Topological susceptibility in lattice QCD with two flavors of dynamical quarks

quark masses at each b. The lattice spacings at these b’s are a’0.22, 0.16 and 0.11 fm at the physical up and down quark mass, which are fixed by the physicalr meson mass. The study is supplemented by simulations of pure SU~3! gauge theory with the same gauge action at 5 values of b with lattice spacings 0.09 fm&a &0.27 fm. We employ a field-theoretic definition of the topological charge together with cooling. For the topological susceptibility in the continuum limit of pure SU~3! gauge theory we obtain x t 5197 216 113 MeV where the error shows statistical and systematic ones added in quadrature. In full QCD x t at heavy sea quark masses is consistent with that of pure SU~3! gauge theory. A decrease of x t toward light quark masses, as predicted by the anomalous Ward-Takahashi identity for U~1! chiral symmetry, becomes clearer for smaller lattice spacings. The cross over in the behavior of x t from heavy to light sea quark masses is discussed.