Positronium scattering by helium

Coupled-state calculations are presented for Ps(1s) + He(1(1)S) scattering in the energy range 0-40 eV. Up to 22 Ps states are employed but only one He state is retained. Pseudostates are used to represent the Ps continuum. First Born estimates of cross sections in which the He is excited or ionized are found to be negligible in the chosen energy range. Calculation shows that ionization of the Ps is the main process at the higher energies. Comparison is made with available experimental measurements of the total cross section and with other theories. At low energies there is not only serious conflict between different experiments but also between the different theoretical approximations. Tn particular, the calculations of Biswas and Adhikari [Phys. Rev. A 59, 363 (1999)] and Peach (unpublished) predict a much smaller elastic cross section at low energies than that obtained in the present work. The relative merits of the different approximations are discussed. [S1050-2947(99)04212-2].