We address the impact of sterile fermions on charged lepton electric dipole moments. We show that in order to have a non-vanishing contribution to electric dipole moments, the minimal extension necessitates the addition of at least two sterile fermion states. Sterile neutrinos can give significant contributions to the charged lepton electric dipole moments if the masses of the non-degenerate sterile states are both above the electroweak scale. In addition, the Majorana nature of neutrinos is also important. Furthermore, we apply the computations of the electric dipole moments for the most minimal realisation of the Inverse Seesaw mechanism, in which the Standard Model is extended by two right-handed neutrinos and two sterile fermion states. We show that the two pairs of (heavy) pseudo-Dirac mass eigenstates can give significant contributions to the electron electric dipole moment, lying close to future experimental sensitivity. We further discuss the possibility of beyond the minimal Inverse Seesaw models and of having a successful leptogenesis in this framework.
We investigate the sensitivity of future lepton colliders to displaced vertices from the decays of long-lived heavy (almost sterile) neutrinos with electroweak scale masses and detectable time of flight. As future lepton colliders we consider the FCC-ee, the CEPC, and the ILC, searching at the Z-pole and at the center-of-mass energies of 240, 350 and 500 GeV. For a realistic discussion of the detector response to the displaced vertex signal and the Standard Model background we consider the ILC's Silicon Detector (SiD) as benchmark for the future lepton collider detectors. We find that displace…
We study the signal of long-lived sterile neutrino at the LHC produced through the decay of the $W$ boson. It decays into charged lepton and jets. The characteristic signature is a hard prompt lepton and a lepton from the displaced decay of the sterile neutrino, which leads to a bundle of displaced tracks with large transverse impact parameter. Different from other studies, we neither reconstruct the displaced vertex nor place requirement on its invariant mass to maintain sensitivity for low sterile neutrino masses. Instead, we focus on the displaced track from the lepton. A difficulty for low…
Extending the Standard Model (SM) with sterile ('right-handed') neutrinos is one of the best motivated ways to account for the observed neutrino masses. We discuss the expected sensitivity of future lepton collider experiments for probing such extensions. An interesting testable scenario is given by 'symmetry protected seesaw models', which theoretically allow for sterile neutrino masses around the electroweak scale with up to order one mixings with the light (SM) neutrinos. In addition to indirect tests, e.g. via electroweak precision observables, sterile neutrinos with masses around the elec…
An anomaly-free U(1) gauge extension of the standard model (SM) is presented. Only one Higgs doublet with a nonzero vacuum expectation is required as in the SM. New fermions and scalars as well as all SM particles transform nontrivially under this U(1), resulting in a model of three active neutrinos and one sterile neutrino, all acquiring radiative masses. Charged-lepton masses are also radiative as well as the mixing between active and sterile neutrinos. At the same time, a residual $Z_2$ symmetry of the U(1) gauge symmetry remains exact, allowing for the existence of dark matter.
We study possible contributions of heavy sterile neutrinos $ν_h$ to the decays $τ^-\to e^{\pm}(μ^{\pm})π^{\mp}π^-$. From the experimental upper bounds on their rates we derive new constraints on the $ν_h-ν_τ$ mixing in the mass region $140.5{MeV}\leq m_{ν_h}\leq 1637$ MeV. We discuss cosmological and astrophysical status of $ν_h$ in this mass region and compare our constraints with those recently derived by the NOMAD collaboration.
We investigate lepton-number-violating decays $K^\mp \rightarrow π^\pm l^\mp l^\mp$ in the presence of sterile neutrinos. We consider minimal interactions with Standard-Model fields through Yukawa couplings as well as higher-dimensional operators in the framework of the neutrino-extended Standard Model Effective Field Theory. We use $SU(3)$ chiral perturbation theory to match to mesonic interactions and compute the lepton-number-violating decay rate in terms of the neutrino masses and the Wilson coefficients of higher-dimensional operators. For neutrinos that can be produced on-shell, the deca…
We consider a tree-level enhancement to the violation of lepton flavour universality in light meson decays arising from modified $W \ell ν$ couplings in the standard model minimally extended by sterile neutrinos. Due to the presence of additional mixings between the active (left-handed) neutrinos and the new sterile states, the deviation from unitarity of the leptonic mixing matrix intervening in charged currents might lead to a tree-level enhancement of $R_{P} = Γ(P \to e ν) / Γ(P \to μν)$, with $P=K, π$. We illustrate these enhancements in the case of the inverse seesaw model, showing that o…
We review the detection of the Delta L=2 semileptonic meson decays of the form M(+) -> M'(-) lepton(+) lepton(-) that we studied in a previous work, mediated by a Majorana neutrino with a mass in the range between the masses of the initial and final mesons. For such range of masses, the Majorana neutrino will go on its mass shell and the two charged leptons will appear at displaced vertices or, if the lifetime is long enough, most secondary vertices will fall outside the detector. We study the consequences of this effect on the experimental searches and limits that can be extracted.
