Coisotropic D8-branes and Model-building

TL;DR

This work expands the landscape of type IIA, four-dimensional chiral vacua by introducing coisotropic D8-branes—D8s wrapping homologically trivial 5-cycles with worldvolume flux that induces D6-brane charge. These branes yield D=4 chirality through D6-like and magnetization mechanisms, and they generate open-closed superpotentials that couple open moduli to untwisted Kahler moduli, offering new moduli-stabilization prospects. In a concrete setting on $T^6/(\Z_2\times\\bZ_2)$ orientifolds, the authors compute chiral spectra, SUSY conditions, gauge couplings, and Yukawa structures, and present MSSM-like and left-right symmetric models that utilize D8-branes to cancel RR tadpoles and constrain Kahler moduli. The paper further explores mirror symmetry implications, mapping coisotropic D8-branes to tilted D5 or magnetized D9-branes in Type I, and uncovers a novel class of D6-branes with non-factorizable RR charge arising under multiple T-dualities, broadening the toolkit for string phenomenology and moduli stabilization.

Abstract

Up to now chiral type IIA vacua have been mostly based on intersecting D6-branes wrapping special Lagrangian 3-cycles on a CY three-fold. We argue that there are additional BPS D-branes which have so far been neglected, and which seem to have interesting model-building features. They are coisotropic D8-branes, in the sense of Kapustin and Orlov. The D8-branes wrap 5-dimensional submanifolds of the CY which are trivial in homology, but contain a worldvolume flux that induces D6-brane charge on them. This induced D6-brane charge not only renders the D8-brane BPS, but also creates D=4 chirality when two D8-branes intersect. We discuss in detail the case of a type IIA Z2 x Z2 orientifold, where we provide explicit examples of coisotropic D8-branes. We study the chiral spectrum, SUSY conditions, and effective field theory of different systems of D8-branes in this orientifold, and show how the magnetic fluxes generate a superpotential for untwisted Kahler moduli. Finally, using both D6-branes and coisotropic D8-branes we construct new examples of MSSM-like type IIA vacua.

Figures (8)

• Figure 1: Coisotropic D8-brane on ${\bf T}^6$.
• Figure 2: Fractional coisotropic D8-brane on ${\bf T}^6/\mathbb{Z}_2 \times \mathbb{Z}_2$.
• Figure 3: Chirality from a D6-D6 system on ${\bf T}^6/\mathbb{Z}_2 \times \mathbb{Z}_2$.
• Figure 4: Chirality from a D6-D8 system on ${\bf T}^6$.
• Figure 5: Chirality from a D8-D8 system on ${\bf T}^6$.