Mirage gauge coupling unification

We use compact D = 4, N = 1, Type IIB orientifolds as a testing ground for recent ideas about precocious gauge coupling unification and a low energy string scale. We find that certain such orientifolds have the interesting property that gauge couplings receive moduli-dependent corrections which mimic the effect of field theoretical logarithmic running. The effective cut-off scale for the logarithmic correction is not Mstring but rather MX = √ αMP lanckMc/Mstring, where Mc is the compactification scale. Thus there is just normal logarithmic running up to Mstring and extra moduli dependent corrections which behave as if there was further running to a higher virtual scale MX . In this mechanism a prominent role is played by anomalous U(1)’s with moduli dependent FayetIliopoulos terms. A vanishing FI-term fixes the modulus dependence of the corrected gauge coupling. We discuss possible ways to implement this mechanism in the context of a simple extension of the MSSM. Agreement identical (the one-loop equations are the same) to the one obtained in SUSY-GUT’s is obtained for Mstring = 10 11 GeV and Mc = 10 9 Gev. This fits with the recent suggestion, based on completely independent arguments, of identifying the string scale with the intermediate scale. Scenarios with a 1 TeV string scale tend to yield too small a value for MX in this context.