| In this dissertation, the idea of universal extra dimensions, in which all the standard model fields propagate, is studied. We focus on the models with one or two universal extra dimensions, compactified on S 1/Z2 or T 2/Z2 orbifolds. After describing the Kaluza-Klein decomposition of these models, we first discuss salient features of the model with two universal extra dimensions. The model with two universal extra dimensions on T2/Z 2 is especially interesting, because of an explanation of the number of generations derived from anomaly cancellations in six dimensions. We pay a special attention to the problem of proton stability. The observed proton stability has a natural origin in this model from a geometrical symmetry of the T2/Z2-orbifold. An extension of the model with two universal extra dimensions, by adding a single additional warped dimension in which only gravity and gauge-singlet neutrinos propagate, is introduced to explain the weakness of four dimensional gravity and the observed neutrino oscillations. For completeness of the exposition, we also briefly review existing studies on the model with one universal extra dimension on S1/Z 2, especially on its collider phenomenology and astronomical implications. We finally consider bounds on the compactification scale of universal extra dimensions imposed by precision measurements. A complete analysis of contributions to precision electroweak experiments in terms of the Peskin-Takeuchi parameters is done, which shows that the compactification scale as low as 250 GeV can be consistent with current experiments if we allow a relatively large Higgs boson mass of 800 GeV. Studies in the literature on bounds from the anomalous muon magnetic moment and various CP-violating/FCNC rare events are briefly recapitulated for a comparison with the bounds from precision electroweak experiments. All of these analyses indicate that the compactification scale of several hundred GeV is consistent with current experiments. |
