F-theory GUTs with U(1) Symmetries: Generalities and Survey
  We study the structure of SU(5) F-theory GUT models that engineer additional U(1) symmetries. These are highly constrained by a set of relations observed by Dudas and Palti (DP) that originate from the physics of 4D anomaly cancellation. Using the DP relations, we find a general tension between unification and the suppression of dimension 5 proton decay when one or more U(1)'s are PQ symmetries and hypercharge flux is used to break the SU(5) GUT group. We then specialize to spectral cover models, whose global completions in F- theory we know how to construct. In that setting, we provide a technical derivation of the DP relations, construct spectral covers that yield all possible solutions to them, and provide a complete survey of spectral cover models for SU(5) GUTs that exhibit two U(1) symmetries. 
Matthew J. Dolan, Joseph Marsano, Natalia Saulina and Sakura   Schafer-Nameki
hep-th