The Mass Effect in Quarkonium Decay

In the NRQCD factorization for productions and decays of a quakonium the difference between the quarkonium mass and the twice of the heavy quark mass is neglected at the leading order of the small velocity expansion. The effect of this difference is included in the relativistic correction. We attempt to define the difference in terms of NRQCD matrix elements and to separate the effect of the difference in several decays. It turns out that the total relativistic correcttion can be estimated by the difference. Numerically the correction will enhance the decay widths considered here.