The Zee model generates neutrino masses at the one-loop level by adding charged SU(2)_L-singlet and extra SU(2)_L-doublet scalars to the standard model of particle physics. As the origin of the nontrivial structure of the lepton flavor mixing, we introduce the softly broken A_4 symmetry to the Zee model. This model is compatible with the tribimaximal mixing which agrees well with neutrino oscillation measurements. Then, a sum rule m_1 e^{i alpha_12} + 2 m_2 + 3 m_3 e^{i alpha_32} = 0 is obtained and it results in Delta m^2_31 < 0 and m_3 > 1.8*10^{-2}eV. The effective mass |(M_nu)_{ee}| for the neutrinoless double beta decay is predicted as | (M_ν)_{ee} | > 1.7*10^{-2}eV. The characteristic particles in this model are SU(2)_L-singlet charged Higgs bosons s^+_alpha (alpha=xi,eta,zeta) which are made from a 3-representation of A_4. Contributions of s^+_alpha to the lepton flavor violating decays of charged leptons are almost forbidden by an approximately remaining Z_3 symmetry; only BR(tau to ebar mu mu) can be sizable by the flavor changing neutral current interaction with SU(2)_L-doublet scalars. Therefore, s^+_alpha can be easily light enough to be discovered at the LHC with satisfying current constraints. The flavor structures of BR(s^-_alpha to ell nu) are also discussed.
The virtual effects of the Zee charged scalar boson on the lepton-family-number (LFN) violating processes are studied. We obtain the constraints on the individual Yukawa coupling constants of the Zee boson to leptons. Using these constraints, we predict the upper bounds on the muonium-antimuonium conversion probability, the branching fractions of the LFN violating decays such as $τ\to eγ$, $τ\to μγ$, $τ^- \to μ^+e^-e^-$ and $τ\to e^+μ^-μ^-$. The contribution of the Zee boson to the muon anomalous magnetic moment is also considered
The Zee model for Dirac neutrinos is one of the simplest models featuring one-loop Dirac neutrino masses. The interactions between the new scalars (two singly-charged fields) and neutrinos induce general neutrino interactions (GNI) which, as a generalisation of the non standard neutrino interactions, constitute an additional tool to probe models beyond the SM like this. In this work, we consider a $U(1)_{B-L}$ gauge symmetry as the responsible for the Diracness of the neutrinos and the radiative character of the neutrino masses. We determine the viable parameter space consistent with neutrino …
To generate small neutrino masses radiatively, the Zee-model introduces two Higgs doublets and one weak-singlet charged Higgs boson to its Higgs sector. From analyzing the renormalization group equations, we determine the possibile range of the lightest CP-even Higgs boson ($h$) mass and the Higgs boson self-couplings as a function of the cut-off scale beyond which either some of the coupling constants are strong enough to invalidate the perturbative analysis or the stability of the electroweak vacuum is no longer guaranteed. Using the results obtained from the above analysis, we find that the…
We study the Zee-Babu two-loop neutrino mass generation model and look for a possible flavor symmetry behind the tri-bimaximal neutrino mixing. We find that there probably exists the mu-tau symmetry in the case of the normal neutrino mass hierarchy, whereas there may not be in the inverted hierarchy case. We also propose a specific model based on a Froggatt-Nielsen-like $Z_5$ symmetry to naturally accomplish the mu-tau symmetry on the neutrino mass matrix for the normal hierarchy case.
