Effects Of Extra Dimensions Research Articles

Symmetries and assisted inflation in the Dvali-Gabadadze-Porrati braneworld model

We present here the general transformation that leaves unchanged the form of the field equations for perfect fluid cosmologies in the Dvali-Gabadadze-Porrati (DGP) brane-world model. Specifically, a prescription for relating exact solutions with different equations of state is provided, and the symmetries found can be used as algorithms for generating new cosmological models from previously known ones. We also present, implicitly, the first known exact DGP perfect fluid spacetime. A particular case of the general transformation is used to illustrate the crucial role played both by the number of scalar fields and the extra-dimensional effects in the occurrence of inflation. In particular, we see that assisted inflation does not proceed at all times for one of the two possible ways in which the brane can be embedded into the bulk.

The effects of non-universal extra dimensions on the radiative lepton flavor decays e and in the two Higgs doublet model

We study the effect of non-universal extra dimensions on the branching ratios of the lepton flavor violating processes \mu\to e\gamma and \tau\to \mu\gamma in the general two Higgs doublet model. We observe that these effects are small for a single extra dimension, however, in the case of two extra dimensions there is a considerable enhancement in the additional contributions.

The effects of non-universal extra dimensions on the fermion electric dipole moments in the two Higgs doublet model

We study the the effects of non-universal extra dimensions on the electric dipole moments of fermions in the two Higgs doublet model. We observe that the t quark and b quark electric dipole moments are sensitive to the extra dimensions, however, in the case of charged lepton electric dipole moments, this sensitivity is relatively weak.

Large extra dimension effects in Higgs boson production at linear colliders and Higgs factories

In the framework of quantum gravity propagating in large extra dimensions, the effects of virtual Kaluza-Klein graviton and graviscalar interference with Higgs boson production amplitudes are computed at linear colliders and Higgs factories. The interference of the almost-continuous spectrum of the KK gravitons with the standard model resonant amplitude is finite and predictable in terms of the fundamental D-dimensional Plank scale M_D and the number of extra dimensions \delta. We find that, for M_D ~ 1 TeV and \delta=2, effects of the order of a few percent could be detected for heavy Higgs bosons (m_H>500 GeV) in Higgs production both via WW fusion in e^+e^- colliders and at \mu^+\mu^- Higgs-boson factories.

Z boson pair-production at LEP

Events stemming from the pair-production of Z bosons in e+e− collisions are studied using 217.4 pb−1 of data collected with the L3 detector at centre-of-mass energies from 200 GeV up to 209 GeV. The special case of events with b quarks is also investigated. Combining these events with those collected at lower centre-of-mass energies, the Standard Model predictions for the production mechanism are verified. In addition, limits are set on anomalous couplings of neutral gauge bosons and on effects of extra space dimensions.

Constraints on large extra dimensions from neutrino oscillation experiments

The existence of bulk sterile neutrinos in theories with large extra dimensions can naturally explain small 4-dimensional Dirac masses for the active neutrinos. We study a model with 3 bulk neutrinos and derive various constraints on the size of the extra dimensions from neutrino oscillation experiments. Our analysis includes recent solar and atmospheric data from SNO and Super-Kamiokande, respectively, as well as various reactor and accelerator experiments. In addition, we comment on possible extensions of the model that could accommodate the LSND results, using natural parameters only. Our most conservative bound, obtained from the atmospheric data in the hierarchical mass scheme, constrains the largest extra dimension to have a radius R < 0.82 microns. Thus, in the context of the model studied here, future gravitational experiments are unlikely to observe the effects of extra dimensions.

PossibleZ-width probe of a “brane-world” scenario for neutrino masses

The possibility that the accurately known value of the Z width might furnish information about the coupling of two neutrinos to the Majoron (Nambu-Goldstone boson of spontaneous lepton number violation) is proposed and investigated in detail. Both the "ordinary" case and the case in which one adopts a "brane" world picture with the Majoron free to travel in extra dimensions are studied. Bounds on the dimensionless coupling constants are obtained, allowing for any number of extra dimensions and any intrinsic mass scale. These bounds may be applied to a variety of different Majoron models. If a technically natural see-saw model is adopted, the predicted coupling constants are far below these upper bounds. In addition, for this natural model, the effect of extra dimensions is to decrease the predicted partial Z width, the increase due to many Kaluza-Klein excitations being compensated by the decrease of their common coupling constant.

