尾田 欣也

We study implications of generalized non-zero Dirichlet boundary condition along with the ordinary Neumann one on a bulk scalar in the Randall-Sundrum warped compactification. First we show profiles of vacuum expectation value of the scalar under the general boundary conditions. We also investigate Goldberger-Wise mechanism in several setups with the general boundary conditions of the bulk scalar field and find that the mechanism can work under non-zero Dirichlet boundary conditions with appropriate vacuum expectation values. Especially, we show that $SU(2)_R$ triplet Higgs in the bulk left-right symmetric model with custodial symmetry can be identified with the Goldberger-Wise scalar.

We clarify and study our previous observation that, under a compactification with boundaries or orbifolding, vacuum expectation value of a bulk scalar field can have different extra-dimensional wave-function profile from that of the lowest Kaluza-Klein mode of its quantum fluctuation, under presence of boundary-localized potentials which would be necessarily generated through renormalization group running. For concreteness, we analyze the Universal Extra Dimension model compactified on orbifold $S^1/Z_2$, with brane-localized Higgs potentials at the orbifold fixed points. We compute the Kaluza-Klein expansion of the Higgs and gauge bosons in an $R_\xi$-like gauge by treating the brane-localized potential as a small perturbation. We also check that the $\rho$ parameter is not altered by the brane localized potential.

We propose a simple five-dimensional extension of the Standard Model (SM) without any Higgs potential nor any extra fields. A Higgs doublet lives in the bulk of a flat line segment and its boundary condition is Dirichlet at the ends of the line, which causes the electroweak symmetry breaking without Higgs potential. The vacuum expectation value of the Higgs is induced from the Dirichlet boundary condition which is generally allowed in higher dimensional theories. The lightest physical Higgs has non-flat profile in the extra dimension even though the vacuum expectation value is flat. As a consequence, we predict a maximal top Yukawa deviation (no coupling between top and Higgs) for the Brane-Localized Fermion and a small deviation, a multiplication of 2\sqrt{2}/\pi\simeq0.9 to the Yukawa coupling, for the Bulk Fermion. The latter is consistent with the electroweak precision data within 90% CL for 430GeV\lesssim m_{KK}\lesssim 500GeV.