Mean-link tadpole improvement of SW and D234 actions

We investigate a tadpole-improved tree-level-script O sign(a³)-accurate action, D234c, on coarse lattices, using the mean link in Landau gauge to measure the tadpole contribution. We find that D234c shows much better rotational invariance than Sheikholeslami-Wohlert, and that mean-link tadpole improvement gives much better hadron mass scaling than plaquette tadpole improvement, with finite-a errors at 0.25 fm of only a few percent. We explore the effects of possible script O sign(α_s) changes to the improvement coefficients, and find that the two leading coefficients can be independently tuned: hadron masses are only sensitive to the clover coefficient C_F, while hadron dispersion relations are only sensitive to the third derivative coefficient C₃. Preliminary non-perturbative tuning of these coefficients yields values that are consistent with the expected size of perturbative corrections.