J. Carlson, Los Alamos National Laboratory
R. B. Wiringa and S. C. Pieper, Argonne National Laboratory
R. Schiavilla and J. Forest, Thomas Jefferson National Accelerator Facility
V. R. Pandharipande, University of Illinois
K. E. Schmidt, Arizona State University
S. Fantoni, Scuola Internazionale Superiore di Studi Avanzati (SISSA), Trieste, Italy

Research Objectives

The goal of this project is to calculate the properties of nuclei and neutron matter with realistic interactions. This project studies the spectra of nuclei and equation of state of neutron matter, and also studies a variety of weak-interaction processes of importance in astrophysics and elsewhere.

Computational Approach

The computational approach is based on a variety of quantum Monte Carlo algorithms. In light nuclei, variational and Green's function Monte Carlo are used to study structure and low-energy reactions. Constrained path-integral algorithms allow us to study many-fermion systems quite accurately. Methods which combine auxiliary-field and diffusion Monte Carlo should allow us to perform accurate simulations for much larger numbers of particles, including studies of finite nuclei and neutron matter.

Accomplishments

We have constructed a new class of three-nucleon interactions which yield significantly improved results for the structure of light nuclei. In particular, these models reproduce the binding of neutron-rich nuclei and the LS splittings much better than previous-generation interactions. We expect these new models to play a significant role in neutron-rich nuclei generally, as well as in the structure and response of neutron-star matter.

We have also developed new codes which combine diffusion and auxiliary field methods for treating many-body nuclear physics problems. Initial tests in neutron drops and neutron matter have been extremely encouraging. The present calculations are more accurate (as tested against exact calculations of smaller systems) than previous efforts at variational calculations of neutron matter. We are continuing to develop these algorithms to be able to study possible phase transitions in neutron matter as well as neutrino scattering, an important ingredient in supernovae.

Finally, we have developed new algorithms for treating low-energy scattering within the Monte Carlo framework. These schemes work very well in determining parameters such as scattering lengths, effective ranges, and positions and widths of resonance. We have used them to explore the parity-violating spin rotation of neutrons passing through helium. This experiment is designed to help determine the weak parity-violating -NN coupling constant, a subject of much current theoretical and experimental interest.

A Quantum Monte Carlo simulation of neutron star matter; individual neutrons are shown with arrows representing each neutron's spin. Different colors represent periodic images.

Significance

Publications

J. Carlson, J. E. Gubernatis, and G. Ortiz, "Issues and observations on applications of the constrained-path Monte Carlo method to many-fermion systems," Phys. Rev. B 59, 12788 (1999).

J. Carlson and R. Schiavilla, "Structure and dynamics of few-nucleon systems," Rev. Mod. Phys. 70, 743 (1998).

R. Schiavilla et al., "Weak capture of protons by protons," Phys. Rev. C 58, 1263 (1998).