Standard model at intersecting D5-branes:: lowering the string scale

Recently a class of type HA orientifold models was constructed yielding just the fermions of the SM at the intersections of D6-branes wrapping a 6-torus. We generalize that construction to the case of type-IIB compactified on an orientifold of T-4 x (C/Z(N2)) with D5-branes intersecting at angles on T4. We construct explicit models in which the massless fermion spectrum is just the one of a three-generation standard model. One of the motivations for these new constructions is that in this case there are 2 dimensions which are transverse to the SM D5-brane configuration. By making those two dimensions large enough one can have a low string scale M-s of order 1-10 TeV and still have a large M-Planck in agreement with observations. From this point of view, these are the first explicit D-brane string constructions where one can achieve having just the fermionic spectrum and gauge group of the SM embedded in a Low String Scale scenario. The cancellation of U(1) anomalies turns out to be quite analogous to the toroidal D6-brane case and the proton is automatically stable due to the gauging of baryon number. Unlike the D6-brane case, the present class of models has N = 0 SUSY both in the bulk and on the branes and hence the spectrum is simpler. (C) 2002 Elsevier Science B.V. All rights reserved.