A sterile neutrino at GeV mass scale is of particular interest in this work. Though not take part in neutrino oscillation, the sterile neutrino can induce flavor violating semileptonic and leptonic decay of $K, D$ and $B$ mesons. We calculated a box diagram contribution in these processes. By making use of current experiment limit of lepton flavor violating decays $M^+_h\to M_l^+\ell_1^+\ell_2^-$ and $M^0\to \ell_1^-\ell_2^+$, we explore the allowed parameter space of $U_{e4}$ and $U_{μ4}$ in different mass ranges. Generally speaking, both channels give a limit to the product of $U_{e4}$ and $…
It has been proposed that two resonances could coincide in the early universe at temperatures $T \sim 0.2 ... 0.5$ GeV: one between two nearly degenerate GeV-scale sterile neutrinos, producing a large lepton asymmetry through freeze-out and decays; another between medium-modified active neutrinos and keV-scale sterile neutrinos, converting the lepton asymmetry into dark matter. Making use of a framework which tracks three sterile neutrinos of both helicities as well as three separate lepton asymmetries, and scanning the parameter space of the GeV-scale species, we establish the degree of fine-…
We determine the sensitivities of short-baseline coherent elastic neutrino-nucleus scattering (CE$ν$NS) experiments using a pion decay at rest neutrino source as a probe for nonunitarity in the lepton sector, as expected in low-scale type-I seesaw schemes. We also identify the best configuration for probing light sterile neutrinos at future ton-scale liquid argon CE$ν$NS experiments, estimating the projected sensitivities on the sterile neutrino parameters. Possible experimental setups at the Spallation Neutron Source, Lujan facility and the European Spallation Source are discussed. Provided t…
We address the impact of sterile fermion states on the anomalous magnetic moment of charged leptons, as well as their contribution to neutrinoless double beta decays. We illustrate our results in a minimal, effective extension of the Standard Model by one sterile fermion state, and in a well-motivated framework of neutrino mass generation, embedding the Inverse Seesaw into the Standard Model. The simple "3+1" effective case succeeds in alleviating the tension related to the muon anomalous magnetic moment, albeit only at the 3$σ$ level, and for light sterile states (corresponding to a }cosmolog…
We assume that the mass matrix of active neutrinos has hierarchical form with small mixings, similar to quark mass matrix. We show that the large mixings between $ν_μ$ and $ν_τ$ as well as between $ν_e$ and certain combination of $ν_μ$ and $ν_τ$ required by the present data can appear due to the presence of a sterile neutrino state. Two realizations of this possibility are considered: (i) Large flavor mixing appears as a result of sterile neutrino ``decoupling'' ($m_{ss} \gg m_a$), so that the active-sterile mixings are negligible. (ii) Sterile neutrino has a mass of O(1) eV and its mixing wit…
It is suggested that there is an underlying symmetry which relates the quark and lepton sectors. Namely, among the mixing matrix elements of CKM for quarks and PMNS for leptons there exist complementarity relations at a high energy scale (such as the see-saw or even the GUT scales). Assuming that the relations would remain during the matrix elements running down to the EW scale. Observable breaking of the rational relation is attributed to existence of sterile neutrinos which mix with the active neutrino to result in the observable PMNS matrix. We show that involvement of a sterile in the (3+1…
In this work we investigate the implication of low-energy precision measurements on the quark-lepton charged currents in general neutrino interactions with sterile neutrinos in effective field theories. The physics in low-energy measurements is described by the low-energy effective field theory extended with sterile neutrinos (LNEFT) defined below the electroweak scale. We also take into account renormalization group running and match the LNEFT onto the Standard Model (SM) effective field theory with sterile neutrinos (SMNEFT) to constrain new physics (NP) above the electroweak scale. The most…
A large lepton asymmetry could be generated in the Early Universe by oscillations of active to sterile neutrinos with a small mixing angle sin 2 θ< 10^-2. The final order of magnitude of the lepton asymmetry ηis mainly determined by its growth in the last stage of evolution when the MSW resonance dominates the kinetic equations. In this paper we present a simple way of calculating the maximum possible lepton asymmetry which can be created. Our results are in good agreement to previous calculations. Furthermore, we find that the growth of asymmetry does not obey any particular power law. We fin…
