Fat branes, orbifolds and doublet-triplet splitting

A simple higher dimensional mechanism of the doublet-triplet splitting is presented in a five dimensional supersymmetric SU(5) GUT on S1/Z2. The splitting of multiplets is realized by a mass term of the Higgs hypermultiplet which explicitly breaks SU(5) gauge symmetry. Depending on the sign of the mass, zero mode Higgs doublets and triplets are localized on either side of the fixed points. The mass splitting is realized due to the difference of magnitudes of the overlap with a brane localized or a bulk singlet field. No unnatural fine-tuning of the parameters is needed. The proton stability is ensured by locality - without symmetries.