Systematic gauge-invariant approach to heavy quarkonium decays

We present a method which, starting directly from QCD, permits a systematic gauge-invariant expansion to be made for all hard processes involving quarkonia in powers of the quark relative velocity, a small natural parameter for heavy quark systems. Our treatment automatically introduces soft gluons in the expansion. Corrections arising from the incorporation of gauge symmetry turn out to be important for decay and fragmentation processes involving $Q\overline{Q}$ systems. The contribution of soft gluons is shown to be of higher order in $v$ and so is neglected for calculations done up to and including $O\left(v^2\right)$.