Unquenched charmonium with nonrelativistic QCD

We present the results from a series of lattice simulations of the charmonium system using a highly improved NRQCD action, both in the quenched approximation and with $n_f=2\mathrm{}$ light dynamical quarks. The spectra show some evidence for quenching effects of roughly 10% in the S and P hyperfine spin splittings—probably too small to account for the severe underestimates in these quantities seen in previous quenched charmonium simulations. We also find estimates for the magnitude of other systematic effects—in particular, the choice of the tadpole factor can alter spin splittings at the 10–20 % level, and $\mathcal{O}\left({\alpha }_s\right)$ radiative corrections may be as large as 40% for charmonium. We conclude that quenching is just one of a collection of important effects that require attention in precision heavy-quark simulations.