Gauge unification in type I/I ' models

We discuss whether the (MSSM) unification of gauge couplings can be accommo dated in string theories with a low (TeV) string scale. This requires eithe r power law running of the couplings or logarithmic running extremely far a bove the string scale. In both cases it is difficult to arrange for the mul tiplet structure to give the MSSM result. For the case of power law running there is also enhanced sensitivity to the spectrum at the unification scal e. For the case of logarithmic running there is a fine tuning problem assoc iated with the light closed string Kaluza Klein spectrum which requires gau ge mediated supersymmetry breaking on the "visible" brane with a dangerousl y low scale of supersymmetry breaking. Evading these problems in low string scale models requires a departure from the MSSM structure, which would imp ly that the success of gauge unification in the MSSM is just an accident. ( C) 2000 Elsevier Science B.V. All rights reserved.