Limits on the neutron-antineutron oscillation time from the stability of nuclei

We have reexamined the question of what limit may be placed on the free space neutron-antineutron oscillation time ${\tau }_{\mathrm{nn}\mathrm{¯}}$ from a measured limit $T_0$ on the annihilation lifetime T of a nucleus. Using several recent antinucleon-nucleus optical potentials consistent with both p¯-atom and low energy scattering data, we obtain a limit ${\tau }_{\mathrm{nn}\mathrm{¯}>(1\mathrm{}\pm 0.3)\times {10}^8}$ sec if $T_0$=2.4×${10}^{31}$ yr. We consider the effect on our results of nuclear medium corrections such as a renormalization of the effective operator for n→n¯ transitions or the presence of additional ΔB=2 decay modes which are absent in free space. It is argued that renormalization effects are not likely to exceed those characteristic of nuclear β decay, and that if additional modes are present, the limit on ${\tau }_{\mathrm{nn}\mathrm{¯}}$ becomes even better.