Neutrino Mass Models Research Articles

Leptogenesis in quasi-degenerate neutrino mass models with non-zero θ13

In the context of neutrino oscillation experiments, six different quasi-degenerate neutrino (QDN) mass models, which we parameterized recently, are found equally relevant. The present attempt tries to explore the possibilities for the discrimination of the six QDN models in the light of baryogenesis via leptogenesis. In this work we investigate all six models to predict observable baryon asymmetry. If leptogenesis is unflavoured or single flavoured, a significant difference is found. Then, only QD-NH-IA and QD-IH-IA are dominant. To get specific results, the choice of Dirac neutrino mass matrix as down-quark type is found most favourable.

Possible roles of Peccei-Quinn symmetry in an effective low energy model

The strong $CP$ problem is known to be solved by imposing Peccei-Quinn (PQ) symmetry. However, the domain wall problem caused by the spontaneous breaking of its remnant discrete subgroup could make models invalid in many cases. We propose a model in which the PQ charge is assigned quarks so as to escape this problem without introducing any extra colored fermions. In the low energy effective model resulting after the PQ symmetry breaking, both the quark mass hierarchy and the CKM mixing could be explained through Froggatt-Nielsen mechanism. If the model is combined with the lepton sector supplemented by an inert doublet scalar and right-handed neutrinos, the effective model reduces to the scotogenic neutrino mass model in which both the origin of neutrino masses and dark matter are closely related. The strong $CP$ problem could be related to the quark mass hierarchy, neutrino masses, and dark matter through the PQ symmetry.

From the Trees to the Forest: A Review of Radiative Neutrino Mass Models

A plausible explanation for the lightness of neutrino masses is that neutrinos are massless at tree level, with their mass (typically Majorana) being generated radiatively at one or more loops. The new couplings, together with the suppression coming from the loop factors, imply that the new degrees of freedom cannot be too heavy (they are typically at the TeV scale). Therefore, in these models there are no large mass hierarchies and they can be tested using different searches, making their detailed phenomenological study very appealing. In particular, the new particles can be searched for at colliders and generically induce signals in lepton-flavor and lepton-number violating processes (in the case of Majorana neutrinos), which are not independent from reproducing correctly the neutrino masses and mixings. The main focus of the review is on Majorana neutrinos. We order the allowed theory space from three different perspectives: (i) using an effective operator approach to lepton number violation, (ii) by the number of loops at which the Weinberg operator is generated, (iii) within a given loop order, by the possible irreducible topologies. We also discuss in more detail some popular radiative models which involve qualitatively different features, revisiting their most important phenomenological implications. Finally, we list some promising avenues to pursue.

The B-L scotogenic models for Dirac neutrino masses

We construct the one-loop and two-loop scotogenic models for Dirac neutrino mass generation in the context of U(1)_{B-L} extensions of standard model. It is indicated that the total number of intermediate fermion singlets is uniquely fixed by the anomaly free condition and the new particles may have exotic B-L charges so that the direct SM Yukawa mass term bar{nu }_Lnu _Roverline{phi ^0} and the Majorana mass term (m_N/2)overline{nu _R^C}nu _R are naturally forbidden. After the spontaneous breaking of the U(1)_{B-L} symmetry, the discrete Z_{2} or Z_{3} symmetry appears as the residual symmetry and gives rise to the stability of intermediate fields as DM candidates. Phenomenological aspects of lepton flavor violation, DM, leptogenesis and LHC signatures are discussed.

Phenomenological analysis of an E6 -inspired seesaw model

We analyse the phenomenology of a model of neutrino masses inspired by $\mathrm{E}_{6}$ GUT unification in which the exotic neutrinos can be present at low scales. The model introduces vector-like isosinglet down-type quarks, vector-like isodoublet leptons, neutrino singlets and two $Z'$ bosons. The seesaw mechanism can be achieved with exotic neutrino masses as low as 100 GeV and Yukawa couplings of order $10^{-3}$. We find that the lightest $Z'$ boson mass is required to be above 2.8 TeV, the exotic quark masses are required to be above 1.3 TeV (810 GeV) if they are collider stable (promptly decaying), and the exotic lepton mass bounds remain at the LEP value of 102 GeV. The model also presents a type-II two-Higgs-doublet model (2HDM) along with two heavy singlet scalars. The 2HDM naturally has the alignment limit enforced thanks to the large VEVs of the exotic scalars, thereby avoiding most constraints.