In scenarios with sterile (right-handed) neutrinos that are subject to an approximate "lepton-number-like" symmetry, the heavy neutrinos (i.e. the mass eigenstates) can have masses around the electroweak scale and couple to the Higgs boson with, in principle, unsuppressed Yukawa couplings while accounting for the smallness of the light neutrinos' masses. In these scenarios, the on-shell production of heavy neutrinos and their subsequent decays into a light neutrino and a Higgs boson constitutes a hitherto unstudied resonant contribution to the Higgs production mechanism. We investigate the rel…
We consider the minimal 3-3-1 model with three sterile neutrinos transforming as singlet under the $SU(3)_L\otimes U(1)_X$ symmetry. This model, with or without sterile neutrinos, predicts flavor violating interactions in both quark and lepton sectors, since all the charged fermions mass matrices can not be assumed diagonal in any case. Here we accommodate the lepton masses and the Pontecorvo-Maki-Nakawaga-Sakata matrix at the same time, and as consequence the Yukawa couplings and the unitary matrices which diagonalize the mass matrices are not free parameters anymore. We study some phenomenol…
A future high-luminosity $Z$-factory will offer the possibility to study rare $Z$ decays, as those leading to lepton flavour violating final states. Processes such as $Z \to \ell_1^\mp \ell_2^\pm$ are potentially complementary to low-energy (high-intensity) observables of lepton flavour violation. In this work we address the impact of new sterile fermions on lepton flavour violating $Z$ decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints on the sterile states. We consider a minimal extension of the Standard Model by one st…
We propose a resonant leptogenesis scenario in a U(1)_{B-L} gauge extension of the standard model to generate large lepton asymmetries for cosmological baryon asymmetry and dark matter. After B-L number is spontaneously broken, inflaton can pick up a small vacuum expectation value for the mass splits of three pairs of quasi-degenerately heavy Majorana neutrinos and the masses of three sterile neutrinos. With thermal mass effects of sphalerons, the observed small baryon asymmetry can be converted from large lepton asymmetries of individual flavors although total lepton asymmetry is assumed zero…
A future high-luminosity $Z$-factory has the potential to investigate lepton flavour violation. Rare decays such as $Z \to \ell_1^\mp \ell_2^\pm$ can be complementary to low-energy (high-intensity) observables of lepton flavour violation. Here we consider two extensions of the Standard Model which add to its particle content one or more sterile neutrinos. We address the impact of the sterile fermions on lepton flavour violating $Z$ decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints. We show that sterile neutrinos can give…
We consider the possibility of the lightest sterile neutrino dark matter which has dipole interaction with heavier sterile neutrinos. The lifetime can be long enough to be a dark matter candidate without violating other constraints and the correct amount of relic abundance can be produced in the early Universe. We find that a sterile neutrino with the mass of around MeV and the dimension-five non-renormalisable dipole interaction suppressed by $Λ_5 \gtrsim 10^{15}$ GeV can be a good candidate of dark matter, while heavier sterile neutrinos with masses of the order of GeV can explain the active…
Matrix element $U_{τN}$ of sterile neutrino N mixing with $ν_τ$ is the least constrained in the literature among the three $U_{αN}$ ($α=e,μ,τ$) mixing parameters characterizing the sterile neutrino phenomenology. We study the contribution of massive dominantly sterile neutrinos to purely leptonic $τ$-decays and semileptonic decays of $τ$ and K, D mesons. We consider some decays allowed in the Standard Model (SM) as well as Lepton Flavor and Lepton Number Violating (LFV, LNV) decays forbidden in the SM. From the existing experimental data on the branching ratios of these processes we derived ne…
We examine medium-enhanced, neutrino scattering-induced decoherent production of dark matter candidate sterile neutrinos in the early universe. In cases with a significant net lepton number we find two resonances, where the effective in-medium mixing angles are large. We calculate the lepton number depletion-driven evolution of these resonances. We describe the dependence of this evolution on lepton numbers, sterile neutrino rest mass, and the active-sterile vacuum mixing angle. We find that this resonance evolution can result in relic sterile neutrino energy spectra with a generic form which …
We address the impact of sterile fermions on charged lepton electric dipole moments. Any experimental signal of these observables calls for scenarios of physics beyond the Standard Model providing new sources of CP violation. In this work, we consider a minimal extension of the Standard Model via the addition of sterile fermions which mix with active neutrinos and we derive the corresponding analytical expressions for the electric dipole moments of charged leptons at two-loop order. Our study reveals that, in order to have a non-vanishing contribution in this framework, the minimal extension n…
