On G-flux, M5 instantons, and U(1)s in F-theory
  Local aspects of singular F-theory compactifications for SUSY GUT model-building are fairly well understood in terms of Higgs bundles and their spectral data. Several global issues remain, however, including a description of G-fluxes, which are key to constructing chiral matter and stabilizing moduli, and the global realization of U(1) symmetries that can forbid phenomenologically unfavorable couplings. In this paper, we sharpen our earlier proposal for describing G-fluxes through "spectral divisors" and introduce a distinguished "Tate divisor", which can be used to describe both G-flux and U(1)s when present. As an application, we give a general discussion of M5-instanton contributions in the presence of G-flux and exemplify this in a concrete example, where we comment on the ability of instanton induced superpotential couplings to stabilize Kahler moduli. 
Joseph Marsano, Natalia Saulina, Sakura Schafer-Nameki
hep-th