We study gauge threshold corrections for local grand unified theory models in IIB/F-theory. Consistency with holomorphy requirements of supergravity and the Kaplunovsky-Louis formula implies that the unification scale is enhanced by the bulk radius R from the string scale to MX=RMS. We argue that the stringy interpretation of this is via a locally uncanceled tadpole sourced by the hypercharge flux. This sources closed string modes propagating into the bulk---equivalently open string gauge coupling running up to the winding scale MX. The enhancement to RMₛ is tied to grand unified theory breaking by a globally trivial hypercharge flux and will occur in all models realizing this mechanism.
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Conlon et al. (2009) studied this question.
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