Dynamical Quark Effects on Light Quark Masses

We present results for light quark masses from a systematic calculation in lattice QCD with two degenerate flavors of dynamical quarks. Simulations are made with a renormalization-group improved gauge action and a mean-field improved clover quark action for sea quark masses corresponding to $m_{\rm PS}/m_{\rm V} \approx 0.8$-0.6 and the lattice spacing a=0.22-0.11 fm. In the continuum limit we find $m_{ud}^{\bar{MS}}(2{\rm GeV})=3.5(3)$ MeV using the $\pi$ and $\rho$ meson masses as physical input, and $m_s^{\bar{MS}}(2{\rm GeV})=89(7)$ MeV or 99(5) MeV with the K or $\phi$ meson mass as additional input. Compared to quenched results, two flavors of dynamical quarks reduce the quark masses by about 25%.