Abstract :
For recently constructed classes of D6-brane models, yielding the Standard Model fermion spectrum and gauge symmetry, we compute lower bounds on the masses of new U(1) fields that such models predict in addition to the hypercharge U(1)Y. In models with extra dimensions, generic uncertainties due to unknown values of the compactification radii of the extra dimensions affect the value of the string scale and thus the predictive power of such models. Using ρ parameter and Z–U(1) mixing-angle constraints we show how to avoid such uncertainties, to provide lower mass bounds for the additional U(1) fields. These are in the region above 750 GeV for mixing angles less than 1.5×10−3 (and as low as 550 GeV for mixing angles of 3×10−3).
