ar X iv : h ep - l at / 9 71 00 84 v 1 2 4 O ct 1 99 7 1 Resolving Exceptional Configurations

Quenched lattice QCD studies using WilsonDirac fermions have encountered large statistical errors in calculations involving very light quarks. A small subset of the gauge configurations seem to play a dominant role in the final results. These “exceptional configurations” have limited studies to quark masses much larger than the up and down quark masses. The large fluctuations resulting from these exceptional configurations appear to be related to the structure of the small eigenvalues of the Wilson-Dirac operator [1–3]. As I will explain, exceptional configurations arise from a specific lattice artifact associated with the Wilson-Dirac formulation. This artifact is not removed by using the standard Sheikholeslami-Wohlert(Clover) O(a) “improved” action [4]. However, introducing a modified quenched approximation (MQA), removes the dominant part of this artifact and eliminates the exceptional configurations, thereby greatly reducing the noise associated with light quarks. The detailed studies that form the basis of this report are presented elsewhere [5,6].