The inert Zee model is an extension of the Zee model for neutrino masses to allow for a solution to the dark matter problem that involves two vector-like fields, a doublet and a singlet of ${\rm SU(2)_L}$, and two scalars, also a doublet and a singlet of ${\rm SU(2)_L}$, all of them being odd under an exact ${\rm Z_2}$ symmetry. The introduction of the ${\rm Z_2}$ guarantees one-loop neutrino masses, forbids tree-level Higgs-mediated flavor changing neutral currents and ensures the stability of the dark matter candidate. Due to the natural breaking of lepton numbers in the inert Zee model, we …
We study a Zee-Babu model in a modular $A_4$ flavor symmetry, in which we search for several predictions such as phases, sum of neutrino masses, and neutrinoless double beta decay, satisfying neutrino oscillation data in a minimum framework of the charge assignments of modular weight. We perform $Δχ^2$ analysis to get our results and find $τ$ is localized nearby at one of the fixed points of $i\times \infty$ for both of normal and inverted mass hierarchies.
A Zee-type neutrino mass matrix model with a badly broken horizontal symmetry SU(3)_H is investigated. By putting a simple ansatz on the symmetry breaking effects of SU(3)_H for transition matrix elements, it is demonstrated that the model can give a nearly bimaximal neutrino mixing with the ratio $Δm^2_{solar}/Δm^2_{atm} \simeq \sqrt{2} m_e/m_μ=6.7 \times 10^{-3}$, which are in excellent agreement with the observed data. In the near future, the lepton-number violating decay $Z\to μ^\pm τ^\mp$ will be observed.
The inert Zee model is an extension of the Zee model for neutrino masses to allow for a solution to the dark matter problem that involves two vector-like fields, a doublet and a singlet of $SU(2)_L$, and two scalars, also a doublet and a singlet of $SU(2)_L$, all of them being odd under an exact $Z_2$ symmetry. The introduction of the $Z_2$ guarantees one-loop neutrino masses, forbids tree-level Higgs-mediated flavor changing neutral currents and ensures the stability of the dark matter candidate. Due to the natural breaking of lepton numbers in the inert Zee model and encouraged by the ambiti…
The inert Zee model is an extension of the Zee model for neutrino masses. This new model explains the dark matter relic abundance, generates a one-loop neutrino masses and forbids tree-level Higgs-mediated flavor changing neutral currents. Although the dark matter phenomenology of the model is similar to that of the inert doublet model, the presence of new vector-like fermions opens the lepton portal as a new dark matter annihilation channel. We study the impact of such a new portal in the low mass regime and show the parameter space allowed by direct and indirect searches of dark matter. Rema…
We study a simple extension of the Zee model, in which a discrete $Z_2$ symmetry imposed in the original model is replaced by a global $U(1)$ symmetry retaining the same particle content. Due to the $U(1)$ symmetry with flavor dependent charge assignments, the lepton sector has an additional source of flavor violating Yukawa interactions with a controllable structure, while the quark sector does not at tree level. We show that current neutrino oscillation data can be explained under constraints from lepton flavor violating decays of charged leptons in a successful charge assignment of the $U(1…
We propose an extension of the standard model with a $U(1)_{\rm B-L}$ global symmetry that accommodates radiative neutrino masses along with dark matter and leptogenesis. The observed matter antimatter asymmetry of the universe is generated through the leptogenesis route keeping the $U(1)_{\rm B-L}$ symmetry intact. The $B-L$ global symmetry is then softly broken, providing the sub-eV neutrino masses. The model then incorporates a MeV scale sterile neutrino like dark matter.
We calculate the production rate of gauge-boson pairs at the SSC in a model with a ``hidden'' electroweak symmetry breaking sector. We show that the signal of electroweak symmetry breaking is lower than the background and that we cannot necessarily rely on gauge boson pairs as a signal of the dynamics of symmetry breaking.
We have studied the predictions for the LSP decay within the framework of a radiatively broken unified supergravity model without R-parity. Assuming that Higgs/slepton mixing is the only source of R-parity breaking and responsible for the observed neutrino oscillations we obtain predictons for the LSP life-time and branching fractions.
We formulate the equivalence between the longitudinal weak-boson and the Goldstone boson as a criterion for sensitively probing the electroweak symmetry breaking mechanism and develop a precise power counting rule for chiral Lagrangian formulated electroweak theories. With these we semi-quatitatively analyze the sensitivities to various effective operators related to electrowaeak symmetry breaking via weak-boson scatterings at the CERN Large Hadron Collider (LHC).
