Bottom and Charm quark masses from lattice NRQCD

The Υ(bb) and ψ(cc) systems possess several properties which permit accurate lattice simulations. They are physically small, allowing the use of small volumes. They are insensitive to the presence of light quarks and to uncertainties in their masses. They are well understood phenomenologically, which aids in the estimation of systematic errors. Their spin-averaged mass splittings are insensitive to the b or c masses, allowing independent tuning of the lattice spacing and bare masses. Their properties have been precisely measured. Finally, they may be efficiently simulated using a nonrelativistic effective action, NRQCD. To compute the spin-averaged spectra we employed an NRQCD action for the heavy quarks, improved to include leading and next-to-leading order corrections for relativity and discretization errors. We used the average plaquette to tadpole improve the link operators. Several collaborations generously provided gauge field configurations for both zero and two light quark flavors, generated using the standard (unimproved) Wilson action for gauge fields and staggered fermions for the light quarks. We removed perturbatively the leading discretization errors in the spectrum due to use of this unimproved gluonic action [1]. The details of our simulations and references for the gauge field configurations appear in Ref. [2].