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

A successful unified description of p̄ nuclear interactions near E=0 is achieved using a p̄ optical potential within a folding model, V_opt∼v̄*ρ, where a p̄p potential v̄ is folded with the nuclear density ρ. The potential v̄ fits very well the measured p̄p-annihilation cross sections at low energies (p_L<200 MeV/c) and the 1s and 2p spin-averaged level shifts and widths for the p̄H 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 p̄-annihilation cross sections measured on Ne. Both v̄ 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 p̄-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 p̄-⁴He observables for E<0 as well as for E>0 are used to assess the reliability of extracting Coulomb modified p̄ nuclear scattering lengths directly from the data. The relationship between different kinds of scattering lengths is discussed and previously published systematics of the p̄ nuclear scattering lengths is updated.