We discuss the link between the chiral symmetry of QCD and the numerical results of the light-front quark model, analyzing both the twist-2 and twist-3 distribution amplitudes of a pion. In particular, we exhibit that the pion twist-3 distribution amplitude can flip the sign of curvature as the cutoff scale for the transverse momentum gets lower than a typical scale of around 1 GeV.
We study a simple left-right symmetric (LRS) extension of the Zee model for neutrino mass generation. An extra $SU(2)_{L/R}$ singlet charged scalar helps in generating a loop-induced Majorana mass for neutrinos in this model. The right-handed neutrinos in this case are very light of the order of a few eV to a few MeV which makes this scenario quite different from other LRS models. We have analyzed the scalar potential and Higgs spectrum in detail, which also play an important role for the neutrino phenomenology. We identified the parameter regions in the model which satisfy the experimentally …
An important open question in fundamental physics concerns the nature of spacetime at distance scales associated with the Planck length. The widespread belief that probing such distances necessitates Planck-energy particles has impeded phenomenological and experimental research in this context. However, it has been realized that various theoretical approaches to underlying physics can accommodate Planck-scale violations of spacetime symmetries. This talk surveys the motivations for spacetime-symmetry research, the SME test framework, and experimental efforts in this field.
We discuss the possible signatures in the electroweak symmetry breaking sector by new strong dynamics at future hadron colliders such as the Tevatron upgrade, the LHC and VLHC, and $e^+e^-$ linear colliders. Examples include a heavy Higgs-like scalar resonance, a heavy Technicolor-like vector resonance and pseudo-Goldstone states, non-resonance signatures via enhanced gauge-boson scattering and fermion compositeness.
We calculate the production rate of gauge-boson pairs at $e^+e^-$ colliders in a model with a ``hidden'' electroweak symmetry breaking sector - i.e. one in which there are a large number of particles in the symmetry breaking sector other than the $W^{\pm}$ \thinspace and the $Z^0$. In such a model, the elastic $W^{\pm}$ \thinspace and $Z^0$ scattering amplitudes are small and structureless, i.e. lacking any discernable resonances, at all energies. We show that two gauge boson fusion signal of electroweak symmetry breaking is swamped by the background. Therefore, we cannot rely on gauge boson p…
Violated mirror symmetry (MS) is capable of reproducing observed qualitative properties of weak mixing for quarks and leptons. In violated MS, lepton phenomenology, that is, small neutrino masses and mixing properties different from those of quarks, requires the Dirac nature of neutrinos and existence of processes that change the total lepton number. Such processes involve heavy mirror neutrinos, and therefore occur at very high energies. CP non-conservation would mean here that the parity conserving MS Lagrangian must be non-invariant to both time reversal T and (according to the CPT theorem)…
We extend the Zee-Babu model introducing local $U(1)_{L_μ- L_τ}$ symmetry with several singly-charged bosons. We find a predictive neutrino mass texture in a simple hypothesis that mixings among singly-charged bosons are negligible. Also lepton flavor violations are less constrained compared with the original model. Then we explore testability of the model focussing on a doubly-charged boson physics at the LHC and the ILC.
A priori mixings of eigenstates in physical states are quantum mechanical effects well known in several realms of physics. The possibility that such effects are also present in particle physics, in the form of mixings that break flavor and parity symmetries, is studied. Applications to non-leptonic and weak radiative decays of hyperons are discussed. The results are very encouraging but should be improved by eventually including the W and Z contributions, assumed small (non-enhanced) in this work.
