THE PRICE OF DERIVING THE STANDARD MODEL FROM THE STRING

It is well known that precision LEP data on sin2-theta(w) and alpha-3 are consistent with supersymmetric grand unification at an energy scale almost two orders of magnitude below that calculated in string models. We examine whether one can raise the unification scale, whilst retaining the successful predictions for sin2-theta(w) and alpha-3, by adding to the standard model non-exotic multiplets of particles. We find that one must introduce extra (3, 2) representations of SU (3) X SU (2). If there is just one such vector-like (3, 2) representation, it must weigh less than about 10(13) GeV. Moreover, there must be at least one other type of light matter representation. We exhibit a "twisted" version of the flipped SU (5) model derived from the string that can accommodate such light representations.