Bulk Fermion Mass Research Articles

A warped scalar portal to fermionic dark matter

We argue that extensions of the SM with a warped extra dimension, together with a new {mathbb {Z}}_2-odd scalar singlet, provide a natural explanation not only for the hierarchy problem but also for the nature of fermion bulk masses and the observed dark matter relic abundance. In particular, the Kaluza-Klein excitations of the new scalar particle, which is required to naturally obtain fermion bulk masses through Yukawa-like interactions, can be the leading portal to any fermion propagating into the bulk of the extra dimension and playing the role of dark matter. Moreover, such scalar excitations will necessarily mix with the Higgs boson, leading to modifications of the Higgs couplings and branching ratios, and allowing the Higgs to mediate the coannihilation of the fermionic dark matter. We study these effects and explore the viability of fermionic dark matter in the presence of these new heavy scalar mediators both in the usual freeze-out scenario and in the case where the freeze-out happens during an early period of matter domination.

Spinor vertices and bubbles in the old conformal bootstrap in AdS/CFT and the fermion mass hierarchy problem: The Yukawa model

Attempts to resolve the longstanding Fermion mass hierarchy (FMH) problem in frames of the AdS/CFT correspondence demand the knowledge of bulk fermion masses. The approach of the "old" conformal bootstrap in AdS context permitted to calculate bulk masses of scalar fields [1], [2]. In the present paper this approach is extended to physically more interesting spin 1/2 bulk fields. Calculation of spinor-scalar vertices is performed in physical AdS space, and unexpectedly simple expressions for spinor-scalar bubbles (2-point one-loop self-energy Witten diagrams) are obtained. The "double-trace from UV to IR flow" subtraction of UV divergences used earlier in calculations of the UV-finite bulk tadpoles is applied to bubbles. This permitted to write down in frame of the "old" conformal bootstrap approach the nontrivial spectral equations for the bulk fermion masses; SU(N) Yukawa model of spinor fields interacting with conformal invariant scalar field in case of four boundary dimensions is considered as a specific example.

Dynamical origin of fermion bulk masses in a warped extra dimension

Extensions of the Standard Model featuring a warped extra dimension compactified on an S1/ℤ2 orbifold, in which the fermions and gauge bosons live in the bulk of the fifth dimension, offer one of the most compelling mechanisms for addressing both the hierarchy problem and the flavor puzzle of the Standard Model. However, the five-dimensional mass terms of bulk fermions must be odd functions on the orbifold, and as such they should be described by a field depending on the coordinate of the extra dimension. We demonstrate the feasibility of dynamically generating these fermion bulk masses with a bulk scalar field in warped extra dimensions. The bulk scalar acquires a vacuum expectation value, which is odd under the orbifold symmetry and gives rise to the fermion bulk masses through non-universal Yukawa-like interactions. Like in the conventional Randall-Sundrum setup, the localization of the different fermion zero modes along the extra dimension naturally explains the observed flavor structure and four-dimensional mass hierarchy of the SM fermions. We study the phenomenological implications of the backreaction on the metric and the modified fermion profiles due to the bulk scalar field on electroweak precision and flavor observables. Using up-to-date data, we show that the contributions to the S, T, and ϵK parameters require the mass of the first Kaluza-Klein gluon resonance to be of order 14 and 10 TeV in the minimal and the custodial model, respectively, regardless of the effect of the backreaction. Furthermore, effective flavor-changing interactions among the SM fermions induced by the bulk scalar are discussed. We also comment on the potential impact of the Higgs portal interaction of the bulk scalar on the couplings of the Higgs boson.

Unified flavor symmetry from warped dimensions

In a model of warped extra-dimensions with all matter fields in the bulk, we propose a scenario which explains all the masses and mixings of the SM fermions. In this scenario, the same flavor symmetric structure is imposed on all the fermions of the Standard Model (SM), including neutrinos. Due to the exponential sensitivity on bulk fermion masses, a small breaking of this symmetry can be greatly enhanced and produce seemingly un-symmetric hierarchical masses and small mixing angles among the charged fermion zero-modes (SM quarks and charged leptons), thus washing out visible effects of the symmetry. If the Dirac neutrinos are sufficiently localized towards the UV boundary, and the Higgs field leaking into the bulk, the neutrino mass hierarchy and flavor structure will still be largely dominated and reflect the fundamental flavor structure, whereas localization of the quark sector would reflect the effects of the flavor symmetry breaking sector. We explore these features in an example based on which a family permutation symmetry is imposed in both quark and lepton sectors.

