SUSY Quivers, Intersecting Branes and the Modest Hierarchy Problem

We present a class of chiral non-supersymmetric D=4 field theories in whichquadratic divergences appear only at two loops. They may be depicted as ``SUSYquivers'' in which the nodes represent a gauge group with extended e.g., N=4SUSY whereas links represent bifundamental matter fields which transform aschiral multiplets with respect to different N=1 subgroups. One can obtain thistype of field theories from simple D6-brane configurations on Type IIA stringtheory compactified on a six-torus. We discuss the conditions under which thiskind of structure is obtained from D6-brane intersections. We also discuss someaspects of the effective low-energy field theory. In particular we computegauge couplings and Fayet-Iliopoulos terms from the Born-Infeld action and showhow they match the field theory results. This class of theories may be ofphenomenological interest in order to understand the modest hierarchy problemi.e., the stability of the hierarchy between the weak scale and a fundamentalscale of order 10-100 TeV which appears e.g. in low string scale models.Specific D-brane models with the spectrum of the SUSY Standard Model and threegenerations are presented.Comment: 36 pages, using JHEP3.cls, 8 figures. References update