One-loop Neutrino Mass Research Articles

Singlet-doublet fermion Dark Matter with Dirac neutrino mass, (g − 2)μ and ∆Neff

We study the possibility of generating light Dirac neutrino mass via scotogenic mechanism where singlet-doublet fermion Dark Matter (DM) plays non-trivial role in generating one-loop neutrino mass, anomalous magnetic moment of muon: (g − 2)μ as well as additional relativistic degrees of freedom ∆Neff within reach of cosmic microwave background (CMB) experiments. We show that the Dirac nature of neutrinos can bring interesting correlations within the parameter space satisfying the (g − 2)μ, DM relic density and the effective relativistic degrees of freedom ∆Neff. While we stick to thermal singlet-doublet DM with promising detection prospects, both thermal and non-thermal origin of ∆Neff have been explored. In addition to detection prospects of the model at DM, (g − 2)μ and other particle physics experiments, it remains verifiable at future CMB experiments like CMB-S4 and SPT-3G.

Anomaly-free dark matter models with one-loop neutrino masses and a gauged U(1) symmetry

We systematically study and classify scotogenic models with a local U(1) gauge symmetry. These models give rise to radiative neutrino masses and a stable dark matter candidate, but avoid the theoretical problems of global and discrete symmetries. We restrict the dark sector particle content to up to four scalar or fermionic SU(2) singlets, doublets or triplets and use theoretical arguments based on anomaly freedom, Lorentz and gauge symmetry to find all possible charge assignments of these particles. The U(1) symmetry can be broken by a new Higgs boson to a residual discrete symmetry, that still stabilizes the dark matter candidate. We list the particle content and charge assignments of all non-equivalent models. Specific examples in our class of models that have been studied previously in the literature are the U(1)D scotogenic and singlet-triplet scalar models breaking to Z2. We also briefly discuss the new phenomenological aspects of our model arising from the presence of a new massless dark photon or massive Z′ boson as well as the additional Higgs boson.

W boson mass in singlet-triplet scotogenic dark matter model

The recent high precision measurement of [Formula: see text] boson mass by CDF-II collaboration points to the contribution(s) of new physics beyond the Standard Model (SM). One of the minimalistic ways to account for the anomalous [Formula: see text] boson mass is by introducing a hyperchargeless real [Formula: see text] triplet scalar whose vacuum expectation value (VEV) explicitly contributes to the [Formula: see text] boson mass at the tree level while the [Formula: see text] boson mass remains the same. Such a triplet can be naturally embedded in a singlet-triplet scotogenic model for one-loop neutrino mass generated by dark sector particles running in the loop. We discuss the detailed phenomenology of the model, obtaining the parameter space consistent with the CDF-II [Formula: see text] boson mass measurements. The dark matter as well as the constraints comings from [Formula: see text], [Formula: see text], [Formula: see text] parameters are also analyzed.

On minimal realization of topological Lorentz structures with one-loop seesaw extensions in A4 modular symmetry

The topological classification of one-loop Weinberg operator at dimension-5 leads to systematic categorization of one-loop neutrino mass models. All one-loop neutrino mass models must fall in one of these categories. Among these topological categories, loop extension of canonical seesaw scenarios is interesting in light of the current LHC run. Apart from one-loop contribution, these extensions result in dominant tree-level contribution to neutrino masses. The immediate remedy to obtain dominant one-loop contribution requires combination of flavor symmetries and enlarged field content. Alternatively, in this work, we propose a minimal way of realizing the topological structures with dominant one-loop contribution using modular variant of the permutation symmetries. In such a realization, no new fields are needed apart from those permitted by the topology itself. For the first time, we have realized one such topological Lorentz structure(T4-2-i) pertaining to one-loop extension of Type-II seesaw using modular A4 symmetry. Here, modular weights play an important role in suppressing tree-level terms and stabilizing the particles running in the loop(Ni , ρ and ϕ), thus, making them suitable dark matter candidates. In this work, we have explored the possibility of fermionic dark matter candidate where right-handed neutrino (N 1) is assumed to be lightest. We have, also, analyzed the compatibility of the model with neutrino oscillation data and obtained model predictions for effective Majorana mass Mee and CP violation. Furthermore, the predictions on relic density of dark matter and its direct detection considering bound on lepton flavor violating process, μ → eγ have, also, been investigated.

