  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.
