Topological Susceptibility in Lattice QCD with Two Flavors of Dynamical Quarks

TL;DR

This study measures the topological susceptibility in lattice QCD with two dynamical quark flavors, showing how it decreases with lighter quark masses and comparing results with pure gauge theory.

Contribution

It provides the first continuum limit estimate of topological susceptibility in full QCD with two flavors using improved actions and compares it with pure gauge theory results.

Findings

Topological susceptibility in pure SU(3) gauge theory: 197^{+13}_{-16} MeV.

In full QCD, susceptibility decreases as sea quark mass decreases.

Clearer decrease of susceptibility observed at smaller lattice spacings.

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$\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 $\chi_t^{1/4} = 197^{+13}_{-16}$ 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.