Topological susceptibility in two-flavor lattice QCD with exact chiral symmetry

We determine the topological susceptibility X-t in two-flavor QCD using the lattice simulations at a fixed topological sector. The topological charge density is unambiguously defined on the lattice using the overlap-Dirac operator which possesses exact chiral symmetry. Simulations are performed on a 163 x 32 lattice at lattice spacing similar to 0.12 fm at six sea quark masses m(q) ranging in m(s)/6 - m(s) with m(s) the physical strange quark mass. The Xt is extracted from the constant behavior of the time-correlation of flavor-singlet pseudo-scalar meson two-point function at large distances, which arises from the finite size effect due to the fixed topology. In the Small m(q) regime, our result of Xt is proportional to m(q) as expected from chiral effective theory. Using the formula X-t = m(q) Sigma/N-f by Leutwyler-Smilga, we obtain the chiral condensate in N-f = 2 QCD as Sigma(MS) over bar (2 GeV) = [252(5)(10) MeV](3), in good agreement with our previous result obtained in the c-regime. (C) 2008 Elsevier B.V. All rights reserved.