Topological Charge Correlators, Spectral Bounds, and Contact Terms
  The structure of topological charge fluctuations in the QCD vacuum is strongly restricted by the spectral negativity of the Euclidean 2-point correlator for $x\neq 0$ and the presence of a positive contact term. Some examples are considered which illustrate the physical origin of these properties. 
H. Thacker, S.J. Dong, T. Draper, I. Horvath, F.X. Lee, K.-F. Liu, and   J.B. Zhang