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Topological Susceptibility in Lattice QCD with Two Flavors of Dynamical Quarks

We present a study of the topological susceptibility in lattice QCD with two degenerate flavors of dynamical quarks. The topological charge is measured on gauge configurations generated with a renormalization group improved gauge action and a mean field improved clover quark action at three values of $β=6/g^2$, corresponding to lattice spacings of $a \approx 0.22$, 0.16 and 0.11 fm, with four sea quark masses at each $β$. The study is supplemented by simulations of pure SU(3) gauge theory with the same gauge action at 5 values of $β$ with lattice spacings 0.09 fm$\simlt a \simlt$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 $χ_t^{1/4} = 197^{+13}_{-16}$ MeV where the error shows statistical and systematic ones added in quadrature. In full QCD $χ_t$ at heavy sea quark masses is consistent with that of pure SU(3) gauge theory. A decrease of $χ_t$ 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 $χ_t$ from heavy to light sea quark masses is discussed.