Saturation of low-energy antiproton annihilation on nuclei

Recent measurements of very low-energy (pL<100 MeV/c) p̄ annihilation on light nuclei reveal apparent suppression of annihilation upon increasing the atomic charge Z and mass number A. Using p̄-nucleus optical potentials Vopt, fitted to p̄-atom energy-shifts and -widths, we resolve this suppression as due to the strong effective repulsion produced by the very absorptive Vopt. The low-energy p̄-nucleus wavefunction is kept substantially outside the nuclear surface and the resulting reaction cross section saturates as function of the strength of Im Vopt. This feature, for E>0, parallels the recent prediction, for E<0, that the level widths of p̄ atoms saturate and, hence, that p̄ deeply bound atomic states are relatively narrow. Antiproton annihilation cross sections are calculated at pL=57 MeV/c across the periodic table, and their dependence on Z and A is classified and discussed with respect to the Coulomb focussing effect at very low energies.