Narrow Results

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 M⁴ ⊗ S¹/Z₂. We show that a nontrivial Z₂-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 orbifolding. 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).

Because of the problems arising from the fermion unification in the traditional grand unified theory and the mass hierarchy between the four-dimensional Planck scale and weak scale, we suggest the low energy gauge unification theory with low high-dimensional Planck scale. We discuss the nonsupersymmetric SU(5) model on M⁴ × S¹/Z₂ × S¹/Z₂ and the supersymmetric SU(5) model on M⁴ × S¹/(Z₂ × Z₂′) × S¹/(Z₂ × Z₂′). The SU(5) gauge symmetry is broken by the orbifold projection for the zero modes, and the gauge unification is accelerated due to the SU(5) asymmetric light KK states. In our models, we forbid the proton decay, still keep the charge quantization, and automatically solve the fermion mass problem. We also comment on the anomaly cancellation and other possible scenarios for low energy gauge unification.

We discuss properties of a three-brane in an asymptotic five-dimensional de Sitter spacetime. It is found that a Minkowski solution can be obtained without fine-tuning. In the model, the tiny observed positive cosmological constant is interpreted as a curvature of five-dimensional manifold, but the Minkowski spacetime, where we live, is a natural three-brane perpendicular to the fifth coordinate axis.

Regarding ultrahigh-energy cosmic rays (UHECRs) as a probe and extra dimensions as a possible ingredient of the fantastic ultrahigh- energy world, we discuss possible interplay between them. On the one hand small extra dimensions and KK bursts present a feasible way to the origin of UHECRs. On the other hand large extra dimensions may change various interactions between particles at ultrahigh energies and therefore impact on air showers created by UHECRs. Conversely, UHECR data may tell us secrets veiled in extra dimensions.

We discuss how various mechanisms, including type-2 Randall-Sundrum and Dvali-Shifman, may be combined to create possibly realistic domain-wall-brane models with one extra infinite dimension.

As two generalizations of Einstein's general relativity, the nontrivial spacetime torsion and the compact extra dimensions have been largely studied in the literature. In this paper, by combining a torsioned Kaluza–Klein scheme and the field-theory cosmic string theory, we discuss that a higher-dimensional torsion component can be expressed in terms of the usual four-dimensional field strength two-form, and this torsion form can then get trapped into the cores of the cosmic strings and further relate to the intrinsic spins of the strings.

In this paper we consider the Kaluza–Klein field equations in the presence of a generic 5D matter tensor which is governed by a conservation equation due to 5D Bianchi identities. Following a previous work, we provide a consistent approach to matter where the problem of huge massive modes is removed, without relaxing the compactification hypotheses; therefore we perform the dimensional reduction either for metric fields and for matter, thus identifying a pure 4D tensor term, a 4D vector term and a scalar one. Hence we are able to write down a consistent set of equations for the complete dynamics of matter and fields; with respect to the pure Einstein–Maxwell system we now have two additional scalar fields: the usual dilaton one plus a scalar source term. Some significant scenarios involving these terms are discussed and perspectives for cosmological applications are suggested.

We consider a scenario with an additional scalar standard model singlet ϕ_S, living in a single extra dimension of the RS1 background. The zero mode of this scalar which is localized in the extra dimension is a dark matter candidate and the annihilation cross section is strongly sensitive to its localization parameter. As a second scenario, we assume that the standard model Higgs field is accessible to the fifth flat extra dimension. At first we take the additional standard model singlet scalar field as accessible to the sixth extra dimension and its zero mode is a possible dark matter candidate. Second, we consider that the new standard model singlet, the dark matter candidate, lives in four dimensions. In both choices the KK modes of the standard model Higgs field play an observable role for the large values of the compactification radius R and the effective coupling λ_S is of the order of 10^-2–10^-1 (10^-6) far from (near to) the resonant annihilation.

We construct a class of closed time-like curves (CTCs) using a compactified extra dimension u. A nonzero metric element g_tu(u) enables particles to travel backwards in global time t. The compactified dimension guarantees that the geodesic curve closes in u. The effective 2D (t and u) nature of the metric ensures that spacetime is flat, therein satisfying all the classical stability conditions as expressed by the energy conditions. Finally, stationarity of the metric guarantees that a particle's energy is conserved. The pathologies that plague many hypothesized metrics admitting CTCs, e.g., an infinite cylinder of matter, a negative energy-distribution, particle acceleration/blue-shifting along the CTC, do not occur within our metric class.

We propose an idea that hidden particles can be separated according to gauge quantum numbers from the visible ones by the difference of boundary conditions (BCs) on extra dimensions. We formulate five-dimensional (5D) gauge theories yielding conjugate BCs besides ordinary ones on S¹/Z₂, and examine physical implications concerning hidden particles on an extension of the Standard Model (SM) coexisting different types of BCs. A model with conjugate boundary conditions is applied on a gauge-Higgs inflation scenario.

