D=4 Chiral String Compactifications from Intersecting Branes

Intersecting Dp-branes often give rise to chiral fermions living on their intersections. We study the construction of four-dimensional chiral theories by considering configurations of type II D(3+n)-branes wrapped on non-trivial n-cycles on T^{2n} x R^{2(3-n)}/Z_N, for $n=1,2,3$. The gauge theories on the four non-compact dimensions of the brane world-volume are generically chiral and non-supersymmetric. We analyze consistency conditions (RR tadpole cancellation) for these models, and their relation to four-dimensional anomaly cancellation. Cancellation of U(1) gauge anomalies involves a Green-Schwarz mechanism mediated by RR partners of untwisted and/or twisted moduli. This class of models is of potential phenomenological interest, and we construct explicit examples of SU(3) x SU(2) x U(1) three-generation models. The models are non-supersymmetric, but the string scale may be lowered close to the weak scale so that the standard hierarchy problem is avoided. We also comment on the presence of scalar tachyons and possible ways to avoid the associated instabilities. We discuss the existence of (meta)stable configurations of D-branes on 3-cycles in (T^2)^3, free of tachyons for certain ranges of the six-torus moduli.