Physics of antiproton nuclear interactions near threshold

Antiproton-nucleus optical potentials fitted to p-atom level shifts and widths are used to calculate the recently reported very low energy (P_L < 100 MeV/c). p cross sections for annihilation on light nuclei. The apparent suppression of annihilation upon increasing the atomic charge Z and mass number A is resolved as due to the strong effective repulsion produced by the very absorptive optical potential which keeps the p-nucleus wavefunction substantially outside the nuclear surface, so that the resulting reaction cross section saturates as function of the strength of Im V_opt. This feature, for E > O, parallels the recent prediction, for E < O, that the level widths of p-atoms saturate and, hence, that p deeply bound atomic states are relatively narrow. Predictions are made for p annhilation cross sections over the entire periodic table at these very low energies and the systematics of the calculated cross sections as function of A, Z and E are discussed and explained in terms of a Coulom-modified strong-absorption model. Finally, optical potentials which fit simultaneously low-energy p-⁴ He observables for E < O as well as for E > O are used to assess the reliability of extracting Coulomb modified p nuclear scattering lenghts directly from the data.