Doublet-Triplet Splitting in an SU(5) Grand Unification
  We present a new solution to the doublet-triplet splitting problem which also works in a supersymmetric SU(5) grand unified theory and is testable at TeV-scale collider experiments. In our model, the SU(5) symmetry is broken through the Hosotani mechanism. Thanks to the phase nature of the Hosotani-breaking, the "missing VEV" can be realized even in an SU(5) model. A general and distinctive prediction of this solution is the existence of light adjoint chiral supermultiplets with masses of the supersymmetry breaking scale. Though these disturb the gauge coupling unification, it can be recovered keeping the unified gauge coupling constant perturbative by introducing additional vector-like particles, which may be also observed in the upcoming collider experiments. 
Toshifumi Yamashita
hep-ph