Light-quark masses from unquenched lattice QCD

We calculate the light meson spectrum and the light quark masses by lattice QCD simulation, treating all light quarks dynamically and employing the Iwasaki gluon action and the nonperturbatively $O(a)$-improved Wilson quark action. The calculations are made at the squared lattice spacings at an equal distance a²0.005, 0.01, and 0.015 fm², and the continuum limit is taken assuming an O(a²) discretization error. The light meson spectrum is consistent with experiment. The up, down, and strange quark masses in the MS̄ scheme at 2 GeV are m̄=(mᵤ+md)/2=3.55_-0.28+0.65 MeV and mₛ=90.1_-6.1+17.2 MeV where the error includes statistical and all systematic errors added in quadrature. These values contain the previous estimates obtained with the dynamical u and d quarks within the error.