SU(5) orientifolds, Yukawa couplings, Stringy Instantons and Proton Decay

Understand

We construct a large class of SU(5) orientifold vacua with tadpole cancellation both for the standard and the flipped case.

• We give a general analysis of superpotential couplings up to quartic order in orientifold vacua and identify the properties of needed Yukawa couplings as well as the baryon number violating couplings.
• We point out that successful generation of the perturbatively forbidden Yukawa couplings entails a generically disastrous rate for proton decay from an associated quartic term in the superpotential, generated from the same instanton effects.
• We search for the appropriate instanton effects that generate the missing Yukawa couplings in the SU(5) vacua we constructed and find them in a small subset of them.