  As one test of the validity of the staggered-fermion fourth-root determinant
trick, we examine the suppression of the topological susceptibility of the QCD
vacuum in the limit of small quark mass. The suppression is sensitive to the
number of light sea quark flavors. Our study is done in the presence of 2+1
flavors of dynamical quarks in the improved staggered fermion formulation.
Variance-reduction techniques provide better control of statistical errors. New
results from staggered chiral perturbation theory account for taste-breaking
effects in the low-quark mass behavior of the susceptibility, thereby reducing
scaling violations from this source. Measurements over a range of quark masses
at two lattice spacings permit a rough continuum extrapolation to remove the
remaining lattice artifacts. The results are consistent with chiral
perturbation theory with the correct flavor counting.
