Two-flavor lattice QCD in the ε regime and chiral random matrix theory

The low-lying eigenvalue spectrum of the QCD Dirac operator in the epsilon regime is expected to match with that of chiral random matrix theory (ChRMT). We study this correspondence for the case including sea quarks by performing two-flavor QCD simulations on the lattice. Using the overlap fermion formulation, which preserves exact chiral symmetry at finite lattice spacings, we push the sea quark mass down to similar to 3 MeV on a 16(3)x32 lattice at a lattice spacing a similar or equal to 0.11 fm. We compare the low-lying eigenvalue distributions and find a good agreement with the analytical predictions of ChRMT. By matching the lowest-lying eigenvalue we extract the chiral condensate, Sigma(M) over bar(S) over bar (2 GeV)=(251 +/- 7 +/- 11 MeV)(3), where errors represent statistical and higher order effects in the epsilon expansion. We also calculate the eigenvalue distributions on the lattices with heavier sea quarks at two lattice spacings. Although the epsilon expansion is not applied for those sea quarks, we find a reasonable agreement of the Dirac operator spectrum with ChRMT. The value of Sigma, after extrapolating to the chiral limit, is consistent with the estimate in the epsilon regime.