Scotogenic neutrino masses with gauged matter parity and gauge coupling unification

Building up on previous work we propose a Dark Matter (DM) model with gauged matter parity and dynamical gauge coupling unification, driven by the same physics responsible for scotogenic neutrino mass generation. Our construction is based on the extended gauge group SU(3)c ⊗ SU(3)L ⊗ U(1)X ⊗ U(1)N, whose spontaneous breaking leaves a residual conserved matter parity, MP, stabilizing the DM particle candidates of the model. The key role is played by Majorana SU(3)L-octet leptons, allowing the successful gauge coupling unification and a one-loop scotogenic neutrino mass generation. Theoretical consistency allows for a plethora of new particles at the ≲ mathcal{O} (10) TeV scale, hence accessible to future collider and low-energy experiments.

Lepton dark matter portal in the inert Zee model

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 this new portal in the low-mass regime and show the parameter space allowed by direct and indirect searches of dark matter. Remarkably, the region for [Formula: see text] GeV is recovered for [Formula: see text]. We also show that future experiments like LZ and DARWIN could probe a large region of the parameter space of the model.

Dirac neutrinos from Peccei-Quinn symmetry: Two examples

We aim to explain the nature of neutrinos using Peccei-Quinn symmetry. We discuss two simple scenarios, one based on a type-II Dirac seesaw and the other in a one-loop neutrino mass generation, which solve the strong CP problem and naturally lead to Dirac neutrinos. In the first setup latest neutrino mass limit gives rise to axion which is in the reach of conventional searches. Moreover, we have both axion as well as WIMP dark mater for our second set up.

Long-lived charged particles and multilepton signatures from neutrino mass models

Lepton number violation (LNV) is usually searched for by the LHC collaborations using the same-sign dilepton plus jet signature. In this paper, we discuss multilepton signals of LNV that can arise with experimentally interesting rates in certain loop models of neutrino mass generation. Interestingly, in such models, the observed smallness of the active neutrino masses, together with the high multiplicity of the final states, leads in large parts of the viable parameter space of such models to the prediction of long-lived charged particles, which leave highly ionizing tracks in the detectors. We focus on one particular one-loop neutrino mass model in this class and discuss its LHC phenomenology in some detail.

Non-standard interactions in radiative neutrino mass models

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 non-standard 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 NSI at tree-level, popular models which fall under the type-I category are shown, somewhat surprisingly, to generate observable NSI at tree-level, while being consistent with direct and indirect constraints from colliders, electroweak precision data and charged-lepton flavor violation (cLFV). We survey such models where neutrino masses arise at one, two and three loops. In the prototypical Zee model which generates neutrino masses via one-loop diagrams involving charged scalars, we find that diagonal NSI can be as large as (8%, 3.8%, 9.3%) for (εee, εμμ, εττ), while off-diagonal NSI can be at most (10−3%, 0.56%, 0.34%) for (εeμ, εeτ, εμτ). In one-loop neutrino mass models using leptoquarks (LQs), (εμμ, εττ) can be as large as (21.6%, 51.7%), while εee and (εeμ, εeτ, εμτ) can at most be 0.6%. Other two- and three-loop LQ models are found to give NSI of similar strength. The most stringent constraints on the diagonal NSI are found to come from neutrino oscillation and scattering experiments, while the off-diagonal NSI are mostly constrained by low-energy processes, such as atomic parity violation and cLFV. We also comment on the future sensitivity of these radiative models in long-baseline neutrino experiments, such as DUNE. While our analysis is focused on radiative neutrino mass models, it essentially covers all NSI possibilities with heavy mediators.

