Light hadron spectroscopy in two-flavor QCD with small sea quark masses -: art. no. 074503

We extend the study of the light hadron spectrum and the quark mass in two-flavor QCD to smaller sea quark mass, corresponding to m(PS)/m(V)=0.60-0.35. Numerical simulations are carried out using the RG-improved gauge action and the meanfield-improved clover quark action at beta=1.8 (a=0.2 fm from rho meson mass). We observe that the light hadron spectrum for small sea quark mass does not follow the expectation from chiral extrapolations with quadratic functions made from the region of m(PS)/m(V)=0.80-0.55. Whereas fits with either polynomial or continuum chiral perturbation theory (ChPT) fail, the Wilson ChPT (WChPT) that includes a(2) effects associated with explicit chiral symmetry breaking successfully fits the whole data: In particular, WChPT correctly predicts the light quark mass spectrum from simulations for medium heavy quark mass, such as m(PS)/m(V)greater than or similar to0.5. Reanalyzing the previous data with the use of WChPT, we find the mean up and down quark mass being smaller than the previous result from quadratic chiral extrapolation by approximately 10%, m(ud)((MS) over bar)(mu=2 GeV)=3.11(17) [MeV] in the continuum limit.