Split Fermion Models Research Articles

Production of black holes and string balls in a two-dimensional split-fermion model

We present cross-sections for the black hole and string ball production in proton-proton collisions in a TeV-scale gravity model with split fermions in two dimensions. The cross-section for black hole and string ball production in the split-fermion model is smaller than in the non-split-fermion model. The drop of the cross-section for the string ball production can be one to two orders of magnitude with the increase of the width of the brane from $L=0$ to 15 $\text{TeV}^{-1}$. The cross-section for string ball production in two-dimensional split fermion model reduces more in comparison to black holes. Black holes are quite hard to be observed at the LHC. In fact, taking into account the current experimental limits on the fundamental Planck scale, black holes cannot be produced at the LHC. Cross-section for string ball production depends significantly on string coupling constant, making it very model dependent. We investigate the range of values of string coupling constant from 0.02 to 0.4. There has been no evidence for production of string balls at $\sqrt{s}=8$ TeV. A two-dimensional split fermion model with a extremely thick brane can account for the absence of signature of string balls for a world with the value of fundamental Planck scale even as low as 1 TeV.

Electric dipole induced by gravity in fat branes

In the fat brane model, also known as the split fermion model, it is assumed that leptons and baryons live in different hypersurfaces of a thick brane in order to explain the proton stability without invoking any symmetry. It turns out that, in the presence of a gravity source M, particles will see different four-dimensional (4D) geometries and hence, from the point of view of 4D-observers, the equivalence principle will be violated. As a consequence, we show that a hydrogen atom in the gravitational field of M will acquire a radial electric dipole. This effect is regulated by the Hamiltonian Hd=−μA⋅δr, which is the gravitational analog of the Stark Hamiltonian, where the electric field is replaced by the tidal acceleration A due to the split of fermions in the brane and the atomic reduced mass μ substitutes the electric charge.

Probing Neutrino Models in Extra Dimensions with Muon to Electron Conversion and Other Charged Lepton Flavor Violating Processes

Among the 27(+2) free parameters in Standard Model(SM) with massive neutrinos, 21(+2) out of them are related to the fermion masses. And most of them have been well measured in the past 50 years. Yet, we have no satisfactory understanding of the origin of flavor. However, models involve extra dimension(s) provide a new framework for understanding the hierarchical pattern of the SM Yukawa couplings. We are particularly interested in the neutrino models in higher dimension which alleviate some theoretical problems in their 4D versions. In this talk, some general features of the charged lepton flavor violating (LFV) processes in the higher dimensional neutrino models is illustrated by a simple 5D split fermion model.

Numerical study of leptogenesis in a 5D split fermion model with bulk neutrinos

We study numerically a 5D hybrid model which incorporates a split fermion scenario and bulk neutrinos. We perform a Monte Carlo analysis of the model in order to find the regions in the parameter space allowing for realization of the leptogenesis. We find that higher order Yukawa terms must be included in order the model to produce a CP violation and net baryon number sufficient for the creation of the observed baryon asymmetry of the Universe.

Neutrino masses via the Zee mechanism in the 5D split fermion model

We study the Zee model in the framework of the split fermion model in $M_4\times S_1/Z_2$ spacetime. Neutrino masses are generated through 1-loop diagrams without the right-handed neutrinos introduced. By assuming an order one anarchical complex 5D Yukawa couplings, all the effective 4D Yukawa couplings are determined by the wave function overlap between the split fermions and the bulk scalars in the fifth dimension. The predictability of the Yukawa couplings is in sharp contrast to the original Zee model in 4D where the Yukawa couplings are unknown free parameters. This setup exhibits a geometrical alternative to the lepton flavor symmetry. By giving four explicit sets of the split fermion locations, we demonstrate that it is possible to simultaneously fit the lepton masses and neutrino oscillation data by just a handful free parameters without much fine tuning. Moreover, we are able to make definite predictions for the mixing angle $\theta_{13}$, the absolute neutrino masses, and the lepton flavor violation processes for each configuration.

Fermion Localization on the orbifold S 1/Z 2

The hierarchical structure of fermion masses of the Standard Model is explained in split fermion models by localizing the fermions at different points in an extra dimension. We consider split fermion models with two bulk scalars compactified on an orbifold. In the static case we find analytical expression for the localizer. We also address the issue of stability of the localizer. We also find exact solutions for the fermion zero modes. We explore the parameter space of the model. We find ample opportunity for construction of phenomenologically viable theories exist.

Thermal leptogenesis in a 5D split fermion scenario with bulk neutrinos

We study the thermal leptogenesis in a hybrid model, which combines the so called split fermion model and the bulk neutrino model defined in five dimensional spacetime. This model predicts the existence of a heavy neutrino pair nearly degenerate in mass, whose decays might generate a CP violation large enough for creating the baryon asymmetry of the universe through leptogenesis. We investigate numerically the constraints this sets on the parameters of the model such as the size of the compactified fifth dimension.