Relic gravity waves from braneworld inflation

We discuss a scenario in which extra dimensional effects allow a scalar field with a steep potential to play the dual role of the inflaton as well as dark energy (quintessence). The post-inflationary evolution of the universe in this scenario is generically characterized by a ``kinetic regime'' during which the kinetic energy of the scalar field greatly exceeds its potential energy resulting in a ``stiff'' equation of state for scalar field matter ${P}_{\ensuremath{\varphi}}\ensuremath{\simeq}{\ensuremath{\rho}}_{\ensuremath{\varphi}}.$ The kinetic regime precedes the radiation dominated epoch and introduces an important new feature into the spectrum of relic gravity waves created quantum mechanically during inflation. The amplitude of the gravity wave spectrum increases with the wave number for wavelengths shorter than the comoving horizon scale at the commencement of the radiative regime. This ``blue tilt'' is a generic feature of models with steep potentials and imposes strong constraints on a class of inflationary braneworld models. Prospects for detection of the gravity wave background by terrestrial and space-borne gravity wave observatories such as LIGO II and LISA are discussed.

Brane fluctuation and anomalous muon magnetic moment

We study the effects of extra dimensions on the muon anomalous magnetic moment with brane fluctuation. Since the coupling is naturally suppressed if brane fluctuation is considered by exponential softening factor for heavier states, the contribution from the whole Kaluza-Klein graviton tower is shown to be finite. The recent BNL E821 result is accomodated with $D=4+\delta$ dimensional gravitational scale, $M_D$, in the range of $M_D \simeq 1.0 - 5.1$ TeV ($\delta=2$), and $M_D \simeq 1.0 - 8.0 $ TeV ($\delta=6$) with the brane tension parameter $f=(4\pi^2 \tau)^{1/4}$, in the range $f = 1 - 10 $ TeV.

Universal extra dimensions and b→sγ

We analyze the effect of flat universal extra dimensions (i.e., extra dimensions accessible to all SM fields) on the process b -> s gamma. With one Higgs doublet, the dominant contribution at one-loop is from Kaluza-Klein (KK) states of the charged would-be-Goldstone boson (WGB) and of the top quark. The resulting constraint on the size of the extra dimension is comparable to the constraint from T parameter. In two-Higgs doublet model II, the contribution of zero-mode and KK states of physical charged Higgs can cancel the contribution from WGB KK states. Therefore, in this model, there is no constraint on the size of the extra dimensions from the process b -> s gamma and also the constraint on the mass of the charged Higgs from this process is weakened compared to 4D. In two Higgs doublet model I, the contribution of the zero-mode and KK states of physical charged Higgs and that of the KK states of WGB are of the same sign. Thus, in this model and for small tan beta, the constraint on the size of the extra dimensions is stronger than in one Higgs doublet model and also the constraint on the mass of the charged Higgs is stronger than in 4D.

Enhancement of the Higgs pair production at the CERN LHC: The minimal supersymmetric standard model and extra dimension effects

The neutral Higgs pair production at the LHC is studied in the MSSM, the large extra dimensional (ADD) model and the Randall-Sundrum (RS) model, where the total cross section can be significantly enhanced compared to that in the SM. The $p_{_T}$, invariant mass and rapidity distributions of each model have been shown to be distinctive: The ADD model raises the $p_T$ and invariant mass distributions at high scales of $p_T$ and invariant mass; in the RS model resonant peaks appear after the SM contribution dies away; the SM and the MSSM distributions drop rapidly at those high scales; in the ADD and the RS models the rapidity distributions congregate more around the center. It is concluded that various distributions of the Higgs pair production at the LHC with restrictive kinematic cuts would provide one of the most robust signals for the extra dimensional effects.

Muon anomalous magnetic moment and the stabilized Randall–Sundrum scenario

We study the effects of extra dimension on the muon anomalous magnetic moment in the stabilized Randall–Sundrum scenario. The effects of the Kaluza–Klein states heavier than the cut-off scale expected to be of order Λ π are neglected. Contribution from the spin-2 Kaluza–Klein states dominates over that from the spin-0 radion. The recent BNL E821 results impose a strict constraint on the parameter space of the model: Λ π ≈0.4–2.1 TeV with k/ M Pl=0.01–0.1. Small k/ M Pl is preferred if Λ π is TeV scale.

Spin configuration of top quark pair production with large extra dimensions at photon-photon colliders

Top quark pair production at photon-photon colliders is studied in low scale quantum gravity scenario. From the dependence of the cross sections on the spin configuration of the top quark and anti-quark, we introduce a new observable, top spin asymmetry. It is shown that there exists a special top spin basis where with the polarized parent electron beams the top spin asymmetry vanishes in the standard model but retains substantial values with the large extra dimension effects. We also present lower bounds of the quantum gravity scale $M_S$ from total cross sections with various combinations of the laser, electron beam, and top quark pair polarizations. The measurements of the top spin state $(t_\uparrow\bar{t}_\downarrow)$ with unpolarized initial beams are shown to be most effective, enhancing by about 5% the $M_S$ bounds with respect to totally unpolarized case.

