Precision LEP data, supersymmetric GUTs and string unification

The precision of sin2θwMS (mZ) extracted from LEP data (0.233±0.001) confirms the prediction of minimal supersymmetric GUTs (0.235±0.004) within the errors of about 2%. Moreover, supersymmetric GUTs with three generations and a heavy top quark also predict mb=5.2±0.3 GeV in perfect agreement with potential model estimates (5.0±0.2 GeV). String unification would require that the effective grand unification scale mGUT be no larger than the effective string unification scale mSU, which is indeed consistent with the LEP data, which indicate mGUT ≅ 2x1016 GeV in a minimal supersymmetric GUT, compared with the theoretical estimate mSU ≅ 1017 GeV. Specific choices of the string model moduli could enforce mGUT=mSU even in minimal supersymmetric GUTs, whilst non-minimal supersymmetric GUTs can reconcile the successful predictions of sin2θw with mGUT = mSU for generic values of the moduli, but tend to have mb too large. (orig.)