Superinstantons and the reliability of perturbation theory in non-Abelian models

In dimension [ital D][le]2 the low temperature behavior of systems enjoying a continuous symmetry is dominated by superinstantons: classical configurations of arbitrarily low energy. Perturbation theory in the background of a superinstanton produces thermodynamic answers for the invariant Green's functions that differ from the standard ones, but only in non-Abelian models and only starting at [ital O](1/[beta][sup 2]). This effect modifies the [beta]-function models and persists in the large [ital N] limit of the [ital O]([ital N]) models.