A near-minimal leptoquark model for reconciling flavour anomalies and generating radiative neutrino masses

We introduce two scalar leptoquarks, the SU(2)L isosinglet denoted ϕ ∼ (3,1, −1/3) and the isotriplet φ ∼ (3,3, −1/3), to explain observed deviations from the standard model in semi-leptonic B-meson decays. We explore the regions of parameter space in which this model accommodates the persistent tensions in the decay observables RD(∗), RK (∗) , and angular observables in b → sμμ transitions. Additionally, we exploit the role of these exotics in existing models for one-loop neutrino mass generation derived from ∆L = 2 effective operators. Introducing the vector-like quark χ ∼ (3,2, −5/6) necessary for lepton-number violation, we consider the contribution of both leptoquarks to the generation of radiative neutrino mass. We find that constraints permit simultaneously accommodating the flavour anomalies while also explaining the relative smallness of neutrino mass without the need for cancellation between leptoquark contributions. A characteristic prediction of our model is a rate of muon-electron conversion in nuclei fixed by the anoma- lies in b → sμμ and neutrino mass; the COMET and Mu2e experiments will thus test and potentially falsify our scenario. The model also predicts signatures that will be tested at the LHC and Belle II.

One-loop neutrino mass model without any additional symmetries

We propose a radiative seesaw model at one-loop level, introducing fields with large multiplet of $SU(2)_L$. Thanks to large representations, any additional symmetries are not needed. In this framework, we formulate lepton and new fermion sector such as mass matrices, LFVs, and muon $g-2$. Furthermore, we show our cut-off scale via RGEs of $SU(2)_L$, and numerical analysis at a benchmark point included in dark matter candidate. Finally, we brief discuss possibility to detect at colliders.

A one-loop neutrino mass model with SU(2)L multiplet fields

We propose a one-loop neutrino mass model with several SU(2)L multiplet fermions and scalar fields in which the inert feature of a scalar to realize the one-loop neutrino mass can be achieved by the cancellation among Higgs couplings thanks to non-trivial terms in the Higgs potential and to present it in a simpler way. Then we discuss our typical cut-off scale by computing renormalization group equation for SU(2)L gauge coupling, lepton flavor violations, muon anomalous magnetic moment, possibility of dark matter candidate, neutrino mass matrix satisfying the neutrino oscillation data. Finally, we search for our allowed parameter region to satisfy all the constraints, and discuss a possibility of detecting new charged particles at the large hadron collider.

One-loop Dirac neutrino mass and mixed axion-WIMP dark matter

We consider the Peccei-Quinn (PQ) mechanism as the one behind the Dirac neutrino masses when these are generated through the $d=5$ effective operator $\bar{L}\tilde{H}N_R\phi$ at one loop level, with $\phi$ being a Standard Model singlet scalar. In this setup, the PQ symmetry guarantees that the one-loop realization of such an effective operator gives the leading contribution to the Dirac neutrino masses by forbidding the contributions arising from its tree level realizations. All the mediators in the one-loop neutrino mass diagrams can be stabilized by a remnant $Z_N$ symmetry from the PQ symmetry breaking, thus forming a dark sector besides the axion sector and leading to mixed axion/WIMP dark matter scenarios.

Charged lepton flavor violation and electric dipole moments in the inert Zee model

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 Z2 symmetry. The introduction of the Z2 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 ambitious experimental program designed to look for charged lepton flavor violation signals and the electron electric dipole moment, we study the phenomenology of the processes leading to these kind of signals, and establish which are the most promising experimental perspectives on that matter.

Generalized one-loop neutrino mass model with charged particles

We propose a radiative neutrino-mass model by introducing 3 generations of fermion pairs $E^{-(N+1)/2} E^{+(N+1)/2}$ and a couple of multi-charged bosonic doublet fields $\Phi_{N/2}, \Phi_{N/2+1}$, where $N=1,3,5,7,9$. We show that the models can satisfy the neutrino masses and oscillation data, and are consistent with lepton-flavor violations, the muon anomalous magnetic moment, the oblique parameters, and the beta function of the $U(1)_Y$ hypercharge gauge coupling. We also discuss the collider signals for various $N$, namely, multi-charged leptons in the final state from the Drell-Yan production of $E^{-(N+1)/2} E^{+(N+1)/2}$. In general, the larger the $N$ the more charged leptons will appear in the final state.

Dark matter stability and one-loop neutrino mass generation based on Peccei\u2013Quinn symmetry

We propose a model which is a simple extension of the KSVZ invisible axion model with an inert doublet scalar. Peccei–Quinn symmetry forbids tree-level neutrino mass generation and its remnant Z_2 symmetry guarantees dark matter stability. The neutrino masses are generated by one-loop effects as a result of the breaking of Peccei–Quinn symmetry through a nonrenormalizable interaction. Although the low energy effective model coincides with an original scotogenic model which contains right-handed neutrinos with large masses, it is free from the strong CP problem.

