We consider a neutrino mass generating model which employs a scalar leptoquark, $Δ$, and a scalar diquark, $S$. The new scalars $Δ$ and $S$ carry the standard model $SU(3)_c\times SU(2)_L\times U(1)_Y$ quantum numbers $(3,1,-1/3)$ and $(6,1,-2/3)$, respectively. The neutrino masses are generated at the two-loop level, as in the Zee-Babu model\cite{Zee-Babu}, and $Δ/S$ plays the role of the doubly/singly charged scalar in the Zee-Babu model. With a moderate working assumption that the magnitudes of the six Yukawa couplings between $S$ and the down-type quarks are of the same order, strong connections are found between the neutrino masses and the charged lepton flavor violating processes. In particular, we study $Z\rightarrow \overline{l} l'$, and $l\rightarrow l' γ$ and find that some portions of the parameter space of this model are within the reach of the planned charged lepton flavor violating experiments. Interesting lower bounds are predicted that $B(Z\rightarrow \overline{l} l')\gtrsim 10^{-16}-10^{-14}(10^{-14})\times(1\mbox{TeV}\cdot m_S/7 m_Δ^2)^2$ and $B(l\rightarrow l' γ)\gtrsim 10^{-17}-10^{-16}(10^{-18}-10^{-16})\times(1 \mbox{TeV}\cdot m_S/7 m_Δ^2)^2$ for neutrino masses being the normal(inverted) hierarchical pattern. The type of neutrino mass hierarchy could also be determined by measuring the charged lepton flavor violating double ratios. Moreover, definite leptoquark decay branching ratios are predicted when there is no Yukawa interaction between the right-handed fermions and $Δ$ ( the branching fraction of $Δ$ to a charged lepton and a quark is 50\%), which could help refine the collider search limit on the scalar leptoquark mass.
We show that the minimal 3-3-1 model cannot accommodate the neutrino masses at tree level using present experimental data. Nevertheless, a modified Zee and the Zee-Babu mechanisms for generating neutrino masses at 1-loop and 2-loop, respectively, are automatically implemented in the minimal 3-3-1 model, without introducing new degrees of freedom to the model. We also present a systematic method for finding solutions to the leptonic sector masses and mixing. As a case study, we accommodate the charged and neutral leptons masses and the PMNS matrix in the 1-loop modified Zee mechanism contained …
We show that the neutrino mass matrix of the Zee-Babu model is able to fit the recent data on neutrino masses and mixing with non-zero $θ_{13}$ in the inverted neutrino mass hierarchy. The results show that the Majorana phases are equal to zero and the Dirac phase ($\de$) is predicted to either $0$ or $π$, i. e, there is no CP violation in the Zee-Babu model at the two loop level. The effective mass governing neutrinoless double beta decay and the sum of neutrino masses are consistent with the recent analysis.
We study the neutrinoless double beta decay in the colored Zee-Babu model. We consider three cases of the colored Zee-Babu model with a leptoquark and a diquark introduced. The neutrino masses are generated at two-loop level, and the constraints given by tree-level flavor violation processes and muon anomalous magnetic moment $(g-2)_μ$ have been considered. In our numerical analysis, we find that the standard light neutrino exchange contribution can be canceled by new physics contribution under certain assumption and condition, leading to a hidden neutrinoless double beta decay. The condition …
We study the extended colored Zee-Babu model introducing a vector-like quark and singlet scalar. The active neutrino mass matrix and muon anomalous magnetic moment are analyzed, which can be fitted to experimental data satisfying the constraints from flavor changing neutral current. Then we discuss signature of our model via vector-like quark production. In addition, the diphoton excess can be explained with the contribution from vector-like quark
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.
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.
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.
We demonstrate that the extension of the Zee-Babu model can generate not only the small neutrino masses but also the baryon number asymmetry in the universe. In particular, we show that the scale of the singlet scalar responsible for the leptogenesis can be of order 1 TeV, that can be tested at the LHC and ILC. We also considered the possible minimal extension of this model to generate the dark matter.
