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

Abstract

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 p ̄ p potential v ̄ is folded with the nuclear density ρ. The potential v ̄ fits very well the measured p ̄ p -annihilation cross sections at low energies ( p L<200 MeV /c) and the 1s and 2p spin-averaged level shifts and widths for the p ̄ H 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 systematics of the p ̄ nuclear scattering lengths is updated.