Topological Transitions      

These codes are Langevin solvers designed to study topological transitions mediated by unstable field configurations such as sphalerons. Such processes may have been responsible for the baryon asymmetry of the universe. Subtleties in these codes relate to the imposition of both global and gauge constraints and handling the cutoff dependence of the results.

This code is a multiplicative noise Langevin solver for the nonlinear sigma model in 1+1 dimensions. The multiplicative nature of the noise and damping is a new idea and stems from the imposition of a global constraint in the theory. The code exists in HPF form at present and is being rewritten in F90/MPI to enhance performance. Typical runs are with 128K lattice sites which needs to be increased to a million sites.

The algorithm is a stochastic Euler method. We are in the process of developing a second order stochastic algorithm which is nontrivial because of the multiplicative nature of the noise.

The primary goal for code development during FY 2001 is to implement the second order multiplicative noise algorithm for this problem. This algorithm has been tested and found to be work very effectively for simulating multiplicative noise Brownian motion. No large production runs at NERSC are planned for FY01.

Salman Habib / LANL / habib@lanl.gov / revised August 00