The exact solution for the neutrino mass matrix of the Zee-Babu model is derived. Tribimaximal mixing imposes conditions on the Yukawa couplings, from which the normal mass hierarchy is preferred. The derived conditions give a possibility of Majorana maximal $\mathrm{CP}$ violation in the neutrino sector. We have shown that non-zero $θ_{13}$ is generated if Yukawa couplings between leptons almost equal to each other. The model gives some regions of the parameters where neutrino mixing angles and the normal neutrino mass hierarchy obtained consistent with the recent experimental data.
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 propose a two-loop induced Zee-Babu type neutrino mass model at the TeV scale. Although there is no dark matter candidate in the original Zee-Babu model, that is contained in our model by introducing an unbroken discrete $Z_2$ symmetry. The discrepancy between the experimental value of the muon anomalous magnetic moment (muon $g-2$) and its prediction in the standard model can be explained by contributions from additional vector-like charged-leptons which are necessary to give non-zero neutrino masses. The mass of vector-like leptons to be slightly above 300 GeV is favored and allowed from …
We provide an updated scan of the allowed parameter space of the two-loop Zee-Babu model for neutrino mass. Taking into account most recent experimental data on $μ\to eγ$ as well as the mixing angle $θ_{13}$ we obtain lower bounds on the masses of the singly and doubly charged scalars of between 1 to 2 TeV, with some dependence on perturbativity and fine-tuning requirements. This makes the scalars difficult to observe at LHC with 14 TeV even with optimistic assumptions on the luminosity, and would require a multi-TeV linear collider to see the scalar resonances. We point out, however, that a s…
We discuss how the Zee-Babu model can be tested combining information from neutrino data, low-energy experiments and direct searches at the LHC. We update previous analysis in the light of the recent measurement of the neutrino mixing angle $θ_{13}$, the new MEG limits on $μ\rightarrow e γ$, the lower bounds on doubly-charged scalars coming from LHC data, and, of course, the discovery of a 125 GeV Higgs boson by ATLAS and CMS. In particular, we find that the new singly- and doubly-charged scalars are accessible at the second run of the LHC, yielding different signatures depending on the neutri…
We consider the baryogenesis picture in the Zee-Babu model. Our analysis shows that electroweak phase transition (EWPT) in the model is a first-order phase transition at the $100$ GeV scale, its strength ranges from 1 to 4.15 and the masses of charged Higgs boson are smaller than $300$ GeV. The EWPT is strengthened by only the new bosons and this strength is enhanced by arbitrary $ξ$ gauge. However, the $ξ$ gauge does not break the first-order EWPT or, in other words, the $ξ$ gauge is not the cause of the EWPT. This leads to the fact that the calculation of EWPT in Landau gauge is enough; and …
We analyze the viability of the Zee-Babu model as an explanation of observed neutrino masses and mixings and the possibility that the model is confirmed or discarded in experiments planned for the very close future. The allowed parameter space is studied analytically by using some approximations and partial data. Then, a complete scanning of all parameters and constraints is performed numerically by using Monte Carlo methods. The cleanest signal of the model will be the detection of the doubly charged scalar at the LHC and its correlation with measurements of the branching ratio of $μ\to eγ$ a…
The smallness of the observed neutrino masses might have a radiative origin. Here we revisit a specific two-loop model of neutrino mass, independently proposed by Babu and Zee. We point out that current constraints from neutrino data can be used to derive strict lower limits on the branching ratio of flavour changing charged lepton decays, such as $μ\to e γ$. Non-observation of Br($μ\to e γ$) at the level of $10^{-13}$ would rule out singly charged scalar masses smaller than 590 GeV (5.04 TeV) in case of normal (inverse) neutrino mass hierarchy. Conversely, decay branching ratios of the non-st…
We investigate non-standard neutrino interactions (NSIs) in the Zee--Babu model. The size of NSIs predicted by this model is obtained from a full scan over the parameter space, taking into account constraints from low-energy experiments such as searches for lepton flavor violation (LFV) and the requirement to obtain a viable neutrino mass matrix. The dependence on the scale of new physics as well as on the type of the neutrino mass hierarchy is discussed. We find that NSIs at the source of a future neutrino factory may be at an observable level in the nu_e to nu_tau and/or nu_mu to nu_tau chan…
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.