Effects of extra dimensions on unitarity and the Higgs boson mass

We study the unitarity constraint on two body Higgs boson elastic scattering in the presence of extra dimensions. The contributions from the exchange of spin-2 and spin-0 Kaluza-Klein states can have a large effect on the partial wave amplitude. The unitarity condition restricts the maximal allowed value for the ratio $r=\sqrt{s}{/M}_{S}$ of the center of mass energy $\sqrt{s}$ to the gravity scale ${M}_{S}$ to be less than one. Although the constraint on the standard Higgs boson mass for r of order one is considerably relaxed, for small r the constraint is similar to that in the standard model. The resulting bound on the Higgs boson mass is not dramatically altered if perturbative calculations are required to be valid up to the maximal allowed value for r.

Effects of extra space-time dimensions on the Fermi constant

Effects of Kaluza-Klein excitations associated with extra dimensions with large radius compactifications on the Fermi constant are explored. It is shown that the current precision determinations of the Fermi constant, of the fine structure constant, and of the W and Z mass put stringent constraints on the compactification radius. The analysis excludes one extra space time dimension below $\sim 1.6$ TeV, and excludes 2, 3 and 4 extra space dimensions opening simultaneously below $\sim$ 3.5 TeV, 5.7 TeV and 7.8 TeV at the $90% CL$. Implications of these results for future collider experiments are discussed.

Large extra dimension effects on the spin configuration of the top quark pair ate+e−colliders

Large extra dimension effects on the spin configuration of the top quark pair at the ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}t\overline{t}$ process are studied. It is shown that the TeV scale quantum gravity effects cause significant deviations from standard model (SM) predictions for the spin configuration in the off-diagonal basis: they lead to substantial cross sections of the like-spin states of the top quark pair, which vanish in the SM; they weaken the pure dominance of the processes, the up-down (down-up) spin states for the left-handed (right-handed) beam. In addition it is shown that the angular cut $\ensuremath{-}0.5&lt;\mathrm{cos}\ensuremath{\theta}&lt;0$ is very effective to determine the sign of the quantum gravity corrections.

Effect of extra dimensions on gauge coupling unification

The effects of extra dimensions on gauge coupling unification is studied. We start with a comparison between power law running of the gauge couplings in models with extra dimensions and logarithmic running that happens in many realistic cases. We then discuss the effect of extra dimensions on various classes of unification models. We identify products of evolution coefficients that dictate the profile of unification in different models. We use them to study under what conditions unification of couplings can occur in both one and two step unification models. We find that Kaluza-Klein modes can help generate interesting intermediate scale models with gauge coupling unification such as the minimal left-right models with the seesaw mechanism with a M W R ∼ 10 13 GeV intermediate scale, useful in understanding neutrino oscillations. We also obtain several examples where the presence of non-canonical normalization of couplings enables us to obtain unification scales around 10 11 GeV. This fits very well into a class of models proposed recently where the string scale is advocated to be at this value from physical arguments.

Polarization effects on the e +e −→ W+W− process with large extra dimensions

We study large extra dimension effects on the polarizations of the W pair and electron beam at the e +e −→ W + W − process. It is shown that the measurements of the cross section for transversely polarized W pair with the right-handed electron beam remarkably enhance the possibilities to see the low scale quantum gravity effects. Higher Linear Collider bounds on the string scale in this model can be obtained by using the left-handed electron beam.

Effective Kähler potential and extra space-time dimensions

The effects of extra space-time dimensions on the Wilsonian effective K\"ahler potential and the perturbative one loop effective K\"ahler potential are determined within the framework of an Abelian gauge theory with $N=2$ supersymmetric field content. The relation between the K\"ahler metric and the effective gauge couplings which leads to the absence of radiative corrections to the K\"ahler potential is expressed as a function of the radius of compactification of a fifth dimension. In general, the quantum corrections to the low energy K\"ahler potential are shown to grow with this radius reflecting the underlying higher dimensional nature of the theory.

Extra spacetime dimensions and unification

We study the effects of extra spacetime dimensions at intermediate mass scales, as expected in string theories with large-radius compactifications, and focus on the gauge and Yukawa couplings within the Minimal Supersymmetric Standard Model. We find that extra spacetime dimensions naturally lead to the appearance of grand unified theories at scales substantially below the usual GUT scale. Furthermore, we show that extra spacetime dimensions provide a natural mechanism for explaining the fermion mass hierarchy by permitting the Yukawa couplings to receive power-law corrections. We also discuss how proton-decay constraints may be addressed in this scenario, and suggest that proton-decay amplitudes may be exactly cancelled to all orders in perturbation theory as a result of new Kaluza-Klein selection rules corresponding to the extra spacetime dimensions.