In this paper we investigate the phenomenology of the U(1) gauge symmetry for right-handed fermions, where three right-handed neutrinos are introduced for anomalies cancellation. Constraints on the new gauge boson $Z_{\mathbf{R}}$ arising from $Z-Z^\prime$ mixing as well as the upper bound of $Z^\prime$ production cross section in di-lepton channel at the LHC are presented. We further study the neutrino mass generation and the phenomenology of $Z_{\mathbf{R}}$-portal dark matter in this model. The lightest right-handed neutrino can be the cold dark matter candidate stabilized by a $Z_2$ flavor…
The model proposed by A. Zee (1986) and K.S. Babu (1988) is a simple radiative seesaw model, in which tiny neutrino masses are generated at the two-loop level. We investigate a supersymmetric extension of the Zee-Babu model under R-parity conservation. The lightest superpartner particle can then be a dark matter candidate. We find that the neutrino data can be reproduced with satisfying current data from lepton flavour violation even in the scenario where not all the superpartner particles are heavy. Phenomenology at the Large Hadron Collider is also discussed.
It is possible to construct models based on warped extra dimensions in which electroweak symmetry breaking takes place without the introduction of any Higgs fields. This breaking can occur through the judiciuous choice of boundary conditions applied to gauge fields living in the bulk. One then finds that the fifth components of these bulk fields act as the Goldstone bosons, even for the would-be zero modes of the Kaluza-Klein tower. In this talk I will discuss the phenomenology of such scenarios, in particular, the problems associated with the construction of realistic models due to the simult…
Zee model leads naturally to two heavy, strongly degenerate and almost maximally mixed neutrinos and to one light neutrino with small mixing. This pattern coincides with the one needed for a solution of the atmospheric neutrino problem by $\n_\m\A \n_τ$ oscillations and for existence of the two component hot dark matter in the Universe. Furthermore, the oscillations $\bar\n_\m\A \bar\n_e$ can be in the range of sensitivity of KARMEN, LSND experiments. Phenomenology of this scenario is considered and possibility to check it in the forthcoming experiments is discussed. Scenario implies large val…
We extend the Zee-Babu model for the neutrino masses and mixings by first incorporating a scalar dark matter $X$ with $Z_2$ symmetry and then $X$ and a dark scalar $φ$ with global $U(1)$ symmetry. In the latter scenario the singly and doubly charged scalars that are new in the Zee-Babu model can explain the large annihilation cross section of a dark matter pair into two photons as hinted by the recent analysis of the Fermi $γ$-ray space telescope data. These new scalars can also enhance the $B ( H \rightarrow γγ)$, as the recent LHC results may suggest. The dark matter relic density can be exp…
We modify the Zee mass matrix by adding a real one parameter perturbation which is purely diagonal and trace-less. We show that in this way we can explain both solar and atmospheric neutrino oscillation data. There is a correlation between the deviation from strict maximality of $|U_{μ3}|= 1/\sqrt{2}$, with the emergence of a small but non-zero $U_{e3}$. We calculate how big a value can $U_{e3}$ get when we restrict ourselves within the allowed regions of solar and atmospheric neutrino masses and mixing angles. We also discuss the impact of a $S_2$ permutation symmetry on our mass matrix and s…
Recently, a new class of realistic models for electroweak symmetry breaking have been constructed, without supersymmetry. These theories have naturally light Higgs bosons and perturbative new physics at the TeV scale. We describe these models in detail, and show that electroweak symmetry breaking can be triggered by a large top quark Yukawa coupling. A rich spectrum of particles is predicted, with a pair of light Higgs doublets accompanied by new light weak triplet and singlet scalars. The lightest of these new scalars is charged under a geometric discrete symmetry and is therefore stable, pro…
The processes proton + antiproton -> gamma + heavy-flavour jet(s) + X and proton + proton -> gamma + heavy-flavour jet(s) + X are studied in the framework of a special two-Higgs-doublet model, the MCPM. As distinguishing feature of this model we find that radiative Higgs-boson production and decay lead to heavy flavour c jets but no b jets in the above processes. Thus, the prediction is that b jets should be given by the normal QCD processes whereas for c jets an excess over the QCD expectation should occur. We present results both for Tevatron and LHC energies.