We consider the feasibility of interpreting the $R_{K^{(*)}}$ anomaly in the colored Zee-Babu model. The model generates neutrino masses at two loops with the help of a scalar leptoquark $S\sim(3,3,-\frac{1}{3})$ and a scalar diquark $ω\sim(6,1,-\frac{2}{3})$, and contributes to the transition $b\to s\ell\ell$ via the exchange of a leptoquark $S$ at tree level. Under constraints from lepton flavor violating (LFV) and flavor changing neutral current (FCNC) processes, and direct collider searches for heavy particles, we acquire certain parameter space that can accommodate the $R_{K^{(*)}}$ anoma…
Contrary to the see-saw models, extended Higgs sectors leading to radiatively-induced neutrino masses do require the extra particles to be at the TeV scale. However, these new states have often exotic decays, to which experimental LHC searches performed so far, focused on scalars decaying into pairs of same-sign leptons, are not sensitive. In this paper we show that their experimental signatures can start to be tested with current LHC data if dedicated multi-region analyses correlating different observables are used. We also provide high-accuracy estimations of the complicated Standard Model b…
We update previous analyses of the Zee-Babu model in the light of new data, e.g., the mixing angle $θ_{13}$, the rare decay $μ\to e γ$ and the LHC results. We also analyse the possibility of accommodating the deviations in $Γ(H\to γγ)$ hinted by the LHC experiments, and the stability of the scalar potential. We find that neutrino oscillation data and low energy constraints are still compatible with masses of the extra charged scalars accessible to LHC. Moreover, if any of them is discovered, the model can be falsified by combining the information on the singly and doubly charged scalar decay m…
Although recent Higgs data from ATLAS and CMS are compatible with a Standard Model (SM) signal at $2σ$ level, both experiments see indications for an excess in the diphoton decay channel, which points to new physics beyond the SM. Given such a low Higgs mass $m_H \sim 125 {\rm GeV}$, another sign indicating the existence of new physics beyond the SM is the vacuum stability problem, i.e., the SM Higgs quartic coupling may run to negative values at a scale below the Planck scale. In this paper, we study the vacuum stability and enhanced Higgs diphoton decay rate in the Zee-Babu model, which was …
We consider a local $U(1)_{B-L}$ extension of Zee-Babu model to explain the recently observed 3.5 keV X-ray line signal. The model has three Standard model (SM)-singlet Dirac fermions with different $U(1)_{B-L}$ charges. A complex scalar field charged under $U(1)_{B-L}$ is introduced to break the $U(1)_{B-L}$ symmetry. After $U(1)_{B-L}$ symmetry breaking a remnant discrete symmetry stabilizes the lightest state of the Dirac fermions, which can be a stable dark matter (DM). The second lightest state, if mass splitting with the stable DM is about 3.5 keV, decays dominantly to the stable DM and …
To explain the matter-antimatter asymmetry in the Zee-Babu (ZB) model, the sphaleron process in the baryogenesis scenario is calculated. It always satisfies the de-coupling condition and the strength of phase transition ($S$) is always greater than $1$ in the presence of triggers other than that in the Standard Model, which are singly ($h^{\pm}$) and doubly ($k^{\pm\pm}$) charged scalar bosons. Sphaleron energies are in the range of 5-10 TeV, in calculation with bubble profiles containing free parameters and assuming nuclear bubbles of $h^{\pm}$ and $k^{\pm\pm}$ are very small. We tested the s…
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.
