Topological Susceptibility and Contact Term in QCD. A Toy Model
  We study a number of different ingredients related to $\theta$ dependence, the non-dispersive contribution in topological susceptibility with the "wrong" sign, topological sectors in gauge theories, and related subjects using a simple "deformed QCD". This model is a weakly coupled gauge theory, which however has all the relevant essential elements allowing us to study difficult and nontrivial questions which are known to be present in real strongly coupled QCD. Essentially we want to test the ideas related to the $U(1)_A$ problem in a theoretically controllable manner using the "deformed QCD" as a toy model. One can explicitly see microscopically how the crucial elements work. 
Evan Thomas and Ariel R. Zhitnitsky
hep-th hep-lat hep-ph