We discuss the phenomenology of doubly and singly charged Higgs bosons (of SU(2)_L-triplet fields) in the simplest A_4-symmetric version of the Higgs Triplet Model. Mass eigenstates of these Higgs bosons are obtained explicitly from the Higgs potential. It is shown that their decays into a pair of leptons have unique flavor structures which can be tested at the LHC if some of their masses are below the TeV scale. Sizable decay rates for tau to bar(mu) e e and tau to bar(e) mu mu can be obtained naturally while other tau to l l' l'', mu to bar(e) e e, and l to l' gamma are almost forbidden in t…
We calculate lepton flavor violating (LFV) Z decays Z \to {e_i^\pm}e_j^\mp (i, j = e, μ, τ; i\neq j) in the Zee model keeping in view the radiative leptonic decays e_i\to e_jγ(i = μ, τ; j = e, μ; i\neq j), μdecay and anomalous muon magnetic moment (μAMM). We investigate three different cases of Zee f_{ij} coupling (A) f_{eμ}^2 = f_{μτ}^2= f_{τe}^2, (B) f_{eμ}^2 \gg f_{τe}^2 \gg f_{μτ}^2, and (C) f_{μτ}^2 \gg f_{eμ}^2 \gg f_{τe}^2 subject to the neutrino phenomenology. Interestingly, we find that, although the case (C) satisfies the large excess value of μAMM, however, it is unable to explain t…
We extend the Zee model by introducing a vector-like lepton doublet and a flavor dependent global $U(1)$ symmetry. Flavor changing neutral currents in the quark sector can be naturally forbidden at tree level due to the $U(1)$ symmetry, while sufficient amount of lepton flavor violation is provided to explain current neutrino oscillation data. In our model, additional sources of CP-violation appear in the lepton sector, but their contribution to electric dipole moments is much smaller than current experimental bounds due to the Yukawa structure constrained by the $U(1)$ symmetry. We find that …
We study the dynamics of electroweak symmetry-breaking in an extension of the Standard Model where the Higgs sector is augmented by the addition of a real (Y = 0) isospin triplet. We show that this scenario exhibits a novel, two-step electroweak phase transition, wherein the first step provides the strongly first order transition as required for electroweak baryogenesis followed by a second step to the Standard Model Higgs phase that also admits a cold dark matter candidate. We analyze the constraints on this scenario from recent results obtained at the Large Hadron Collider for the Higgs diph…
We study a realization of the topology of the Zee model for the generation of neutrino masses at one-loop with a minimal set of vector-like fermions. After imposing an exact $Z_2$ symmetry to avoid tree-level Higgs-mediated flavor changing neutral currents, one dark matter candidate is obtained from the subjacent inert doublet model, but with the presence of new co-annihilating particles. We show that the model is consistent with the constraints coming from lepton flavor violation processes, oblique parameters, dark matter and neutrino oscillation data.
We review and compare theoretically and phenomenologically a number of possible family symmetries, which when combined with unification, could be important in explaining quark, lepton and neutrino masses and mixings, providing new results in several cases. Theoretical possibilities include Abelian or non-Abelian, symmetric or non symmetric Yukawa matrices, Grand Unification or not. Our main focus is on anomaly-free U(1) family symmetry combined with SU(5) unification, although we also discuss other possibilities. We provide a detailed phenomenological fit of the fermion masses and mixings for …
We consider a conformal complex singlet extension of the Standard Model with a Higgs portal interaction. Two different scenarios depending on whether the global U(1) symmetry is broken or not have been studied. In the unbroken phase, the decay of the complex singlet is protected by the global U(1) symmetry which leads to an ideal cold dark matter candidate. In the broken phase, we are able to provide a second Higgs at $554\,\rm{GeV}$. In addition, gauging the global U(1) symmetry, we can construct an asymptotically safe U(1)' leptophobic model. We combine the notion of asymptotic safety with c…
We propose a generalized Zee-Babu model with a global $U(1)$ B-L symmetry, in which we classify the model in terms of the number of the hypercharge $N/2$ of the isospin doublet exotic charged fermions. Corresponding to each of the number of $N$, we need to introduce some multiply charged bosons in order to make the exotic fields decay into the standard model fields. We also discuss the muon anomalous magnetic moment and the diphoton excess depending on $N$, and we show what kind of models are in favor of these phenomenologies.