Motivated by searches for so-called leptonic scalars at the LHC and the recent measurement of the $W$ boson's mass at the Tevatron, we revisit the phenomenology of the Zee-Babu model for neutrino masses and the ability to differentiate it from the Type II Seesaw model at the LHC. We conclude that this task is much more difficult than previously believed. All inputs equal in the two scenarios, we find that total and differential rates for producing pairs of doubly and singly charged scalars are identical in shape and only differ in normalization. The normalization is given by the ratio of hadro…
Though models with the radiative neutrino mass generation are phenomenologically attractive, the complicated relationship between the flavour structure of additional Yukawa matrices and the neutrino mass matrix sometimes is a barrier to explore the models. We introduce a simple prescription to analyze the relation in a class of models with the asymmetric Yukawa structure. We then apply the treatment to the Zee-Babu model as a concrete example of the class and discuss the phenomenological consequences of the model. The combined studies among the neutrino physics, the lepton flavour violation, a…
In this paper, we interpret the 750 GeV diphoton excess in the Zee-Babu extension of the two-Higgs-doublet model by introducing a top partner ($T$)/bottom partner ($B$). In the alignment limit, the 750 GeV resonance is identified as the heavy CP-even Higgs boson ($H$), which can be sizably produced via the QCD process $pp \to T\bar{T}$ or $pp \to B\bar{B}$ followed by the decay $T\to Ht$ or $B \to Hb$. The diphoton decay rate of $H$ is greatly enhanced by the charged singlet scalars predicted in the Zee-Babu extension and the total width of $H$ can be as large as 7 GeV. Under the current LHC c…
The present work introduces two possible extensions of the Standard Model Higgs sector. In the first case, the Zee-Babu type model for the generation of neutrino mass is augmented with a scalar triplet and additional singly charged scalar singlets. The second scenario, on the other hand, generalizes the Type-II seesaw model by replicating the number of the scalar triplets. A $\mathbb{Z}_3$ symmetry is imposed in case of both the scenarios, but, allowed to be violated by terms of mass dimension two and three for generating neutrino masses and mixings. We examine how the models so introduced can…
We suggest new setup where SUSY breaking spurion F-term possesses lepton number. This setup not only modifies sparticle mass spectra but also realizes several new models, where neutrino mass is naturally induced through radiative corrections. We here suggest two new models; the first one is (i): pseudo-Dirac/Schizophrenic neutrino model, and the second one is (ii): pure Majorana neutrino model. We will also show this setup can naturally apply to the supersymmetric Zee-Babu model.
At present, there are outstanding discrepancies between standard model predictions and measurements of the muon's $g-2$ and several $B$-meson properties. We resolve these anomalies by considering a two-Higgs-doublet model extended to include leptoquarks and a dark Higgs boson $S$. The leptoquarks modify $B$-meson decays and also induce an $S γγ$ coupling, which contributes to the muon's $g-2$ through a Barr-Zee diagram. We show that, for TeV-scale leptoquarks and dark Higgs boson masses $m_{S} \sim 10-200~\text{MeV}$, a consistent resolution to all of the anomalies exists. The model predicts i…
New physics in the lepton sector may account for neutrino masses, affect electroweak precision observables, induce charged-lepton flavour violation, and shift dipole moments. The low-energy predictions of different models are most conveniently compared within the formalism of effective field theory. To illustrate the benefits of this approach, we derive the Wilson coefficients for a set of representative models: the fermionic seesaw mechanisms (type I and III), the Zee model, and a minimal leptoquark model. In each case, the Weinberg and the dipole operators have qualitatively different origin…
We discussed how neutrino masses and oscillations are radiatively generated in an $SU(3)_L$ $\times$ $U(1)_N$ gauge model with a symmetry based on $L_e-L_μ-L_τ$ ($\equiv$$L^\prime$). The model is characterized by lepton triplets $ψ^i=(ν^i,\ell^{-i},E^{-i})$, where $E^{-i}$ are negatively charged heavy leptons, an $SU(3)_L$ triplet Higgs scalar $ξ$ and a singlet Higgs scalar $k^{++}$. These Higgs scalars can be interpreted as a Zee's and Zee-Babu's scalar for radiative mechanisms. We demonstrated that the mass hierarchy of $Δm_{atm}^2$ $\gg$ $Δm_\odot^2$ arise as a consequence of the dynamical …
