A kind of prediction from superstring model building

Assuming that the Standard Model of particle physics arises from the$E_8\times E_8$ Heterotic String Theory, we try to solve the discrepancybetween the unification scale predicted by this theory ($\approx g_{GUT}\times5.27\cdot 10^{17}$ GeV) and the value deduced from LEP experiments ($\approx2\cdot 10^{16}$ GeV). A crucial ingredient in our solution is the presence atlow energies of three generations of supersymmetric Higgses and vector-likecolour triplets. As a by-product our analysis gives rise to a strategy whichmight be useful when constructing realistic models.Comment: Phenomenological discussion expanded, references added, Latex, 25 pages, 3 figure