Radiative Decay Width of J=- æ(2) from Nf=2 Lattice QCD

The large radiative production rate for pseudoscalar mesons in the J=psi radiative decay remains elusive. We present the first lattice QCD calculation of partial decay width of J=psi radiatively decaying into eta(2), the SU(2) flavor singlet pseudoscalar meson, which confirms QCD UA(1) anomaly enhancement to the coupling of gluons with flavor singlet pseudoscalar mesons. The lattice simulation is carried out using Nf = 2 lattice QCD gauge configurations at the pion mass m pi approximate to 350 MeV. In particular, the distillation method has been utilized to calculate light quark loops. The results are reported here with the mass m eta(2) = 718(8) MeV and the decay width Gamma(J=psi -gamma eta(2)) = 0.385(45) keV. By assuming the dominance of UA(1) anomaly and flavor singlet-octet mixing angle theta = -24.5 degrees, the production rates for the physical eta and eta i in J=psi radiative decay are predicted to be 1.15(14) x 10-3 and 4.49(53) x 10-3, respectively, which agree well with the experimental measurement data. Our study manifests the potential of lattice QCD studies on the light hadron production in J=psi radiative decays.