Remarks on the �pp interaction very close to the threshold

Abstract

We examine possible sources for the disagreement between recent results on theρ parameter and optical potential predictions at very low energies. We find that a proper incorporation of the antineutron-neutron threshold has an extremely small effect, due to annihilation. An application to the protonium atom is discussed. The accuracy of the Bethe formula which serves to estimate the parameter from the forward differential cross-section is considered. It turns out to overestimateρ, but not enough to account for the discrepancy. We make also direct comparisons with the differential cross-sections which show in one case, a neat deviation in the interference region. In other cases the disagreement is not compelling.