Quenched Chiral Logs, the $\eta'$ Mass, and the Hairpin Diagram
  Limits on the size of quenched chiral logs in the pion mass for Wilson fermions are investigated. The smallness of chiral logs is shown to be a result of the suppression of the hairpin diagram for small $p^2$, such that the value of the hairpin on the pion mass shell is much smaller than the physical $m_{\eta'}^2$. A direct calculation of the topological susceptibility from the same data gives $m_{\eta'}\approx 1$ GeV. 
A. Duncan, E. Eichten, S. Perrucci, and H. Thacker