Anomalous U(1)'s and Proton Stability in Brane Models

We consider the most general generation-independent U(1) gauge symmetry consistent with the presence of Yukawa couplings for all quarks and leptons in the SUSY version of the Standard Model. This U(1) has generically mixed anomalies with SM groups, which cannot be cancelled by the Green-Schwarz mechanism of heterotic D=4 strings. We argue that these anomalies can in principle be cancelled by the generalized Green-Schwarz mechanism present in field theories corresponding to D-branes at singularities. Moreover, unlike the heterotic case, once the U(1) symmetry is broken it may remain as an exact perturbative global symmetry in the low energy theory. Applying this scheme to the SUSY SM we find that gauging such a general U(1):1) B and L violating operators at least up to dim=3,4,5,6 are generically forbidden; 2)The $\mu$-term is generically supressed. We also study the properties of a U(1) symmetry whose mixed anomalies with the different SM gauge groups are in the ratio of the beta function coefficients $\beta_a$. This relation has been shown to hold in certain orientifold models. In all cases the U(1) remains as a global symmetry at the orientifold singularity, the SM Higgs can break it at the electroweak scale, making possible to relate the blowing-up of the singularity with electroweak symmetry breaking.