Ps-atom scattering at low energies

TL;DR

This paper develops a pseudopotential model for low-energy positronium-atom scattering, extending previous cross section calculations below ionization threshold, and finds weak van der Waals effects and positive scattering lengths for Ar and Kr.

Contribution

It introduces a pseudopotential approach based on phase shifts to analyze low-energy Ps-atom scattering, extending prior models to energies below ionization threshold.

Findings

Cross sections decrease with velocity below 1 a.u.

Weak van der Waals interaction compared to polarization effects.

Positive scattering lengths for Ar and Kr, no Ramsauer-Townsend minimum.

Abstract

A pseudopotential for positronium-atom interaction, based on electron-atom and positron-atom phase shifts, is constructed, and the phase shifts for Ps-Kr and Ps-Ar scattering are calculated. This approach allows us to extend the Ps-atom cross sections, obtained previously in the impulse approximation [Phys. Rev. Lett. 112, 243201 (2014)], to energies below the Ps ionization threshold. Although experimental data are not available in this low-energy region, our results describe well the tendency of the measured cross sections to drop with decreasing velocity at $v<1$ a.u. Our results show that the effect of the Ps-atom van der Waals interaction is weak compared to the polarization interaction in electron-atom and positron-atom scattering. As a result, the Ps scattering length for both Ar and Kr is positive, and the Ramsauer-Townsend minimum is not observed for Ps scattering from these targets. This makes Ps scattering quite different from electron scattering in the low-energy region, in contrast to the intermediate energy range from the Ps ionization threshold up to $v\sim 2$ a.u., where the two are similar.