Witten-Veneziano Relation, Quenched QCD, and Overlap Fermions
  The quarks in quenched QCD have an anomalous self-interaction in the flavor singlet Goldstone boson channel. This coupling is extracted from a graph with disconnected quark lines, and is used to infer the mass of the eta-prime meson in full QCD. When the fermions are described by an overlap action, the Witten-Veneziano relation is an exact relation between the topological susceptibility (as defined through fermionic zero modes) and the inferred value of the eta-prime mass. Using an overlap action we compute the hairpin amplitude and determine the fermion zero-mode susceptibility, the inferred eta-prime mass and other parameters characterizing the low energy chiral properties of quenched QCD. 
Thomas DeGrand, Urs M. Heller (MILC collaboration)