Coisotropic D8-branes and model-building

Up to now chiral type IIA vacua have been mostly based on intersectingD6-branes wrapping special Lagrangian 3-cycles on a CY three-fold. We arguethat there are additional BPS D-branes which have so far been neglected, andwhich seem to have interesting model-building features. They are coisotropicD8-branes, in the sense of Kapustin and Orlov. The D8-branes wrap 5-dimensionalsubmanifolds of the CY which are trivial in homology, but contain a worldvolumeflux that induces D6-brane charge on them. This induced D6-brane charge notonly renders the D8-brane BPS, but also creates D=4 chirality when twoD8-branes intersect. We discuss in detail the case of a type IIA Z2 x Z2orientifold, where we provide explicit examples of coisotropic D8-branes. Westudy the chiral spectrum, SUSY conditions, and effective field theory ofdifferent systems of D8-branes in this orientifold, and show how the magneticfluxes generate a superpotential for untwisted Kahler moduli. Finally, usingboth D6-branes and coisotropic D8-branes we construct new examples of MSSM-liketype IIA vacua.Comment: 63 pages, 11 figures. Typos corrected and comments adde