[en] We discuss the role of Abelian gauge symmetries in type IIB orientifold compactifications and their F-theory uplift. Particular emphasis is placed on U(1)s which become massive through the geometric Stueckelberg mechanism in type IIB. We present a proposal on how to take such geometrically massive U(1)s and the associated fluxes into account in the Kaluza-Klein reduction of F-theory with the help of non-harmonic forms. Evidence for this proposal is obtained by working out the F-theory effective action including such non-harmonic forms and matching the results with the known type IIB expressions. We furthermore discuss how world-volume fluxes on D3-brane instantons affect the instanton charge with respect to U(1) gauge symmetries and the chiral zero mode spectrum. The classical partition function of M5-instantons in F-theory is discussed and compared with the type IIB results for D3-brane instantons. The type IIB match allows us to determine the correct M5 partition function. Selection rules for the absence of chiral charged zero modes on M5-instantons in backgrounds with G₄ flux are discussed and compared with the type IIB results. The dimensional reduction of the democratic formulation of M-theory is presented in the appendix.

[en] We study the superpotential induced by Euclidean D3-brane instantons carrying instanton flux, with special emphasis on its significance for the stabilization of Kaehler moduli and Neveu-Schwarz axions in type IIB orientifolds. Quite generally, once a chiral observable sector is included in the compactification, arising on intersecting D7-branes with world-volume flux, resulting charged instanton zero modes prevent a class of instantons from participating in moduli stabilization. We show that instanton flux on Euclidean D3-branes can remove these extra zero modes and help in reinstating full moduli stabilization within a geometric regime. We comment also on the F-theoretic description of this effect of alleviating the general tension between moduli stabilization and chirality. In addition, we propose an alternative solution to this problem based on dressing the instantons with charged matter fields, which is unique to F theory and cannot be realized in the weak coupling limit.

[en] We analyse the non-perturbative superpotential due to M5-brane instantons in F-theory compactifications on Calabi–Yau fourfolds. The M5 partition function is obtained via holomorphic factorisation by explicitly performing the sum over chiral 3-form fluxes. Comparison with the partition function of fluxed Euclidean D3-brane instantons in Type IIB orientifolds allows us to fix the spin structure on the intermediate Jacobian of the M5-instanton. We furthermore analyse the contribution of the M5-instanton to the superpotential in the presence of G₄ gauge flux, where the superpotential is dressed with matter fields. We explicitly evaluate the pullback of G₄ onto the M5-brane as a measure for the presence of charged instanton zero modes. This accounts for the M5 charge both under massless U(1)s, if present, and under what corresponds in Type II language to geometrically massive U(1)s.