Intersecting Brane Models of Particle Physics and the Higgs Mechanism

We analyze a recently constructed class of D-brane theories with the fermion spectrum of the SM at the intersection of D6-branes wrapping a compact toroidal space. We show how the SM Higgs mechanism appears as a brane recombination effect in which the branes giving rise to $U(2)_L\times U(1)$ recombine into a single brane related to $U(1)_{em}$. We also show how one can construct D6-brane models which respect some supersymmetry at every intersection. These are quasi-supersymmetric models of the type introduced in hep-th/0201205 which may be depicted in terms of SUSY-quivers and may stabilize the hierarchy between the weak scale and a fundamental scale of order 10-100 TeV present in low string scale models. Several explicit D6-brane models with three generations of quarks and leptons and different SUSY-quiver structure are presented. One can prove on general grounds that if one wants to build a (factorizable) D6-brane configuration with the SM gauge group and $N = 1$ SUSY (or quasi-SUSY), also a massless $(B-L)$ generator must be initially present in any model. If in addition we insist on lef- and right-handed fermions respecting the same N=1 SUSY, the brane configurations are forced to have intersections giving rise to Higgs multiplets, providing for a rationale for the very existence of the SM Higgs sector