The scalar sector of split-fermion models

In the split-fermion model, fermions are localized by scalars. We discuss single-scalar, two-scalar and three-scalar models. In all of these models, we find the localizer. We also calculate the Higgs profile.

Split fermion quasinormal modes

In this paper we use the conformal properties of the spinor field to show how we can obtain the fermion quasinormal modes for a higher-dimensional Schwarzschild black hole. These modes are of interest in so-called split fermion models, where quarks and leptons are required to exist on different branes in order to keep the proton stable. As has been previously shown, for brane-localized fields, the larger the number of dimensions the faster the black hole damping rate. Moreover, we also present the analytic forms of the quasinormal frequencies in both the large angular momentum and the large mode number limits.

Quark-lepton symmetry in five dimensions

We construct a complete five dimensional quark-lepton symmetric model, with all fields propagating in the bulk. The extra dimension forms an ${S}^{1}/{Z}_{2}\ifmmode\times\else\texttimes\fi{}{Z}_{2}^{\ensuremath{'}}$ orbifold with the zero mode fermions corresponding to standard model quarks localized at one fixed point. Zero modes corresponding to left (right)-chiral leptons are localized at (near) the other fixed point. This localization pattern is motivated by the symmetries of the model. Shifting the right-handed neutrinos and charged leptons slightly from the fixed point provides a new mechanism for understanding the absence of relations of the type ${m}_{e}={m}_{u}$ or ${m}_{e}={m}_{d}$ in quark-lepton symmetric models. Flavor changing neutral currents resulting from Kaluza-Klein gluon exchange, which typically arise in the quark sector of split fermion models, are suppressed due to the localization of quarks at one point. The separation of quarks and leptons in the compact extra dimension also acts to suppress the proton decay rate. This permits the extra dimension to be much larger than that obtained in a previous construct, with the bound $1/R\ensuremath{\gtrsim}30\text{ }\text{ }\mathrm{TeV}$ obtained.

Generating the baryon asymmetry of the universe in split fermion models

The origin of the matter-antimatter asymmetry of the universe is one of the major unsolved problems in cosmology and particle physics. In this paper, we investigate the recently proposed possibility that split fermion models -- extra dimensional models where the standard model fermions are localized to different points around the extra dimension -- could provide a means to generate this asymmetry during the phase transition of the localizing scalars. After setting up the scenario that we consider, we use a single fermion toy model to estimate the reflection coefficients for scattering off the phase boundary using a more realistic scalar profile than previous work resulting in a different Kaluza Klein spectrum. The value we calculate for $n_B/s$ is consistent with the mechanism being the source of the baryon asymmetry of our universe provided the $B-L$ violating processes have an efficiency of order $10^{-5}$.

Higgs localization in split fermion models

The flavor puzzle of the Standard Model is explained in split fermion models by having the fermions localized and separated in an extra dimension. Many of these models assume a certain profile for the Higgs VEV, usually uniform, or confined to a brane, without providing a dynamical realization for it. By studying the effect of the coupling between the Higgs and the localizer fields, we obtain these scenarios as results, rather than ansaetze. Moreover, we discuss other profiles and show that they are phenomenologically viable.

Flavor constraints on split fermion models

We examine the contributions to rare processes that arise in models where the standard model fermions are localized at distinct points in compact extra dimensions. Tree-level flavor changing neutral current interactions for the Kaluza-Klein gauge field excitations are induced in such models, and hence strong constraints are thought to exist on the size of the additional dimensions. We find a general parametrization of the model which does not depend on any specific fermion geography and show that typical values of the parameters can reproduce the fermion hierarchy pattern. Using this parametrization, we reexamine the contributions to neutral meson mixing, rare meson decays, and single top-quark production in ${e}^{+}{e}^{\ensuremath{-}}$ collisions. We find that it is possible to evade the stringent bounds for natural regions of the parameters, while retaining finite separations between the fermion fields and without introducing a new hierarchy. The resulting limits on the size of the compact dimension can be as low as ${\mathrm{TeV}}^{\ensuremath{-}1}.$

Realistic construction of split fermion models

The Standard Model flavor structure can be explained in theories where the fermions are localized on different points in a compact extra dimension. We show that models with two bulk scalars compactified on an orbifold can produce such separations in a natural way. We study the shapes and overlaps of the fermion wave functions. We show that, generically, realistic models of Gaussian overlaps are unnatural since they require very large Yukawa couplings between the fermions and the bulk scalars. We give an example of a five dimensional two scalar model that accounts naturally for the observed quark masses, mixing angles and CP violation.