Daniel R. Bolton

Charge Symmetry Breaking in the n p ---> d pi0 reaction

The asymmetry in the angular distribution of np->dpi{sup 0} attributable to charge symmetry breaking is calculated using heavy baryon chiral perturbation theory. Recent developments in power counting have proven successful in describing total cross sections, and we apply them to the asymmetry calculation. Reducibility in one of the leading-order diagrams is examined. We compare the updated theory with experimental results for a range of physically reasonable parameters and find overprediction for the entire range.

Nonlinear Scattering of a Bose-Einstein Condensate on a Rectangular Barrier

We consider the nonlinear scattering and transmission of an atom laser or Bose-Einstein condensate (BEC) on a finite rectangular potential barrier. The nonlinearity inherent in this problem leads to several physical features beyond the well-known picture from single-particle quantum mechanics. We find numerical evidence for a denumerably infinite string of bifurcations in the transmission resonances as a function of nonlinearity and chemical potential, when the potential barrier is wide compared to the wavelength of oscillations in the condensate. Near the bifurcations, we observe extended regions of near-perfect resonance, in which the barrier is effectively invisible to the BEC. Unlike in the linear case, it is mainly the barrier width, not the height, that controls the transmission behavior. We show that the potential barrier can be used to create and localize a dark soliton or dark-soliton train from a phononlike standing wave.