Cluster expansion and resurgence in Polyakov model

Abstract: In Polyakov model, a non-perturbative mass gap is formed at leading order semi-classics by instanton effects. By using the notions of critical points at infinity, cluster expansion and Lefschetz thimbles, we show that a third order effect in semi-classics gives an imaginary ambiguous contribution to mass gap, which is supposed to be real and unambiguous. This is troublesome for the original analysis, and it is difficult to resolve this issue directly in QFT. However, we find a new compactification of Polyakov model to quantum mechanics, by using a background 't Hooft flux (or coupling to TQFT). The compactification has the merit of remembering the monopole-instantons of the full QFT within Born-Oppenheimer (BO) approximation, while the periodic compactification does not. In QM, we prove the resurgent cancellation of the ambiguity in 3-instanton sector against ambiguity in the Borel resummation of the perturbation theory around 1-instanton. Assuming that this result holds in QFT, we provide a large-order asymptotics of perturbation theory around perturbative vacuum and instanton.