Gauge-top unification

Higher-dimensional models of grand unification allow us to relate the top Yukawa coupling y t to the gauge coupling g. The tree level relation y t = g at the scale of grand unification implies, in the framework of the MSSM, a rather small ratio of Higgs expectation values tan β. We find that, in the presence of localized Fayet-Iliopoulos terms, y t is suppressed against g because the bulk fields acquire non-trivial profiles whose overlap is smaller than in the case of flat profiles. This increases the prediction for tan β to moderately large values. Thus tan β is related to the geometry of compact space. We also discuss explicit realizations of such settings in orbifold compactifications of the heterotic string. It turns out that anisotropic compactifications, allowing for an orbifold GUT interpretation, are favored. © 2009 SISSA.