Radiative improvement of the lattice NRQCD action using the background   field method and application to the hyperfine splitting of quarkonium states
  We present the first application of the background field method to Non-Relativistic QCD (NRQCD) on the lattice in order to determine the one-loop radiative corrections to the coefficients of the NRQCD action in a manifestly gauge-covariant manner. The coefficient of the $\sigma\cdot B$ term in the NRQCD action is computed at the one-loop level; the resulting shift of the hyperfine splitting of bottomonium is found to bring the lattice predictions in line with experiment. 
T. C. Hammant, A. G. Hart, G. M. von Hippel, R. R. Horgan, C. J.   Monahan
hep-lat