Mirage Gauge Coupling Unification and TeV Scale Strings

Abstract

We consider gauge coupling unification in models with TeV scale strings and large compact dimensions realized as type IIB string orientifolds. Following an observation by Ibanez we show that the gauge couplings at low energies can behave as if they effectively unify at $M_U \sim 2 \times 10^{16} GeV$ with $\alpha_U \sim 1/24$. This requires the $\sigma$ model anomaly coefficients $b_a^{i'}$ not to be all equal and their ratio to the $\beta$-functions of minimally supersymmetric Standard Model $\beta_a$ to be a constant independent of the gauge group. If, in addition, $b_a^{i'}$ have a gauge group independent constant piece the relation between the unified gauge coupling and the dilaton VEV is modified so that there can be weakly coupled gauge theories arising from strongly coupled strings.