Comments on holographic star and the dual QGP

We study static AdS star in generic dimension. The dependence of the mass limit to the bulk fermion mass is explored. In the bulk conformal limit, the mass limit saturates at a value identical to the mass limit of a radiation star or the AdS space filling with pure radiation. The temperature and entropy of the degenerate AdS star in the bulk conformal limit is zero in contrast to the radiation star. Holographically, the universal mass limit corresponds to the upper limit of the deconfinement temperature in the dual gauge picture. The QGP at this temperature is dual to the large black hole and the heat capacity is positive. When the fermion mass increases, the mass limit falls into the range of the small black holes. We found that even though the small black hole has negative heat capacity, the AdS box allows possibilities that it remains in thermal equilibrium with the radiation as long as the size of the black hole is not smaller than a critical size. Consequently, the dual QGP with negative heat capacity can be produced and remains stable thermodynamically at temperature below a saturation temperature $T_{2}$. The QGP with negative heat capacity produced at higher temperature will still condensate completely into a gas of confined hadron.

AMS-02 and next-to-minimal universal extra dimensions

The anomaly detected by AMS-02 and PAMELA in the cosmic-ray positron flux when interpreted as arising from dark matter annihilation suggests that dark matter may interact differently with hadrons and leptons so as to remain compatible with cosmic-ray antiproton data. Such a scenario is readily accommodated in models with extra spatial dimensions. We study indirect detection of Kaluza–Klein (KK) dark matter in universal extra dimensions with brane-localized terms and fermion bulk masses: next-to-minimal universal extra dimensions. So that an excess of antiprotons is not produced in explaining the positron anomaly, it is necessary that the KK bulk masses in the lepton and hadron sectors be distinct. Even so, we find that cosmic-ray data disfavor a heavy KK photon dark matter scenario. Also, we find these scenarios with flavor-universal bulk masses to be in conflict with dijet and dilepton searches at the LHC.

The muon anomalous magnetic moment in the Randall-Sundrum model

We calculate the anomalous magnetic moment of the muon in the minimal Randall-Sundrum model with standard model fields in five-dimensional (5D) warped space and a brane-localized Higgs. We use a fully 5D framework to compute the one-loop matching coefficients of the effective theory at the electroweak scale. The extra contribution to the anomalous magnetic moment from the model-independent gauge-boson exchange contributions \Delta a_\mu is approximately 8.8*10^-11 (1TeV/T)^2, where 1/T denotes the location of the TeV brane in conformal coordinates, and is related to the mass of the lowest gauge boson KK excitation, which is roughly 2.5T. The result constitutes the first complete determination of the gauge-boson contribution to g-2 and is robust against the variation of the bulk fermion masses and 5D Yukawa coupling. We also determine the strongly model-parameter dependent effect of Higgs-exchange diagrams.

Phenomenology of universal extra dimensions with bulk-masses and brane-localized terms

We present a general model with universal extra dimensions in the presence of the bulk fermion masses and boundary localized kinetic terms, which are generically allowed by symmetries of five dimensional gauge theory. We compute current bounds on the size of allowed universal bulk mass and universal brane-localized terms.

LHC sensitivity to a Kaluza-Klein gluon in the twob-jets decay channel

We examine a possibility to discover a Kaluza-Klein (KK) excitation of gluon in a warped extra-dimension model at the Large Hadron Collider, focusing on a decay channel of the KK gluon into a $b$-quark pair. It is known that, in a certain extension of the warped extra-dimension model, the third generation quarks could strongly couple to the KK gluon, as a result of appropriate bulk fermion mass parameters. Taking account of kinematical cuts to reduce background events, we show the model parameter space, which leads to a significance larger than $5\ensuremath{\sigma}$ with the integrated luminosity of $10\text{ }(100)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$.