GUT and Flavor Models for Neutrino Masses and Mixing

In the recent years experiments have established the existence of neutrino oscillations and most of the oscillation parameters have been measured with a good accuracy. However, in spite of many interesting ideas, no real illumination was sparked on the problem of flavor in the lepton sector. In this review, we discuss the state of the art of models for neutrino masses and mixings formulated in the context of flavor symmetries, with particular emphasis on the role played by grand unified gauge groups.

A testable radiative neutrino mass model with multi-charged particles

We propose a radiatively-induced neutrino mass model at one-loop level by introducing a pair of doubly-charged fermions and a few multi-charged bosons. We investigate the contributions of the model to neutrino masses, lepton-flavor violations, muon g−2, oblique parameters, and collider signals, and find a substantial fraction of the parameter space that can satisfy all the constraints. Furthermore, we discuss the possibility of detecting the doubly-charged fermions at the LHC.

Reconsidering the one leptoquark solution: flavor anomalies and neutrino mass

We reconsider a model introducing a scalar leptoquark ϕ ∼ (3,1, −1/3) to explain recent deviations from the standard model in semileptonic B decays. The leptoquark can accommodate the persistent tension in the decays overline{B}to {D}^{left(ast right)}tau overline{nu} as long as its mass is lower than approximately 10 TeV, and we show that a sizeable Yukawa coupling to the right-chiral tau lepton is necessary for an acceptable explanation. A characteristic prediction of this scenario is a value of {R}_{D^{*}} slightly smaller than the current world average. Agreement with the measured overline{B}to {D}^{left(ast right)}tau overline{nu} rates is mildly compromised for parameter choices addressing the tensions in b → sμμ, where the model can significantly reduce the discrepancies in angular observables, branching ratios and the lepton-flavor-universality observables RK and {R}_{K^{*}} . The leptoquark can also reconcile the predicted and measured value of the anomalous magnetic moment of the muon and appears naturally in models of radiative neutrino mass derived from lepton-number violating effective operators. As a representative example, we incorporate the particle into an existing two-loop neutrino mass scenario derived from a dimension-nine operator. In this specific model, the structure of the neutrino mass matrix provides enough freedom to explain the small masses of the neutrinos in the region of parameter space dictated by agreement with the anomalies in overline{B}to {D}^{left(ast right)}tau overline{nu} , but not the b → s transition. This is achieved without excessive fine-tuning in the parameters important for neutrino mass.

Symmetry restoration due to preheating and lepton number asymmetry

We study a possible symmetry restoration due to the radiative effect of particles which are explosively produced in preheating after inflation. As its application, we consider a scenario for leptogenesis based on the lepton number asymmetry generated in the right-handed neutrino sector through the inflaton decay. The scenario is examined in a one-loop radiative neutrino mass model extended with singlet scalars.

Radiative neutrino mass model with a TeV scale gauge symmetry

The radiative neutrino mass model can relate neutrino masses to dark matter at TeV regions. In order for thermal leptogenesis to work well in this model, both nely degenerate mass of the right-handed neutrinos and a tiny neutrino Yukawa coupling are necessary for resonant out-of-equilibrium decay. We show these features could be realized through an extension of the model with a U(1) gauge symmetry. This extension can also bring about a small quartic scalar coupling between the Higgs doublet scalar and an inert doublet scalar, and the origin of the Z2 symmetry which guarantees the stability of dark matter.

