Topological Susceptibility in Lattice QCD with Two Flavors of Dynamical Quarks

Abstract

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 $\beta =6/g^2$, corresponding to lattice spacings of $a\approx 0.22$, 0.16 and 0.11 fm, with four sea quark masses at each $\beta$. The study is supplemented by simulations of pure SU(3) gauge theory with the same gauge action at 5 values of $\beta$ with lattice spacings 0.09 fm$\text{simlt}a\text{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 ${\chi }_t^{1/4}=197_{-16}^{+13}$ MeV where the error shows statistical and systematic ones added in quadrature. In full QCD ${\chi }_t$ at heavy sea quark masses is consistent with that of pure SU(3) gauge theory. A decrease of ${\chi }_t$ 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 ${\chi }_t$ from heavy to light sea quark masses is discussed.

Comments: RevTeX, 17 pages, 10 figures