Periodic Theta Vacuum Model and the QCD Instanton Vacuum

We formulate a toy model for the $\theta$ vacuum that embodies the n-point moments of the even and odd topological charge densities. The partition function is $2\pi$ periodic only after resumming over all winding modes. We apply the same analysis to the quenched instanton vacuum at finite $\theta$ and derive saddle-point results for the vacuum energy density that are manifestly $2\pi$ periodic whatever $\theta$. We briefly discuss the consequences of unquenching the fermions on our results.