Eta And Eta ' Mesons From Lattice Qcd

The large mass of the ninth pseudoscalar meson, the eta', is believed to arise from the combined effects of the axial anomaly and the gauge field topology present in QCD. We report a realistic, 2 + 1-flavor, lattice QCD calculation of the eta and eta' masses and mixing which confirms this picture. The physical eigenstates show small octet-singlet mixing with a mixing angle of theta = -14.1(2.8)degrees. Extrapolation to the physical light quark mass gives, with statistical errors only, m(eta) = 573(6) MeV and m(eta') = 947(142) MeV, consistent with the experimental values of 548 and 958 MeV.