Positron and electron scattering on atoms and molecules-modified effective range theory revisited

New experiments on the very low-energy electron and positron scattering allow us to verify the old question on applicability of modified effective range theory (MERT). We perform it using an analytical solution of the Schroedinger equation with the long-range polarization potential. In this work two atomic (He, Ar) and molecular (H2, CH4) targets are studied using this approach. Total cross sections were used for obtaining parameters characterizing the scattering phase shifts related to the short-range interaction potential; differential cross-sections were used for comparison. Differently from previous works, we conclude that MERT with few (2-3) partial waves applies very well up to energies of few eV in all four targets studied. For positrons, reliable experimental data indicate occurrence of zeros in the s-wave phase shifts for all four targets. This should be recognized as Ramsauer-Townsend minima. (authors)