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 b (b) over bar-spectrum using quenched gauge configurations at beta = 6.0 and two different NRQCD actions which have been corrected to order mv(4) and mv(6). 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 1 (P) over bar - 1(3)S(1) splitting we determine the inverse lattice spacings to be 2.44(4) GeV and 2.44(5) GeV for the mv(4) and mv(6) 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. (C) 1997 Published by Elsevier Science B.V.