The b-τ\tau-unification in GUTs with non-chiral matter

It is shown, that the presently accepted value for b-quark mass can be obtained from the requirement of the exact b-$\tau$ unification in the both non-SUSY and SUSY non-chiral extended GUTs.Comment: 10 pages, LaTeX, 4 LaTeX figures include

Mass hierarchy and localization of gravity in extra time

We consider Randall-Sundrum model with localized gravity, replacing the extra compact space-like dimension by a time-like one. In this way the solution to the hierarchy problem can be reconciled with a correct cosmological expansion of the visible universe, just as a trivial result of the sign flip of cosmological constants in the bulk and on the 3-branes relative to the case of extra space-like dimension. Some phenomenological aspects of the proposed scenario related to the tachyonic nature of Kaluza-Klein states of graviton are also discussed.Comment: 10 pages, JHEP format, no figures, typos corrected, to appear in Phys. Lett.

Towards the Non-Chiral Extension of SM and MSSM

We show that in some SU(N) type GUTs with the complementary pairs of the conjugated fermion multiplets there naturally appear the relatively light ($M\ll M_{GUT}$) vectorlike fermions which considerably modify the desert physics. In the non-SUSY case they can provide for the unification of the standard coupling constant $\alpha_1$, $\alpha_2$ and $\alpha_S$ whereas in the SUSY case they can increase the unification point up to the string unification limit and decrease $\alpha_S(M_Z)$ down to the value predicted from the low energy physics.Comment: LaTeX, 1 figur

Composite quarks and leptons in higher space-time dimensions

A new approach towards the composite structure of quarks and leptons in the context of the higher dimensional unified theories is proposed. Owing to the certain strong dynamics, much like an ordinary QCD, every possible vectorlike multiplets of composites appear in higher dimensional bulk space-time, however, through a proper Sherk-Schwarz compactification only chiral set of composite quarks and leptons survive as the massless states in four dimensions. In this scenario restrictions related with the 't Hooft's anomaly matching condition are turned out to be avoided and, as a result, the composite models look rather simple and economic. We demonstrate our approach by an explicit construction of model of preons and their composites unified in the supersymmetric SU(5) GUT in five space-time dimensions. The model predicts exactly three families of the composite quarks and leptons being the triplets of the chiral horizontal symmetry SU(3)_h which automatically appears in the composite spectrum when going to ordinary four dimensions.Comment: 13 pages, tcilatex, no figures, v2 - misprints correcte

Unifying flipped SU(5) in five dimensions

It is shown that embedding a four-dimensional flipped SU(5) model in a five-dimensional SO(10) model, preserves the best features of both flipped SU(5) and SO(10). The missing partner mechanism, which naturally achieves both doublet-triplet splitting and suppression of d=5 proton decay operators, is realized as in flipped SU(5), while the gauge couplings are unified as in SO(10). The masses of down quarks and charged leptons, which are independent in flipped SU(5), are related by the SO(10). Distinctive patterns of quark and lepton masses can result. The gaugino mass M_1 is independent of M_3 and M_2, which are predicted to be equal.Comment: revised version-to appear in PRD, 23 pages, 3 figures, ReVTeX

Generalized Froggatt-Nielsen Mechanism

In this paper, we propose a Generalized Froggatt-Nielsen mechanism in which non-renormalizable operators involving a GUT group and $U(1)_H$ non-singlet Higgs field are introduced. Thus the GUT gauge symmetry breaking and the generation of hierarchical flavor hierarchy have a common origin in this mechanism. In this Generalized Froggatt-Nielsen mechanism, we propose universality conditions for coefficients corresponding to different contractions in the group productions. We find that the predictions in Generalized Froggatt-Nielsen mechanism for SU(5) GUT is different to that of ordinary Froggatt-Nielsen mechanism. Such Generalized Froggatt-Nielsen mechanism can be used in GUT models when ordinary Froggatt-Nielsen mechanism is no longer available. We study the application of Generalized Froggatt-Nielsen mechanism in SO(10) model. We find that realistic standard model mass hierarchy and mixings can be obtained both in SU(5) and SO(10) GUT models with such Generalized Froggatt-Nielsen mechanism.Comment: 4 pages, no figure

Lifting a Realistic SO(10) Grand Unified Model to Five Dimensions

It has been shown recently that the problem of rapid proton decay induced by dimension five operators arising from the exchange of colored Higgsinos can be simply avoided in grand unified models where a fifth spatial dimension is compactified on an orbifold. Here we demonstrate that this idea can be used to solve the Higgsino-mediated proton decay problem in any realistic SO(10) model by lifting that model to five dimensions. A particular SO(10) model that has been proposed to explain the pattern of quark and lepton masses and mixings is used as an example. The idea is to break the SO(10) down to the Pati-Salam symmetry by the orbifold boundary conditions. The entire four-dimensional SO(10) model is placed on the physical SO(10) brane except for the gauge fields, the 45 and a single 10 of Higgs fields, which are placed in the five-dimensional bulk. The structure of the Higgs superpotential can be somewhat simplified in doing so, while the Yukawa superpotential and mass matrices derived from it remain essentially unaltered.Comment: 17 pages, version to be published in Phys. Rev. D with expanded discussion of the suppression of dim-5 proton decay operator

Vanishing of cosmological constant and fully localized gravity in a Brane World with extra time(s)

We construct an intersecting brane configuration in six-dimensional space with one extra space-like and one extra time-like dimensions. With a certain additional symmetry imposed on the extra space-time we have found that effective four-dimensional cosmological constant vanishes automatically, providing the static solution with gravity fully localized at the intersection region as there are no propagating massive modes of graviton. In this way, the same symmetry allows us to eliminate tachyonic states of graviton from the spectrum of the effective four-dimensional theory, thus avoiding phenomenological difficulties comming from the matter instability usually induced in theories with extra time-like dimensions.Comment: 15 pages, comments and references added, version accepted in Phys. Lett.

Topped MAC with extra dimensions?

We perform the most attractive channel (MAC) analysis in the top mode standard model with TeV-scale extra dimensions, where the standard model gauge bosons and the third generation of quarks and leptons are put in D(=6,8,10,...) dimensions. In such a model, bulk gauge couplings rapidly grow in the ultraviolet region. In order to make the scenario viable, only the attractive force of the top condensate should exceed the critical coupling, while other channels such as the bottom and tau condensates should not. We then find that the top condensate can be the MAC for D=8, whereas the tau condensation is favored for D=6. The analysis for D=10 strongly depends on the regularization scheme. We predict masses of the top (m_t) and the Higgs (m_H), m_t=172-175 GeV and m_H=176-188 GeV for D=8, based on the renormalization group for the top Yukawa and Higgs quartic couplings with the compositeness conditions at the scale where the bulk top condenses. The Higgs boson in such a characteristic mass range will be immediately discovered in H -> WW^(*)/ZZ^(*) once the LHC starts.Comment: REVTEX4, 24 pages, 21 figures, to appear in PRD. The title is changed in PRD. One reference added, typos correcte

Family Unification on an Orbifold

We construct a family-unified model on a Z_2xZ_2 orbifold in five dimensions. The model is based on a supersymmetric SU(7) gauge theory. The gauge group is broken by orbifold boundary conditions to a product of grand unified SU(5) and SU(2)xU(1) flavor symmetry. The structure of Yukawa matrices is generated by an interplay between spontaneous breaking of flavor symmetry and geometric factors arising due to field localization in the extra dimension.Comment: 13 page