Bounds on the Fermion-Bulk masses in models with universal extra dimensions

In models with extra dimensions, vectorlike Dirac masses for fermion fields are generically allowed. These masses are independent of electroweak symmetry breaking and do not contribute to the known masses for the quarks and leptons. They control the profile of the bulk wave functions, the mass spectra of Kaluza-Klein modes, and interactions that could be tested in experiments. In this article, we study the effects of bulk masses in electroweak precision measurements and in dark matter and collider searches, to set bounds on the bulk mass parameters in models with a flat universal extra dimension, namely, Split-UED. We find the current bound on the universal bulk-mass to be smaller than (0.2-0.3)/R, where R is the radius of the extra dimension. Similar but slightly relaxed bounds are obtained in the non-universal bulk mass case. The LHC is expected to play an important role in constraining the remaining parameter space.

Naturally minute quantum correction to the cosmological constant descended from the hierarchy

We demonstrate that an extremely small but positive quantum correction, or the Casimir energy, to the cosmological constant can arise from a massive bulk fermion field in the Randall-Sundrum model. Specifically, a cosmological constant doubly descended from the Planck-electroweak hierarchy and as minute as the observed dark energy scale can be naturally achieved without fine-tuning of the bulk fermion mass. To ensure the stabilization of the system, we discuss two stabilization mechanisms under this setup. It is found that the Goldberger-Wise mechanism can be successfully introduced in the presence of a massive bulk fermion, without spoiling the smallness of the quantum correction.

A vector-like fourth generation with a discrete symmetry from Split-UED

Split-UED allows for the possibility that the lowest lying KK excitations of the Standard Model fermions can be much lighter than the corresponding gauge or Higgs KK states. This can happen provided the fermion bulk masses are chosen to be large, in units of the inverse compactification radius, 1/R, and negative. In this setup, all of the other KK states would be effectively decoupled from low energy physics. Such a scenario would then lead to an apparent vector-like fourth generation with an associated discrete symmetry that allows us to accommodate a dark matter candidate. In this paper the rather unique phenomenology presented by this picture will be examined.

PAMELA and Fermi LAT signals from long-lived Kaluza-Klein dark matter

We propose a simple extension of the minimal universal extra dimension model by introducing a small curvature. The model is formulated as a small anti--de Sitter curvature limit of the five-dimensional standard model (SM) in the Randall-Sundrum background geometry. While the lightest Kaluza-Klein (KK) particle can be thermal relic dark matter as usual in the universal extra dimension model, the KK parity is explicitly broken in the presence of the small curvature and the KK dark matter decays into the SM fermions with a long lifetime. Couplings of the KK dark matter with SM fermion pairs in the five-dimensional bulk are controlled by fermion bulk masses. By tuning bulk masses of quarks, we can suppress KK dark matter decay into quarks. With a suitable choice of bulk masses for leptons, KK dark matter decay into leptons can account for the cosmic-ray electron/positron excesses reported by the recent PAMELA and Fermi LAT satellite experiments.

Fermion localization in a generalized Randall-Sundrum model

A generalized two-brane Randall-Sundrum warped braneworld model has solutions of the warp factor for both positive or negative cosmological constant on the visible 3-brane which can resolve the naturalness problem in connection with the fine-tuning of Higgs mass in the standard model of elementary particles. To explore the location of the standard model fermions in such a generalized warped model, we, in this work, determine the dependence of the localization profile of a bulk fermion on the brane cosmological constant brane tension and the bulk fermion mass. Our results reveal that for a positive and small value of the induced cosmological constant a bulk fermion is localized close to the brane. On the other hand for a visible brane with negative cosmological constant and positive tension, the fermions are localized inside the bulk leading to phenomenologically interesting possibilities.

Sequestering CP violation and GIM-violation with warped extra dimensions

We propose a model of spontaneous CP violation to address the strong CP problem in warped extra dimensions that relies on sequestering flavor and CP violation. We assume that brane-localized Higgs Yukawa interactions respect a U(3) flavor symmetry that is broken only by bulk fermion mass and Yukawa terms. All CP violation arises from the vev of a CP-odd scalar field localized in the bulk. To suppress radiative corrections to , the doublet quarks in this model are localized on the IR brane. We calculate constraints from flavor-changing neutral currents (FCNCs), precision electroweak measurements, CKM unitarity, and the electric dipole moments in this model and predict to be at least about 10−12.