Lepton Number Violation at the LHC in Radiative Neutrino Mass Models with Leptoquarks
Oklahoma State University（俄克拉荷马州立大学）；The University of Tokyo（东京大学）；Kavli Institute for the Physics and Mathematics of the Universe (WPI)（Kavli宇宙物理与数学研究所（WPI））
We explore the possibility to explain a bosonic dark matter candidate with a gauge singlet inside the loop to generate the neutrino mass matrix at two-loop level. The mass matrix is suppressed by a small mixing that comes from the bound on {direct detection experiments of the dark matter, and equivalent of the three-loop neutrino model due to the small mixing between neutral inert bosons. Here, our setup is} the Zee-Babu type scenario with $Z_3$ discrete symmetry, in which we consider the neutrino oscillation data, lepton flavor violations, muon $g-2$, $μ-e$ conversion rate, lepton flavor-chan…
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 investigate the possibility of using neutrino trident production to probe leptophilic charged scalars at future high intensity neutrino experiments. We show that under specific assumptions, this production process can provide competitive sensitivity for generic charged scalars as compared to common existing bounds. We also investigate how the recently proposed mixed-flavour production - where the two oppositely charged leptons in the final state need not be muon flavoured - can give a 20-50% increase in sensitivity for certain configurations of new physics couplings as compared to tradition…
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 consider an extension of Zee-Babu model to explain the smallness of neutrino masses. (1) We extend the lepton number symmetry of the original model to local $B-L$ symmetry. (2) We introduce three Dirac dark matter candidates with flavor-dependent $B-L$ charges. After the spontaneous breaking of $B-L$, a discrete symmetry $Z_6$ remains, which guarantees the stability of dark matter. Then the model can explain the 3.5 keV X-ray line signal with decaying dark matter. We also introduce a real scalar field which is singlet under both the SM and $U(1)_{B-L}$ and can explain the current relic abun…
We review how the muon anomalous magnetic moment ($g-2$) and the quest for lepton flavor violation are intimately correlated. Indeed the decay $μ\to e γ$ is induced by the same amplitude for different choices of in- and outgoing leptons. In this work, we try to address some intriguing questions such as: Which hierarchy in the charged lepton sector one should have in order to reconcile possible signals coming simultaneously from $g-2$ and lepton flavor violation? What can we learn if the $g-2$ anomaly is confirmed by the upcoming flagship experiments at FERMILAB and J-PARC, and no signal is see…
We study the detection prospects of relatively light charged scalars in radiative Majorana neutrino mass models, such as the Zee model and its variants using scalar leptoquarks, at current and future neutrino telescopes. In particular, we show that these scalar mediators can give rise to Glashow-like resonance features in the ultra-high energy neutrino (UHE) event spectrum at the IceCube neutrino observatory and its high-energy upgrade IceCube-Gen2. The same scalars can also give rise to observable non-standard neutrino interactions (NSI), and we show that the UHE neutrinos provide a complemen…
Muonium is a bound state composed of an antimuon and an electron, and it constitutes a hydrogen-like atom. Because of the absence of the hadronic matter in the bound state, the muonium is a useful probe to explore new physics being free from the hadronic uncertainties. The process of the muonium-to-antimuonium transition is considered to be effective to identify fundamental interactions which relate to the lepton flavor and lepton number violation. New experiments are being planned at J-PARC in Japan and CSNS in China, and it is expected to attract more attention in the near future. In this pa…
Models of radiative Majorana neutrino masses require new scalars and/or fermions to induce lepton number violating interactions. We show that these new particles also generate observable neutrino nonstandard interactions (NSI) with matter. We classify radiative models as type-I or II, with type-I models containing at least one Standard Model (SM) particle inside the loop diagram generating neutrino mass, and type-II models having no SM particle inside the loop. While type-II radiative models do not generate tree-level NSI, popular models which fall under the type-I category are shown, somewhat…
