  Chiral perturbation theory predicts that in quantum chromodymamics light
dynamical quarks suppress the topological (instanton) susceptibility. We
investigate this suppression through direct numerical simulation using the
Asqtad improved lattice fermion action. This action holds promise for carrying
out nonperturbative simulations over a range of quark masses for which chiral
perturbation theory is expected to converge. To test the effectiveness of the
action in capturing instanton physics, we measure the topological
susceptibility as a function of quark masses with 2+1 dynamical flavors. Our
results, when extrapolated to zero lattice spacing, are consistent with
predictions of leading order chiral perturbation theory.
  Included in our study is a comparison of three methods for analyzing the
topological susceptibility: (1) the Boulder hypercubic blocking technique with
the Boulder topological charge operator, (2) the more traditional Wilson
cooling method with the twisted plaquette topological charge operator and (3)
the improved cooling method of de Forcrand, Perez, and Stamatescu and their
improved topological charge operator.
