No abstract

We report on an attempt to solve the gauge hierarchy problem in the framework of higher dimensional gauge theories. Both classical Higgs mass and quadratically divergent quantum correction to the mass are argued to vanish. Hence the hierarchy problem in its original sense is solved. The remaining finite mass correction is shown to depend crucially on the choice of boundary condition for matter fields, and a way to fix it dynamically is presented. We also point out that on the simply-connected space S 2 even the finite mass correction vanishes. *

We pursue the possibility of the scenario in which the Higgs field is identified with the extra-space component of a bulk gauge field. The space-time we take is M 4 ⊗ S 1 /Z 2 . We show that a non-trivial Z 2 -parity assignment allows some of the extra-space component to have radiatively induced VEV, which strongly modifies the mass spectrum and gauge symmetry of the theory, realized by oribifolding. In particular we investigate the dynamical mass generation of zero-mode fermion and spontaneous gauge symmetry breaking due to the VEV. The gauge theories we adopt are a prototype model SU(2) and SU (3) model, of special interest as the realistic minimal scheme to incorporate the standard model SU(2) × U(1).

We show that precision measurements of the oblique correction parameters S and T can be used to derive an upper bound on the number of extra generations of heavy fermions Neg. The bound depends on the type of heavy neutrino masses. We have obtained Neg≤2 in the case of Dirac neutrinos and Neg≤5 in the case of neutrinos with Majorana masses, under some assumptions on the flavor and color contents of extra generations.

We argue how the pseudo Dirac scenario for neutrinos leads to rich neutrino oscillation phenomena, including oscillation inside each generation. The pseudo Dirac scenario is generalized by incorporating generation mixings and formulas for the various neutrino oscillations are derived. As an application we compare the formulas with the corresponding data. We find that the observed pattern of mixings, such as almost maximal mixing in the atmospheric neutrino oscillation, is naturally explained in the generalized pseudo Dirac scenario with small generation mixings. We, however, also point out that there remain some problems to be settled for this scenario to be viable. The possible theoretical framework to realize the pseudo Dirac scenario is also briefly commented on.