Elastic scattering and positronium formation in low-energy positron-helium collisions

Abstract

Ab initio variational calculations are made of the elastic scattering and positronium formation cross sections for positron-helium scattering below the first excitation threshold of the target. Three variants of the Kohn variational method are used with very flexible trial wavefunctions and a very accurate helium wavefunction. The elastic scattering and positronium formation cross sections are believed to have converged to within 5% and 10%, respectively. The calculated positronium formation cross section displays a similar energy dependence to that of recent experimental results but with a 25% difference in magnitude. However, the theoretical total cross section, both above and below the positronium formation threshold, agrees with the experimental measurements to within 10%. Threshold effects in the s-wave cross sections manifest themselves as a significant feature in the total cross section. Similarities with the corresponding results for positron-hydrogen scattering are discussed.