Positronium formation in low-energy positron-helium scattering

Detailed investigations were made of positron collisions with helium atoms in the Ore gap using a two-channel version of the Kohn variational method with very flexible trial wave functions and accurate correlated helium-target wave functions. Accurate values of the s-wave elastic scattering and positronium-formation cross sections are presented, together with preliminary values of the cross sections for p-wave scattering. The s-wave positronium-formation cross section in helium displays a remarkable similarity in both magnitude and energy dependence to the corresponding cross section in positron-hydrogen scattering, with a rapid rise from the positronium-formation threshold and a more gradual rise thereafter. Its small magnitude is consistent with the findings of a recent analysis of experimental measurements of the positronium-formation cross section. Calculations of the s-wave elastic-scattering cross section above and below the positronium-formation threshold reveal a Wigner ''rounded step'' at the threshold itself.