Chiral logarithms in quenched QCD

From a strong coupling investigation of quenched QCD, and an analysis of a previous numerical simulation of quenched SU(2), we show that the infrared logarithms expected in the chiral limit of complete QCD are not suppressed by the suppression of all internal quark loops. This result explains the apparent nonvanishing of π-mass extrapolations at zero quark mass, previously observed in several numerical studies. It has practical consequences for the precise determination from the lattice of physical quark masses. It calls for a better understanding of the nature of quenched QCD