Long lived light scalars as probe of low scale seesaw models

We point out that in generic TeV scale seesaw models for neutrino masses with local B–L symmetry breaking, there is a phenomenologically allowed range of parameters where the Higgs field responsible for B–L symmetry breaking leaves a physical real scalar field with mass around GeV scale. This particle (denoted here by H3) is weakly mixed with the Standard Model Higgs field (h) with mixing θ1≲mH3/mh, barring fine-tuned cancellation. In the specific case when the B–L symmetry is embedded into the TeV scale left–right seesaw scenario, we show that the bounds on the h–H3 mixing θ1 become further strengthened due to low energy flavor constraints, thus forcing the light H3 to be long lived, with displaced vertex signals at the LHC. The property of left–right TeV scale seesaw models are such that they make the H3 decay to two photons as the dominant mode. This is in contrast with a generic light scalar that mixes with the SM Higgs boson, which could also have leptonic and hadronic decay modes with comparable or larger strength. We discuss the production of this new scalar field at the LHC and show that it leads to testable displaced vertex signals of collimated photon jets, which is a new distinguishing feature of the left–right seesaw model. We also study a simpler version of the model where the SU(2)R breaking scale is much higher than the O(TeV) U(1)B–L breaking scale, in which case the production and decay of H3 proceed differently, but its long lifetime feature is still preserved for a large range of parameters. Thus, the search for such long-lived light scalar particles provides a new way to probe TeV scale seesaw models for neutrino masses at colliders.

Testing the low scale seesaw and leptogenesis

Heavy neutrinos with masses below the electroweak scale can simultaneously generate the light neutrino masses via the seesaw mechanism and the baryon asymmetry of the universe via leptogenesis. The requirement to explain these phenomena imposes constraints on the mass spectrum of the heavy neutrinos, their flavour mixing pattern and their CP properties. We first combine bounds from different experiments in the past to map the viable parameter regions in which the minimal low scale seesaw model can explain the observed neutrino oscillations, while being consistent with the negative results of past searches for physics beyond the Standard Model. We then study which additional predictions for the properties of the heavy neutrinos can be made based on the requirement to explain the observed baryon asymmetry of the universe. Finally, we comment on the perspectives to find traces of heavy neutrinos in future experimental searches at the LHC, NA62, BELLE II, T2K, SHiP or a future high energy collider, such as ILC, CEPC or FCC-ee. If any heavy neutral leptons are discovered in the future, our results can be used to assess whether these particles are indeed the common origin of the light neutrino masses and the baryon asymmetry of the universe. If the magnitude of their couplings to all Standard Model flavours can be measured individually, and if the Dirac phase in the lepton mixing matrix is determined in neutrino oscillation experiments, then all model parameters can in principle be determined from this data. This makes the low scale seesaw a fully testable model of neutrino masses and baryogenesis.

Common origin of nonzero θ13 and dark matter in an S4 flavor symmetric model with inverse seesaw mechanism

We study an inverse seesaw model of neutrino mass within the framework of $S_4$ flavour symmetry from the requirement of generating non-zero reactor mixing angle $\theta_{13}$ along with correct dark matter relic abundance. The leading order $S_4$ model gives rise to tri-bimaximal type leptonic mixing resulting in $\theta_{13}=0$. Non-zero $\theta_{13}$ is generated at one loop level by extending the model with additional scalar and fermion fields which take part in the loop correction. The particles going inside the loop are odd under an in-built $Z^{\text{Dark}}_2$ symmetry such that the lightest $Z^{\text{Dark}}_2$ odd particle can be a dark matter candidate. Correct neutrino and dark matter phenomenology can be achieved for such one loop corrections either to the light neutrino mass matrix or to the charged lepton mass matrix although the latter case is found to be more predictive. The predictions for neutrinoless double beta decay is also discussed and inverted hierarchy in the charged lepton correction case is found to be disfavoured by the latest KamLAND-Zen data.

Dark matter, muon g−2 , electric dipole moments, and Z→ℓi+ℓj− in a one-loop induced neutrino model

We study a simple one-loop induced neutrino mass model that contains both bosonic and fermionic dark matter candidates and has the capacity to explain the muon anomalous magnetic moment anomaly. We perform a comprehensive analysis by taking into account the relevant constraints of charged lepton flavor violation, electric dipole moments, and neutrino oscillation data. We examine the constraints from lepton flavor-changing $Z$ boson decays at one-loop level, particularly when the involved couplings contribute to the muon $g-2$. It is found that $\text{BR}(Z\to \mu\tau)\simeq (10^{-7}$ - $10^{-6})$ while $\text{BR}(\tau\to\mu\gamma)\lesssim 10^{-11}$ in the fermionic dark matter scenario. The former can be probed by the precision measurement of the $Z$ boson at future lepton colliders.

