Configurations of intersecting D6-branes in type IIA superstring theory define string models of particle physics. Open string states supported at the intersection of the branes naturally give chiral fermions, a key ingredient of the Standard Model. If orientifold planes are included, the models can display the massless spectrum of gauge bosons and chiral fermions of the Standard Model. Compactification moduli are adjustable parameters that give rise to undesirable massless scalars and must be stabilized. Flux compactifications achieve moduli stabilization and give rise to an extremely large landscape of string vacua. The existence of vacua in which the vacuum energy matches the presently observed value becomes statistically plausible.
Intersecting D6-branes
In this section we consider a D-brane configuration that has a set of features that make it a good starting point for the construction of a string model of particle physics. Since we need fermions, we use a ten-dimensional superstring theory. In this theory, six of the ten dimensions, x4, …, x9, are taken to form a small compact space of finite volume. This is necessary in order to have an effectively four-dimensional spacetime (with coordinates x0, x1, x2, and x3). The compact space is as simple as possible: each dimension is turned into a circle, so that the resulting space is a six-dimensional torus T6.
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