Saturation of low-energy antiproton annihilation on nuclei

Abstract

Recent measurements of very low-energy ($p_L<100$ MeV/c) $\bar{p}$ annihilation on light nuclei reveal apparent suppression of annihilation upon increasing the atomic charge $Z$ and mass number $A$. Using $\bar{p}$-nucleus optical potentials $V_{\mathrm{opt}}$, fitted to $\bar{p}$-atom energy-shifts and -widths, we resolve this suppression as due to the strong effective repulsion produced by the very absorptive $V_{\mathrm{opt}}$. The low-energy $\bar{p}$-nucleus wavefunction is kept substantially outside the nuclear surface and the resulting reaction cross section saturates as function of the strength of Im $V_{\mathrm{opt}}$. 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. Antiproton annihilation cross sections are calculated at $p_L=57$ 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.

10 pages, 3 figures, slightly revised version, PLB in press