0 Unified optical - model approach to low - energy antiproton annihilation on nuclei and to antiprotonic atoms

A successful unified description of ¯ p nuclear interactions near E = 0 is achieved using a ¯ p optical potential within a folding model, V opt ∼ ¯ v * ρ, where a ¯ pp potential ¯ v is folded with the nuclear density ρ. The potential ¯ v fits very well the measured ¯ pp annihilation cross sections at low energies (p L < 200 MeV/c) and the 1s and 2p spin-averaged level shifts and widths for the ¯ pH atom. The density-folded optical potential V opt reproduces satisfactorily the strong-interaction level shifts and widths over the entire periodic table, for A > 10, as well as the few low energy ¯ p annihilation cross sections measured on Ne. Both ¯ v and V opt are found to be highly absorptive, which leads to a saturation of reaction cross sections in hydrogen and on nuclei. Predictions are made for ¯ p annihilation 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 is discussed and explained in terms of a Coulomb-modified strong-absorption model. Finally, optical potentials which fit simultaneously low-energy ¯ p− 4 He observables for E < 0 as well as for E > 0 are used to assess the reliability of extracting Coulomb modified ¯ p nuclear scattering lengths directly from the data. The relationship between different kinds of scattering lengths is discussed and previously published sys-tematics of the ¯ p nuclear scattering lengths is updated.