In this short letter, we find that a magnetic transition dipole moment between tau and sterile neutrinos can account for the XENON1T excess events. Unlike the ordinary neutrino dipole moment, the introduction of the new sterile mass scale allows for astrophysical bounds to be suppressed. Interestingly, the best-fit regions that are compatible with the SN1987A imply either boron-8 or CNO neutrinos as the source flux. We find that sterile neutrinos of either $\sim$ 260 keV or in the $\sim$(500 - 800) keV mass range are capable of evading astrophysical constraints while being able to successfully…
We consider the simplest extension of the standard electroweak model by one sterile neutrino that allows for neutrino masses and mixing. We find that its leptonic sector contains much less free physical parameters than previously realized. In addition to the two neutrino masses, the lepton mixing matrix in charged current interactions involves (n-1) free physical mixing angles for n generations. The mixing matrix in neutral current interactions of neutrinos is completely fixed by the two masses. Both interactions conserve CP. We illustrate the phenomenological implications of the model by vacu…
We study primordial nucleosynthesis abundance yields for assumed ranges of cosmological lepton numbers, sterile neutrino mass-squared differences and active-sterile vacuum mixing angles. We fix the baryon-to-photon ratio at the value derived from the cosmic microwave background (CMB) data and then calculate the deviation of the 2H, 4He, and 7Li abundance yields from those expected in the zero lepton number(s), no-new-neutrino-physics case. We conclude that high precision (< 5% error) measurements of the primordial 2H abundance from, e.g., QSO absorption line observations coupled with high prec…
We explore the possibility of having a successful leptogenesis through oscillations between new sterile fermion states added to the Standard Model field content in a well motivated framework, naturally giving rise to the required mass splitting between the sterile states through a small total lepton number violation. We propose a framework with only two sterile states forming a pseudo-Dirac state, in which their mass difference as well as the smallness of the neutrino masses are due to two sources of lepton number violation with $ΔL=2$, corresponding to an Inverse Seesaw framework extended by …
The generation of lepton asymmetry below the electroweak scale has a considerable impact on production of dark matter sterile neutrinos. Oscillations or decays of the heavier sterile neutrinos in the neutrino minimal standard model can give rise to the requisite lepton asymmetry, provided the masses of the heavier neutrinos are sufficiently degenerate. We study the renormalization group evolution of the mass difference of these singlet fermions to understand the degree of necessary fine-tuning. We construct an example of the model that can lead to a technically natural realization of this low-…
Among the class of models with small mixing angles between sterile and active neutrinos, we place constraints on the effective muon-to-sterile neutrino magnetic and electric dipole transition moments from the combined MiniBooNE results for the sterile neutrino mass range of $10\;\mathrm{MeV}<m_s<500\; \mathrm{MeV}$. Our results are valid for models with CP-violating interactions and for Dirac and Majorana sterile neutrinos. In addition, we show that such dipole electromagnetic interactions cannot be the main source of the anomalous events in the MiniBooNE experiment because they fail to reprod…
We consider the cosmological effects of sterile neutrinos with the masses of $150- 450$ MeV. The decay of sterile neutrinos changes the thermal history of the Universe and affects the energy density of radiation at the recombination and the big bang nucleosynthesis (BBN) results. We derive severe constraints on the parameters of sterile neutrinos from the primordial abundances of helium-4 and deuterium. We also find that in a particular model the constraints can be considerably relaxed by assuming a large lepton asymmetry in the active neutrinos. In this case, the consistent parameters result …
We investigate quantitatively the extent to which having a primordial leptonic asymmetry (n_nu \neq n_nubar) relaxes the bounds on light sterile neutrinos imposed by BBN and LSS. We adopt a few assumptions that allow us to solve the neutrino evolution equations over a broad range of mixing parameters and asymmetries. For the general cases of sterile mixing with the electron or muon neutrino, we identify the regions that can be reopened. For the particular case of a LSND-like sterile neutrino, soon to be rejected or confirmed by MiniBooNE, we find that an asymmetry of the order of 10^-4 is need…
