Y spectrum from NRQCD with improved action

We explore the effect of higher order operators in the non-relativistic formulation of QCD (NRQCD). We calculated masses in the bb-spectrum using quenched gauge configurations at β = 6.0 and two different NRQCD actions which have been corrected to order mv4 and mv6. The two-point functions are calculated in a gauge invariant fashion. We find the general structure of the spectrum to be the same in the two cases. Using the 1P - 13S1 splitting we determine the inverse lattice spacings to be 2.44(4) GeV and 2.44(5) GeV for the mv4 and mv6 actions, respectively. We do observe shifts in the spin splittings. The hyperfine splitting is reduced by approximately 4 MeV, while the fine splitting is down by up to 10 MeV, albeit with large statistical errors.