SYSTEMATIC GAUGE-INVARIANT APPROACH TO HEAVY QUARKONIUM DECAYS

We present a method which, starting directly from QCD, permits a syste matic gauge-invariant expansion to be made for all hard processes invo lving quarkonia in powers of the quark relative velocity, a small natu ral parameter for heavy quark systems. Our treatment automatically int roduces soft gluons in the expansion. Corrections arising from the inc orporation of gauge symmetry turn out to be important for decay and fr agmentation processes involving <Q(Q)over bar> systems. The contributi on of soft gluons is shown to be of higher order in v and so is neglec ted for calculations done up to and including O(v(2)).