We consider a compactification of extra dimensions and numerically calculate Casimir energy which is provided by the mass of Kaluza–Klein modes. For the extra space we consider a torus with shape moduli and show that the corresponding vacuum energy is represented as a function of the moduli parameter of the extra dimensions. By assuming that the Casimir energy may be identified with cosmological constant, we evaluate the size of extra dimensions in terms of the recent data given by the Wilkinson Microwave Anisotropy Probe (WMAP) measurement and the supernovae observations. We suggest that the observed cosmological constant may probe the shape moduli of the extra space by the study of the Casimir energy of the compactified extra dimensions.

A five-dimensional FRW-type Kaluza–Klein cosmological model is taken to study the role of extra dimension in the expansion of the universe. Relation between scale factors corresponding to conventional four dimensions and the extra dimension has been established. Field equations are solved in order to find out the effect of pressure corresponding to these scale factors. Conditions for accelerated expansion are derived.

Since our universe is a system with the temperature so it is interesting to investigate the effects of temperature in the brane-world theories. In this paper we consider the effect of temperature on a real scalar field with potential V(ϕ) = -½aϕ² + ¼bϕ⁴ in a flat extra dimension. We find analytical expressions for the coefficients of the potential. We study the effect of the temperature on the domain wall (thick brane solution). We find an expression for the critical temperature which separates a symmetric phase from the broken symmetry phase. By inclusion of fermions, we also investigate temperature effect on the localization of fermions in extra dimension. It is found that the inclusion of fermion reduces the critical temperature.

We propose a mechanism that the soft supersymmetry breaking masses and μ parameter can be induced from the dynamical rearrangement of local U(1) symmetries in a five-dimensional model. It offers to a solution to μ problem if there is a large hierarchy among the relevant U(1) charge of Higgsinos and that of hidden fields which stabilize the extra-dimensional component of U(1) gauge boson.

We study the generation of μ parameter from the dynamical rearrangement of local U(1) symmetry in a five-dimensional model and discuss phenomenological implications on the θ parameter, under the assumption that supersymmetry is broken by the Scherk–Schwarz mechanism.

The double-lepton polarization asymmetries for the exclusive B → K₁l⁺l^- transition is studied in the universal extra dimension. It is shown that most of the double-lepton polarization asymmetries are sensitive to the model parameters. Any experimental measurements on double-lepton polarizations and their comparison with theoretical results can give valuable information on the physics beyond the standard model.

The phenomenon of fermion production in a 5D FRW space–time with a warped extra dimension is studied by considering the canonical method based on Bogoliubov transformation connecting the "in" with the "out" states. For an exactly solvable model, two sets of exact solution are expressed in terms of Bessel functions. Studying the semiclassical Hamilton–Jacobi equation, the "in" and "out" states are identified. The pair creation probability and the mean number of created particles are calculated from the Bogoliubov coefficients. It is shown that the particle creation probability and the number density of created particles depend on the ratio of the scale factors associated with the ordinary space {x} and the extra dimension. It is also shown that the contraction of the extra dimension induces particle creation like the ordinary space expansion.

We consider the Kaluza–Klein (KK) graviton and its decays in the presence of the light radion, the modulus of extra dimension, appearing naturally light due to classical scale invariance in warped gravity. Due to the presence of a sizable wave function overlap in the extra dimension between the KK graviton and the radion, the KK graviton can decay largely into a pair of radions, each of which decays into a pair of collimated photons or photon-jets. Focusing on the setup where the KK graviton has suppressed couplings to the Higgs boson and fermions, we discuss the bounds on the light radion in the parameter space where the photon-jets are relevant. Moreover, we pursue the possibility of distinguishing the scenario from the case with direct photon contributions by the angular distributions of photons in the LHC Run 2. Roles of KK graviton and radion as mediators of dark matter interactions, extensions with brane kinetic terms and the model with a holographic composite Higgs are also addressed.

One of the simplest ways to extend 4D cosmological models is to add another spatial dimension to make them 5D. In particular, it has been shown that the simplest of such 5D models, i.e. one in which the right-hand side of the Einstein equation is empty, induces a 4D nonempty universe. Accordingly, the origin of matter in a real 4D universe might be mathematically attributed to the existence of one (fictitious) extra spatial dimension. Here we consider the case of an empty 5D universe possessing a cosmological constant Λ and obtain exact solutions for both positive and negative values of the Λ. It is seen that such a model can naturally reduce to a power law ΛCDM model for the real universe. Further, it can be seen that the arbitrary constants and functions appearing in this model are endowed with definite physical meanings.

Killing symmetries are revisited in $d=5$ bulk geometric torsion (GT) perturbation theory to investigate the perihelion precession. Computation reveals a nonperturbative (NP) modification to the precession known in General Relativity (GR). Remarkably the analysis reassures our proposed holographic correspondence between a perturbative GT in bulk and a boundary GR coupled to $B_2\wedge F_2$. In fact, the topological correction is sourced by a non-Newtonian potential in GR and we identify it with an “electro-gravito” (EG) dipole. Interestingly, the dipole correction is shown to possess its origin in a $4$-form underlying a propagating GT and leads to a NP gravity in $d=4$.