Flavor violation in warped extra dimensions and CP asymmetries in B decays

We show that CP asymmetries in b {yields} s hadronic decays are potentially affected by the presence of massive color-octet particles strongly coupled to the third generation quarks. Theories with warped extra dimensions provide natural candidates in the Kaluza-Klein excitations of gluons in scenarios where flavor-breaking by bulk fermion masses results in the localization of fermion wave-functions. Topcolor models, in which a new gauge interaction leads to top-condensation and a large top mass, also result in the presence of these color-octet states with TeV masses. We find that large effects are possible in modes such as B {yields} {phi}K{sub s}, B {yields} {eta}{prime}K{sub s} and B {yields} {pi}{sup 0}K{sub s} among others.

Flavor violation in warped extra dimensions and CP asymmetries in B decays

We show that CP asymmetries in b→s hadronic decays are potentially affected by the presence of massive color–octet particles strongly coupled to the third generation quarks. Theories with warped extra dimensions provide natural candidates in the Kaluza–Klein excitations of gluons in scenarios where flavor-breaking by bulk fermion masses results in the localization of fermion wave-functions. Topcolor models, in which a new gauge interaction leads to top-condensation and a large top mass, also result in the presence of these color–octet states with TeV masses. We find that large effects are possible in modes such as B→φKs, B→η′Ks and B→π0Ks among others.

$B\bar B$ mixing with the bulk fields in the Randall-Sundrum model

We calculate the $B\Bbar$ mixing in the Randall-Sundrum bulk model. In this model, all the Standard Model fields except the Higgs can reside in the bulk. Two suggestive models of "mixed" and "relaxed" scenarios are considered. We find that the enhancement of the loop function is 0.51% for the "relaxed" and 1.07% for the "mixed" scenario when the first 4th KK modes are included, for a bulk fermion mass parameter $\nu=-0.3$.

Top quark Kaluza-Klein mode mixing in the Randall-Sundrum bulk standard model and constraint from Δϱ

The Randall-Sundrum (RS) scenario with all the standard model (SM) fermions and gauge bosons in the bulk is phenomenologically studied. Even though the simple assumption of universal bulk fermion mass m Θ leads to the same Kaluza-Klein (KK) mass spectrum for all the SM fermions and thus suppresses new contributions to Flavor-Changing-Neutral-Current (FCNC) and the ϱ parameter, large Yukawa coupling of the top quark generates its KK mode mixing and breaks the degeneracy: unacceptably large contribution to Δϱ occurs. With a different bulk fermion mass to SU(2) singlet bottom quark, we demonstrate that there exists some parameter space to satisfy the Δϱ constraint.

Top quark Kaluza-Klein mode mixing in the Randall-Sundrum bulk standard model andB→Xsγ

We study top quark Kaluza-Klein (KK) mode mixing in the Randall-Sundrum scenario with all the SM fermions and gauge bosons in the bulk. Even though the simple assumption of a universal bulk fermion mass ${m}_{\ensuremath{\psi}}$ leads to the same KK mass spectrum for all SM fermions and thus suppresses new contributions to the flavor changing neutral current and the $\ensuremath{\rho}$ parameter, the large Yukawa coupling of the top quark generates mixing among its KK modes and breaks the degeneracy: An unacceptably large contribution to $\ensuremath{\Delta}\ensuremath{\rho}$ occurs. In order to satisfy the $\ensuremath{\Delta}\ensuremath{\rho}$ constraint, we relax the model by assigning a different bulk fermion mass to the SU(2) singlet bottom quark and demonstrate that there exists some limited parameter space where the $\ensuremath{\Delta}\ensuremath{\rho}$ constraint is satisfied. It is also shown that the current measurement of $\mathrm{Br}(\stackrel{\ensuremath{\rightarrow}}{B}{X}_{s}+\ensuremath{\gamma})$ can be accommodated in this modified model, and the one sigma level precision constrains the effective weak scale ${k}_{\mathrm{EW}}$ such as ${k}_{\mathrm{EW}}\ensuremath{\gtrsim}3\mathrm{TeV}$ for ${m}_{\ensuremath{\psi}}/k=\ensuremath{-}0.4,$ where ${k}_{\mathrm{EW}}$ is the warp-suppressed ${\mathrm{AdS}}_{5}$ curvature.