Light neutrino masses from a non-Hermitian Yukawa theory

Working within the context of PT-symmetric quantum mechanics, we begin by describing a non-Hermitian extension of QED that is both Lorentz invariant and consistent with unitarity. We show that the non-Hermitian Dirac mass matrix of this theory exhibits an exceptional point, corresponding to an effectively massless theory whose conserved current is either right- or left-chiral dominated. With this inspiration, we are able to construct a non- Hermitian model of light Dirac neutrino masses from Hermitian and anti-Hermitian Yukawa couplings that are both of order unity. We finish by highlighting potential phenomenological implications of this model.

Pseudo-majoron as light mediator of singlet scalar dark matter

In the singlet-triplet majoron model of neutrino mass, lepton number is spontaneously broken. If it is also softly broken, then a naturally light pseudoscalar particle ηI exists. It may then act as a light mediator for a real singlet scalar χ with odd dark parity. It is itself unstable, but decays dominantly to two neutrinos through its triplet scalar component, thereby not disturbing the cosmic microwave background (CMB). It also mixes with the standard-model Higgs boson only in one loop, thereby not contributing significantly to the elastic scattering of χ off nuclei in dark-matter direct-search experiments.

A radiative neutrino mass model with SIMP dark matter

We propose the first viable radiative seesaw model, in which the neutrino masses are induced radiatively via the two-loop Feynman diagram involving Strongly Interacting Massive Particles (SIMP). The stability of SIMP dark matter (DM) is ensured by a ℤ5 discrete symmetry, through which the DM annihilation rate is dominated by the 3 → 2 self-annihilating processes. The right amount of thermal relic abundance can be obtained with perturbative couplings in the resonant SIMP scenario, while the astrophysical bounds inferred from the Bullet cluster and spherical halo shapes can be satisfied. We show that SIMP DM is able to maintain kinetic equilibrium with thermal plasma until the freeze-out temperature via the Yukawa interactions associated with neutrino mass generation.

Probing left-right seesaw models using beam polarization at an e+e− collider

We show that the longitudinal beam polarization option at a future electron-positron collider provides an unambiguous distinction between low-scale seesaw models of neutrino mass. This is possible due to the fact that the pair production cross section of the heavy neutrinos in seesaw models is sensitive to the polarization of the initial lepton beams, and for a suitable choice of the polarization, shows a clear enhancement over the unpolarized cross section. More interestingly, the choice of the beam polarization for which the enhancement is maximum is governed by the size of the light-heavy neutrino mixing parameter. We also find that using this effect, one can probe a previously uncharted parameter space of the left-right seesaw model, which is complementary to the existing searches at both energy and intensity frontiers.

Overview on Neutrino Theory and Phenomenology

In this talk, I give an overview on recent theoretical and phenomenological studies of massive neutrinos. First of all, the present status of neutrino mixing parameters is summarized. The phenomenology of neutrino oscillations is then discussed, and current understanding of lepton flavor mixing is presented. Finally, I consider the seesaw models of neutrino masses and briefly mention the direct searches at the CERN Large Hadron Collider (LHC).

The seesaw portal in testable models of neutrino masses

A Standard Model extension with two Majorana neutrinos can explain the measured neutrino masses and mixings, and also account for the matter-antimatter asymmetry in a region of parameter space that could be testable in future experiments. The testability of the model relies to some extent on its minimality. In this paper we address the possibility that the model might be extended by extra generic new physics which we parametrize in terms of a low-energy effective theory. We consider the effects of the operators of the lowest dimensionality, d = 5, and evaluate the upper bounds on the coefficients so that the predictions of the minimal model are robust. One of the operators gives a new production mechanism for the heavy neutrinos at LHC via higgs decays. The higgs can decay to a pair of such neutrinos that, being long-lived, leave a powerful signal of two displaced vertices. We estimate the LHC reach to this process.