Evidence for neutrino oscillations is pointing to the existence of tiny but finite neutrino masses. Such masses may be naturally generated via radiative corrections in models such as the Zee model where a singlet Zee-scalar plays a key role. We minimally extend the Zee model by including a right-handed singlet neutrino ν_R. The radiative Zee-mechanism can be protected by a simple U(1)_X symmetry involving only the ν_R and a Zee-scalar. We further construct a class of models with a single horizontal U(1)_FN (a la Frogatt-Nielsen) such that the mass patterns of the neutrinos and leptons are natu…
The search for the weakly-coupled Higgs sector at future colliders consists of three phases: discovery of a Higgs candidate, verification of the Higgs interpretation of the signal, and precision measurements of Higgs sector properties. The discovery of one Higgs boson with Standard Model properties is not sufficient to expose the underlying structure of the electroweak symmetry breaking dynamics. It is critical to search for evidence for a non-minimal Higgs sector and/or new physics associated with electroweak symmetry breaking dynamics.
A sterile neutrino with a mass of a few keV can play the role of a warm dark matter(DM). This can be realized in seesaw models with 3 left- and 3 right-handed neutrinos. It is possible to identify the keV neutrino to be one of the right-handed neutrinos leaving the other two to be much more heavier, the $ν$SM model. We show that with this realization of keV neutrino DM, the model has an approximate Friedberg-Lee symmetry providing a natural explanation for the lightness of the right-handed neutrino. We also find that in this model the mixing parameters couple light and heavy neutrinos are stro…
We extend the colored Zee-Babu model with a gauged $U(1)_{B-L}$ symmetry and a scalar singlet dark matter (DM) candidate $S$. The spontaneous breaking of $U(1)_{B-L}$ leaves a residual $Z_2$ symmetry that stabilizes the DM and generates tiny neutrino mass at the two-loop level with the color seesaw mechanism. After investigating dark matter and flavor phenomenology of this model systematically, we further focus on its imprint on two of cosmic-ray anomalies: the Fermi-LAT gamma-ray excess at the Galactic Center (GCE) and the PeV ultra-high energy (UHE) neutrino events at the IceCube. We found t…
We present a TeV scale seesaw mechanism for exploring the dark matter and neutrino phenomenology in the light of recent neutrino and cosmology data. A different realization of the Inverse seesaw(ISS) mechanism with $A_{4}$ flavor symmetry is being implemented as a leading contribution to the light neutrino mass matrix which usually gives rise to vanishing reactor mixing angle $θ_{13}$. Using a non-diagonal form of Dirac neutrino mass matrix and $3σ$ values of mass square differences we parameterize the neutrino mass matrix in terms of Dirac Yukawa coupling "$y$". We then use type II seesaw as …
We know $L_e-L_μ-L_τ$ symmetry gives $m^2_1= m^2_2 >> m^2_3$ pattern in Zee model. $Δm^2_\odot$ emerges from a small breaking of this symmetry. Because this symmetry is broken very weakly $θ_\odot$ does not deviate much from $\tan^2 θ_\odot=1$ which is its value in the symmetric limit. This gives a mismatch with LMA solution where mixing is large but not exactly maximal. We confront this property of Zee mass matrix by phenomenologically analyzing recent results from solar and atmospheric neutrino oscillation experiments at various confidence levels. We conclude that LOW type solution is compat…
We study a model of neutrino within the framework of the minimal extended seesaw (MES), which plays an important role in active and sterile neutrino phenomenology in (3+1) scheme. The $A_4$ flavor symmetry is augmented by an additional $Z_4\times Z_3$ symmetry to constraint the Yukawa Lagrangian of the model. We use non-trivial Dirac mass matrix, with broken $μ-τ$ symmetry, as the origin of leptonic mixing. Interestingly, such a structure of mixing naturally leads to the non-zero reactor mixing angle $θ_{13}$. Non-degenerate mass structure for right-handed neutrino $M_R$ is considered so that …
In the simplest examples of models with a discrete quark-lepton symmetry, an electroweak symmetry breaking sector with more than one Higgs doublet is necessary to obtain the correct mass relations between quarks and leptons. A two Higgs doublet model has flavour-nonconserving Yukawa couplings, which are proportional to the masses of the quark-lepton symmetric partners of the fermions. We describe how flavour changing leptonic decays can occur, with branching ratios not far beyond that currently measureable, enabling investigation of the phenomenology of such models.
