The lightest Higgs mass in supersymmetric models with extra dimensions

In the four-dimensional supersymmetric standard model extended with gauge singlets the lightest Higgs boson mass, MH, has an important contribution proportional to the squared of the superpotential coupling λ of singlets to Higgs fields, λ SH1·H2. The requirement of perturbativity up to the unification scale yields an upper bound on MH∼140 GeV. In extensions to theories with (longitudinal) extra dimensions at the TeV where such coupling exists and massive Kaluza-Klein states fall into N=2 representations, if either of the Higgs or singlet fields live in the bulk of the extra dimensions, the β-function of λ is suppressed due to the absence of anomalous dimension of hypermultiplets to leading order. This implies a slower running of λ and an enhancement of its low energy value. The MH upper bound increases to values MH≲165 GeV.