-Houghton (UNC Chapel Hill)
Why did the universe start in such an extremely improbable state?
Wed., Apr. 2, 4:00-5:00, Riddick 400P Conference Room
I will discuss a recent theory for the initial conditions based on dynamical evolution of gravitational and matter degrees of freedom, the need to address these fundamental problems within the framework of a multiverse and possible astrophysical imprints of the nonlocal entanglement left over from the earliest times. I will briefly focus on the discovery of the void last year which tested one of the predictions of this theory.

Sheila Kannappan (UNC Chapel Hill)
Building Disk Galaxies in a Violent Universe
Thurs., Apr. 10, 3:30-4:30, Riddick 400P Conference Room
Hierarchical models suggest that galaxies form through a series of mergers that create spheroids of stars, while disks of gas and stars grow around these spheroids during the quiet times between galaxy encounters. This picture has been questioned in light of the fact that many spiral galaxies have disky "pseudobulges" rather than dynamically hot, spheroidal classical bulges. I will present "smoking gun" evidence that disk regrowth occurs, and I will show results suggesting that outer disk growth may occur in tandem with pseudobulge growth via frequent episodes of gas inflow and central star formation enhancement. This process may subsume a merger-formed spheroid within a larger disk+pseudobulge system.

Rebecca Surman (Union College)
Neutrinos from Black Hole Accretion Disks
Wed., May 7, 3:00-4:00 pm, Riddick 400P Conference Room
A potential astrophysical source of MeV neutrinos is a rapidly accreting disk around a stellar-mass black hole (AD-BH). In one possible formation mechanism, a subset of core-collapse supernovae will form an AD-BH rather than the canonical protoneutron star. We examine the neutrino fluxes emitted in this alternate scenario and investigate the consequences for galactic supernova neutrino detection. A second possible progenitor for an AD-BH is the collision of compact binaries. We examine the neutrino physics of this scenario starting with the results of a three dimensional numerical model of a black hole-neutron star merger. We calculate the neutrino and antineutrino fluxes emitted from the resulting AD-BH and discuss their impact on the system, focusing on their influence on the outflow nucleosynthesis.