We study the possibility of simultaneously addressing neutrino phenomenology and the dark matter in the framework of inverse seesaw. The model is the extension of the standard model by the addition of two right handed neutrinos and three sterile fermions which leads to a light sterile state with the mass in the keV range along with three light active neutrino states. The lightest sterile neutrino can account for a feasible dark matter(DM) candidate. We present a $S_{4}$ flavor symmetric model which is further augmented by $Z_{4}\times Z_{3}$ symmetry to constrain the Yukawa Lagrangian. The str…
We explore the phenomenology of an $SU(2)$-singlet doubly charged scalar at the high and low energy frontier. Such a particle is predicted in different new physics models, like left-right symmetric models or the Zee-Babu model. Nonetheless, since its interactions with Standard Model (SM) leptons are gauge invariant, it can be consistently studied as a UV complete SM extension. Its signatures range from same-sign di-lepton pairs to flavour changing decays of charged leptons to muonium-antimuonium oscillations. In this article, we use a systematic effective-field-theory approach for studying the…
We present a five dimensional supersymmetric SO(10) model compactified on an orbifold $S^{(1)}/Z_2\tm Z_2'$. The gauge symmetry $G_{422}\equiv SU(4)_c\tm SU(2)_L\tm SU(2)_R$, realized on one of the fixed points (branes), is spontaneously broken to the MSSM via the higgs mechanism. Employing a flavor ${\cal U}(1)$ symmetry and suitably extending the 'matter' sector enables us to understand large mixings in the neutrino sector via a democratic approach, versus the small CKM mixings. A residual ${\cal R}$-symmetry on $G_{422}$ brane helps eliminate the troublesome dimension five nucleon decay, wh…
It has been shown that tribimaximal mixing can be obtained by some particular breaking pattern of the $A_4$ symmetry, wherein the extra $A_4$ triplet Higgs scalars pick up certain fixed vacuum expectation value (VEV) alignments. We have performed a detailed analysis of the different possible neutrino mass matrices within the framework of the $A_4$ model. We take into account all possible singlet and triplet Higgs scalars which leave the Lagrangian invariant under $A_4$. We break $A_4$ spontaneously, allowing the Higgs to take any VEV in general. We show that the neutrino mixing matrix deviates…
We formulate general conditions on $3\times 3$ neutrino mass matrices under which a degenerate pair of neutrinos at a high scale would split at low scale by radiative corrections involving only the standard model fields. This generalizes the original observations of Wolfenstein on pseudo Dirac neutrinos to three generations. A specific model involving partially broken discrete symmetry and solving the solar and atmospheric anomalies is proposed. The symmetry pattern of the model naturally generates two large angles one of which can account for the large angle MSW solution to the solar neutrino…