We study the possibility to observe sterile neutrinos with masses in the range between 5 GeV and 20 GeV at the LHC, using the exclusive semileptonic modes involving pions, namely W to lepton + N to n pions + lepton+lepton (n = 1, 2, 3). The two pion and three pion modes require extrapolations of form factors to large time-like $q^2$, which we do using vector dominance models as well as light front holographic QCD, with remarkable agreement. This mass region is difficult to explore with inclusive dilepton+dijet modes or trilepton modes and impossible to explore in rare meson decays. While parti…
We propose a new mechanism to block the active-sterile neutrino oscillations in the Early Universe. We show that a typical consequence of theories where the lepton number is spontaneously broken is the existence of a coherent cosmological Majoron field with a strength proportional to the lepton and baryon numbers of the Universe. This field interacts with leptons and changes the potentials relevant for neutrino oscillations. If the scale of lepton number symmetry breaking is of the order of 1 GeV then a Majoron field and lepton number asymmetry of the order of the baryon asymmetry are strong e…
The phenomenon of neutrino flavor oscillations motivates searches for sterile neutrinos in a broad range of masses and mixing-parameter values. A sterile neutrino $N$ that mixes predominantly with the $τ$ neutrino is particularly challenging experimentally. To address this challenge, we propose a new method to search for a $ν_τ$-mixing with $N$ lighter than the $τ$ lepton. The method uses the large $e^+e^-\toτ^+τ^-$ samples collected at $B$-factory experiments to produce the $N$ in $τ$-lepton decays. We exploit the long lifetime of a sterile neutrino in this mass range to suppress background a…
The mixing of active neutrinos with their sterile counterparts with keV mass is known to have a potentially major impact on the energy loss from the supernova core. By relying on a set of three static hydrodynamical backgrounds mimicking the early accretion phase and the Kelvin-Helmoltz cooling phase of a supernova, we develop the first self-consistent, radial- and time-dependent treatment of sterile and tau neutrinos mixing in the dense stellar core. We follow the flavor evolution by including ordinary matter effects, collisional production of sterile neutrinos, as well as reconversions of st…
We study the violation of lepton flavour universality in light meson decays due to the presence of non-zero mixings between the active neutrinos with new sterile states. The modified $W \ell ν$ vertices, arising from a non-unitarity leptonic mixing matrix intervening in charged currents, might lead to a tree-level enhancement of $R_{P} = Γ(P \to e ν) / Γ(P \to μν)$, with $P=K, π$. These enhancements are illustrated in the case of the inverse seesaw, showing that one can saturate the current experimental bounds on $Δr_{K}$ (and $Δr_π$), while in agreement with the different experimental and obs…
We present here a study of the impact of a heavy neutrino (or heavy neutral lepton) on leptonic and semileptonic kaon decays. We used a simplified model consisting of 3 light neutrinos responsible for neutrino oscillations and a heavy sterile neutrino. We found that it can lead to large deviations from the Standard Model predictions for leptonic decays, in conflict with experimental measurements of $K_{e2}$ and the lepton universality test $R_K$. This allows to derive new constraints on the leptonic mixing for heavy sterile neutrinos. No tension was found when considering the semileptonic deca…
We study lepton number violating and lepton number conserving semi-leptonic decays of heavy baryons $Λ_b^0$ to three charged leptons and a neutrino. The decays occur through two intermediate quasi-degenerate GeV-scale sterile neutrinos of either Majorana or Dirac type that can be on-shell. Interference between the intermediate heavy neutrinos leads to CP violation in the final states. Effect of neutrino oscillations between the heavy states are also considered in observables of interests, \emph{i.e.,} branching ratio, and the CP-asymmetry. Given the present constraints on the heavy-to-light mi…
Motivated by the first observation of coherent-elastic neutrino-nucleus scattering at the COHERENT experiment, we confront the neutrino dipole portal giving rise to the transition of the standard model neutrinos to sterile neutrinos with the recently released CENNS 10 data from the liquid argon as well as the CsI data of the COHERENT experiment. Performing statistical analysis of those data, we show how the transition magnetic moment can be constrained for the range of the sterile neutrino mass between 10 keV and 40 MeV.
The quasi-degeneracy of heavier sterile neutrino masses in the Neutrino Minimal Standard Model ($ν$MSM) facilitates the production of lepton asymmetry below the electroweak scale. The first order loop corrections to this mass-difference has been computed in this work along with a numerical estimate of the contribution.