From high-scale leptogenesis to low-scale one-loop neutrino mass generation

We show that a high-scale leptogenesis can be consistent with a low-scale one-loop neutrino mass generation. Our models are based on the SU(3)c×SU(2)L×U(1)Y×U(1)B−L gauge groups. Except a complex singlet scalar for the U(1)B−L symmetry breaking, the other new scalars and fermions (one scalar doublet, two or more real scalar singlets/triplets and three right-handed neutrinos) are odd under an unbroken Z2 discrete symmetry. The real scalar decays can produce an asymmetry stored in the new scalar doublet which subsequently decays into the standard model lepton doublets and the right-handed neutrinos. The lepton asymmetry in the standard model leptons then can be partially converted to a baryon asymmetry by the sphaleron processes. By integrating out the heavy scalar singlets/triplets, we can realize an effective theory to radiatively generate the small neutrino masses at the TeV scale. Furthermore, the lightest right-handed neutrino can serve as a dark matter candidate.

Leptoquark mechanism of neutrino masses within the grand unification framework

We demonstrate the viability of the one-loop neutrino mass mechanism within the framework of grand unification when the loop particles comprise scalar leptoquarks (LQs) and quarks of the matching electric charge. This mechanism can be implemented in both supersymmetric and non-supersymmetric models and requires the presence of at least one LQ pair. The appropriate pairs for the neutrino mass generation via the up-type and down-type quark loops are S_3–R_2 and S_{1,,3}–tilde{R}_2, respectively. We consider two distinct regimes for the LQ masses in our analysis. The first regime calls for very heavy LQs in the loop. It can be naturally realized with the S_{1,,3}–tilde{R}_2 scenarios when the LQ masses are roughly between 10^{12} and 5 times 10^{13} GeV. These lower and upper bounds originate from experimental limits on partial proton decay lifetimes and perturbativity constraints, respectively. Second regime corresponds to the collider accessible LQs in the neutrino mass loop. That option is viable for the S_3–tilde{R}_2 scenario in the models of unification that we discuss. If one furthermore assumes the presence of the type II see-saw mechanism there is an additional contribution from the S_3–R_2 scenario that needs to be taken into account beside the type II see-saw contribution itself. We provide a complete list of renormalizable operators that yield necessary mixing of all aforementioned LQ pairs using the language of SU(5). We furthermore discuss several possible embeddings of this mechanism in SU(5) and SO(10) gauge groups.

Scale-invariant models with one-loop neutrino mass and dark matter candidates

We construct a list of minimal scale-invariant models at the TeV scale that generate one-loop neutrino mass and give viable dark matter candidates. The models generically contain a singlet scalar and a ${Z}_{2}$-odd sector comprised of singlet, doublet, and/or triplet SU(2) multiplets. The dark matter may reside in a single multiplet or arise as an admixture of several multiplets. We find 15 independent models, for which the dark matter is a viable candidate and neutrino mass results from a diagram with just one of the irreducible scale-invariant one-loop topologies. Further ``nonpure'' cases give hybrid one-/two-loop masses. All models predict new TeV scale physics, including a singlet scalar that generically mixes with the Higgs boson.

Scotogenic model for co-bimaximal mixing

We present a scotogenic model, i.e. a one-loop neutrino mass model with dark right-handed neutrino gauge singlets and one inert dark scalar gauge doublet $\eta$, which has symmetries that lead to co-bimaximal mixing, i.e. to an atmospheric mixing angle $\theta_{23} = 45^\circ$ and to a $CP$-violating phase $\delta = \pm \pi/2$, while the mixing angle $\theta_{13}$ remains arbitrary. The symmetries consist of softly broken lepton numbers $L_\alpha$ ($\alpha = e,\mu,\tau$), a non-standard $CP$ symmetry, and three $Z_2$ symmetries. We indicate two possibilities for extending the model to the quark sector. Since the model has, besides $\eta$, three scalar gauge doublets, we perform a thorough discussion of its scalar sector. We demonstrate that it can accommodate a Standard Model-like scalar with mass $125\, \mathrm{GeV}$, with all the other charged and neutral scalars having much higher masses.