  Data relative to antineutron and antiproton annihilation on large nuclei in
the range 75-200 MeV/c present two unexpected features: (a) antineutron and
antiproton cross sections have a similar size, (ii) the rise of the antineutron
cross section at decreasing energy is much steeper than predictable for an
inelastic process of purely strong nature at that energy. The observed behavior
of antineutron-nucleus annihilations is similar to what would be expected for
antiproton-nucleus annihilations, where Coulomb attraction focusses antiproton
trajectories towards the nucleus, enhancing the inelastic cross section by a
factor O(1/p) with respect to antineutrons on the same target. This results in
a 1/p^2 behavior at small energies. The presence of a similar enhancement in
the antineutron case may only be justified by an interaction with a longer
range than strong interactions. Excluding a Coulomb force because of the
antineutron neutrality, and taking into account that an intrinsic electric
dipole is forbidden for the antineutron, the next choice is an electric dipole
that is induced by the nuclear electric field. Recent theoretical works have
shown that a non-negligible electric polarization may be induced in a neutron
by QED vacuum polarization. Assuming this as a possibility, we have used a
simple model to calculate the polarization strengths that are needed to fit the
available data in terms of this effect. These are within the magnitude
predicted by the vacuum polarization model. We have also discussed alternative
scenarios that could induce an electric polarization of the antineutron as a
consequence of the interplay between strong and e.m. interactions.