We consider an SU(3)'_c\times SU(2)'_L\times U(1)'_Y mirror sector where the field content and dimensionless couplings are a copy of the SU(3)_c\times SU(2)_L\times U(1)_Y ordinary sector. Our model also contains three gauge-singlet fermions with heavy Majorana masses and an [SU(2)_L\times SU(2)'_L]-bidoublet Higgs scalar with seesaw-suppressed vacuum expectation value. The mirror sterile neutrino masses will have a form of canonical seesaw while the ordinary active neutrino masses will have a form of double and linear seesaw. In this canonical and double-linear seesaw scenario, we can expect …
We investigate how hypothetical particles - sterile neutrinos - can be produced in the interior of exploding supernovae via the resonant conversion of $\barν_μ$ and $\bar ν_τ$. The novelty of our treatment lies in the proper account of the resulting lepton number diffusion. We compute the yield of sterile neutrinos and find that even after taking into account back reaction, sterile neutrinos can carry out a sizeable fraction of the total energy of the explosion comparable to that of active neutrinos. The production is, however, exponentially sensitive to the temperature in the inner supernovae…
We solve the problem of coherent Mikheyev-Smirnov-Wolfenstein (MSW) resonant active-to-sterile neutrino flavor conversion driven by an initial lepton number in the early universe. We find incomplete destruction of lepton number in this process and a sterile neutrino energy distribution with a distinctive cusp and high energy tail. These features imply alteration of the non-zero lepton number primordial nucleosynthesis paradigm when there exist sterile neutrinos with rest masses ~ 1 eV. This could result in better light element probes of (constraints on) these particles.
The possibility of light sterile neutrinos allows for the resonant production of lepton number in the early universe through matter-affected neutrino mixing. For a given a mixing of the active and sterile neutrino states it has been found that the lepton number generation process is chaotic and strongly oscillatory. We undertake a new study of this process' sensitivity to initial conditions through the quantum rate equations. We confirm the chaoticity of the process in this solution, and moreover find that the resultant lepton number and the sign of the asymmetry produces a fractal in the para…
We study the contribution of massive dominantly sterile neutrinos, $N$, to the Lepton Number and Lepton Flavor Violating semileptonic decays of $τ$ and $B, D, K$-mesons. We focus on special domains of sterile neutrino masses $m_{N}$ where it is close to its mass-shell. This leads to an enormous resonant enhancement of the decay rates of these processes. This allows us to derive stringent limits on the sterile neutrino mass $m_{N}$ and its mixing $U_{αN}$ with active flavors. We apply a joint analysis of the existing experimental bounds on the decay rates of the studied processes. In contrast t…
It is well known that the Standard Model of the Electroweak interactions rests on a metastable vacuum. This can only be fixed by means of new physics. Presently neutrino physics provides the most intriguing framework to formulate new physics. This is so because, in addition to the problem of the lightness of the active standard neutrinos, currently MiniBooNE experimental result may be indicating that sterile neutrinos exist and are light, too. In this case, it is reasonable to expect that the framework that yields light active and sterile neutrinos could stabilize the vacuum, too. In order to …
We address the impact of sterile fermions on the lepton flavor violating decays of quarkonia as well as of the $Z$ boson. We compute the relevant Wilson coefficients and show that the ${\rm B}(V\to\ell_α\ell_β)$, where $V=ϕ,ψ^{(n)}$, $Υ^{(n)},Z$ can be significantly enhanced in the case of large sterile fermion masses and a non-negligible active-sterile mixing. We illustrate that feature in a specific minimal realization of the inverse seesaw mechanism, known as $(2,3)$-ISS, and in an effective model in which the presence of non-standard sterile fermions is parameterized by means of one heavy …
The existence of sterile neutrino is hinted by simultaneous explanation of diverse neutrino anomalies. We suggest that the quasi Goldstone fermions (QGF) arising in supersymmetric theory as a result of spontaneous breaking of global symmetry like the Peccei-Quinn symmetry or the lepton number symmetry can play a role of the sterile neutrino. The smallness of mass of QGF ($m_S \sim 10^{-3}-10$ eV) can be related to the specific choice of superpotential or Kähler potential (e.g., no-scale kinetic terms for certain superfields). Mixing of QGF with neutrinos implies the $R$-parity violation. It ca…
