ar X iv : h ep - p h / 97 10 35 0 v 1 1 4 O ct 1 99 7 SLAC - PUB - 7675 October 1997

The observation of a possible excess of both neutral and charged current events at HERA has brought the discussion of leptoquarks(LQs) and their properties off the back burner, though they have been subjects of study for quite some time. For more than a decade any discussion of leptoquark models has been historically based on the classic work by Buchmüller, Rückl and Wyler (BRW). As these authors showed, LQs can be either spin-0 (scalars) or spin-1 (vectors) and may carry fermion number, F = 3B + L = 0 or ±2. In that paper the authors provided a set of basic assumptions under which consistent leptoquark models can be constructed; these can be restated as: (a) LQ couplings must be invariant with respect to the Standard Model (SM) gauge interactions, (b) LQ interactions must be renormalizable, (c) LQs couple to only a single generation of SM fermions, (d) LQ couplings to fermions are chiral, (e) LQ couplings separately conserve Baryon and Lepton numbers, (f) LQs only couple to the SM fermions and gauge bosons. Amongst these assumptions, both (a) and (b) are can be considered sacrosanct whereas (c)-(e) are data driven by a host of low energy processes as discussed by Davidson, Bailey and Campbell and by Leurer. Assumption (f) effectively requires that the leptoquark be the only new component added to the SM particle spectrum,