INSTANTONS, CHIRAL-SYMMETRY BREAKING AND THE FATE OF THE ULTIMATE QUENCHED CALCULATION

We investigate how topological zero-modes affect hadron physics in the quenched approximation. We remark that in a finite volume the exact z ero-modes render tile chiral condensate pathological and explain why t his is not observed in lattice calculations. We then provide art argum ent, supported by a model calculation, to show how tire mixed 'zero-mo des' produce a density of modes, lambda, in tire large volume limit th at diverges as 1/root lambda. This implies that the quark physics of q uenched QCD becomes pathological in the continuum limit. We also show how similar arguments lead to a non-standard but regular spectrum in f ull QCD.