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

We determine the topological susceptibility χtχt 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 163×32163×32 lattice at lattice spacing ∼ 0.12 fm at six sea quark masses mqmq ranging in ms/6−msms/6−ms with msms the physical strange quark mass. The χtχt 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 mqmq regime, our result of χtχt is proportional to mqmq as expected from chiral effective theory. Using the formula χt=mqΣ/Nfχt=mqΣ/Nf by Leutwyler–Smilga, we obtain the chiral condensate in Nf=2Nf=2 QCD as ΣMS¯(2 GeV)=[252(5)(10) MeV]3, in good agreement with our previous result obtained in the ϵ-regime.