Physics of Antiproton Nuclear Interactions near Threshold

Antiproton-nucleus optical potentials fitted to $\bar p$-atom level shifts and widths are used to calculate the recently reported very low energy ($p_{L}<100$ MeV/c) $\bar p$ 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 $\bar p$-nucleus wavefunction substantially outside the nuclear surface, so that the resulting reaction cross section saturates as function of the strength of Im $V_{{\rm opt}}$. This feature, for $E >0$, parallels the recent prediction, for $E < 0$, that the level widths of $\bar p$ atoms saturate and, hence, that $\bar p$ deeply bound atomic states are relatively narrow. Predictions are made for $\bar p$ 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 $E$ are discussed and explained in terms of a Coulomb-modified strong-absorption model. Finally, optical potentials which fit simultaneously low-energy $\bar p - ^4$He observables for $E < 0$ as well as for $E > 0$ are used to assess the reliability of extracting Coulomb modified $\bar p$ nuclear scattering lengths directly from the data.