“Monodromy in QCD: Insights From Supersymmetric Theories with Soft Breakings”
The nature of $\theta$-dependence in QCD is a longstanding question. Witten long ago argued that in large N, instantons are suppressed ant that QCD should exhibit a branched structure. Supersymmetric theories with soft breakings provide a setting in which one can address some of these questions. In this talk, we’ll see that with small soft breakings, Witten’s branched picture is realized. On the other hand, instantons are sometimes under control and when they are, they are not suppressed. The behavior of the theory with large soft breakings – i.e. real QCD, plausibly might yield a branched structure, or a more conventional periodicity as suggested by instantons. We suggest lattice experiments to settle the question.