Type II (orientifold) compactifications provide an efficient way to encode the matter content of Standard Model-like setups in terms of the geometry of complex subspaces.
Besides the perturbative closed string sector, type II theories admit non-perturbative objects, the so-called D-branes. These are objects extended in spacetime on which open strings can end. The way they mutually intersect gives rise to (charged) matter and GUT gauge groups.
Strings can
- have both ends on the same D-brane
- they can stretch between two different D-branes
- propagate (as closed strings) from one D-brane to another
Due to their intrinsic tension, stretched strings give rise to massive excitation modes. When two or more branes intersect, then the strings stretched between them also give rise to massless modes. One finds the following for the above cases:
- The massless modes lead to non-abelian gauge theory (more precisely a dimensional reduced Super Yang-Mills gauge theory) on the D-brane.
- Matter is induced by open strings located at the intersection of two D-branes. Yukawa couplings can arise when three branes intersect.
- Gravity which corresponds to closed strings is not confined to the branes but interacts with them.
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