Small Neutrino Masses Research Articles

Extended scotogenic model of neutrino mass and proton decay

The current article presents an extension of the classical scotogenic neutrino mass paradigm, where the three issues in particle physics: dark matter, smallness of neutrino mass, and stability of the proton are interconnected. The scenario encompasses the neutrino mass as well as the proton decay as a consequence of an existence of the dark matter. The study successfully achieves the correlation between the naturally small neutrino masses and naturally long proton lifetime in the present paradigm. Furthermore, all relevant cosmological, collider, and flavor physics constraints are incorporated in the detailed analysis. The scotogenic fermionic dark matter with the mass in the range from 100 GeV to 10 TeV successfully satisfies all relevant constraints. The valid range of 2.51×10−5<λ<2×10−3 is obtained for the 2HDM λ coupling. We give a brief discussion, as well as outline some of the future prospects. Published by the American Physical Society 2024

Signatures of bulk neutrinos in the early Universe

Neutrino masses and quantum gravity are strong reasons to extend the standard model of particle physics. A large extra dimension can be motivated by quantum gravity and can explain the small neutrino masses with new singlet states that propagate in the bulk. In such a case, a Kaluza-Klein tower of sterile neutrinos emerges. We revisit constraints on towers of sterile neutrinos that come from cosmological observables such as the effective number of noninteracting relativistic species and the dark matter density. These limits generically rule out micron-sized extra dimensions. We explore the weakening of these constraints to accommodate an extra dimension close to the micron size by assuming that the Universe is reheated after inflation to a low temperature. We discuss how such a possibility can be distinguished in the event of a positive signal in a cosmological observable. Published by the American Physical Society 2024

Understanding small neutrino mass and its implication

We have derived previously the relations between the neutrino masses and mixing angles in a dispersive analysis on the mixing of neutral leptonic states. The only involved assumption is that the electroweak symmetry of the Standard Model (SM) is restored at a high energy scale in some new physics scenario, which diminishes the box diagrams responsible for the mixing. Here we include corrections to the analysis up to three loops, arising from exchanges of additional neutral and charged scalars in the electroweak symmetric phase. The solution to the dispersion relation for the μ−e+-μ+e− mixing generates a typical neutrino mass mν∼O(1)eV in the SM unambiguously. The solution also favors the normal ordering of the neutrino masses over the inverted one, and links the large electroweak symmetry restoration, i.e., new physics scale to the small neutrino mass.

Probing lepton number violation and Majorana nature of neutrinos at the LHC

Observation of lepton number (L) violation by two units at colliders would provide evidence for the Majorana nature of neutrinos. We study signals of L-violation in the context of two popular models of neutrino masses, the type-II seesaw model and the Zee model, wherein small neutrino masses arise at the tree-level and one-loop level, respectively. We focus on L-violation signals at the LHC arising through the process pp → ℓ±ℓ′± + jets within these frameworks. We obtain sensitivity to L-violation in the type-II seesaw model for triplet scalar masses up to 700 GeV and in the Zee model for charged scalar masses up to 4.8 TeV at the high-luminosity LHC with an integrated luminosity of 3 ab−1.

Testing the number of neutrino species with a global fit of neutrino data

We present the first experimental constraints on models with many additional neutrino species with a similar Yukawa coupling with their right-handed partners among the additional Higgsed sectors by an analysis of current neutrino data. These types of models are motivated as a solution to the hierarchy problem by lowering the species scale of gravity to TeV. Additionally, they offer a natural mechanism to generate small neutrino masses and provide interesting dark matter candidates. This study analyzes data from DayaBay, KamLAND, MINOS, NOνA, and KATRIN. We do not find evidence for the presence of any additional neutrino species, therefore we report lower bounds on the allowed number of neutrino species realized in nature. For the normal/inverted neutrino mass ordering, we can give a lower bound on the number of neutrino species of O(30) and O(100), respectively, over a large range of the parameter space. Published by the American Physical Society 2024

CP violation in lepton-number-conserving processes through heavy Majorana neutrinos at future lepton colliders

Small neutrino masses confirmed in the neutrino oscillation experiments indicate the need for new physics beyond the standard model. Seesaw mechanism is an interesting way to extend the standard model for explaining the neutrino masses. In a low-scale type-I seesaw mechanism, the tiny masses of neutrinos can be explained by heavy Majorana neutrino masses. Heavy Majorana neutrinos can lead to lepton-number-violating processes and the induced CP violation can contribute to the baryon asymmetry in the Universe. Heavy Majorana neutrinos can also lead to lepton-number-conserving processes and in this paper, we study the CP violation in lepton-number-conserving processes through heavy Majorana neutrinos at future lepton colliders. New possible observations of CP violation can also be connected to evidences of new physics beyond the standard model.

Contribution of BSM Particles on Electric Dipole Moment of Charged Leptons in the Minimal Left-Right Symmetric Model

In this paper, the contribution of Beyond the Standard Model (BSM) particles on the Electric Dipole Moment (EDM) of charged lepton is calculated in a minimal Left-Right Symmetric Model. First-order quantum correction i.e., one loop correction, from relevant diagrams in the unitary gauge using Feynman parameterization is used because there is no tree-level contribution in Electric Dipole Moment due to non-renormalizability of the dipole term in the Lagrangian. This study mainly focuses on the BSM Higgs sector contribution to the EDM of charged leptons due to phenomenological interest. CP violation is directly connected to the EDM of fundamental particles. Matter-antimatter asymmetry is a mystery of the universe that demands CP symmetry to be violated. The standard model cannot explain the observed CP violation. Standard Model (SM) is so far the best description of nature. Despite having tremendous success, e.g., Electro-weak Symmetry Breaking and Higgs discovery, the Standard model is not complete. Moreover, experimentally, we observed small neutrino mass and neutrino mixing. The Seesaw Mechanism can explain neutrino mass smallness and mixing introducing a large Majorana mass. Left-right symmetric model is the most natural extension of SM which can accommodate the Seesaw mechanism. So EDM contributions from BSM particles in the Left-Right symmetric Model are a good probe of new physics beyond the SM. These contributions then can be used to do phenomenology and test the Model.

B − L model with D 4 × Z 4 × Z 2 symmetry for fermion mass hierarchies and mixings* *This research was funded by Tay Nguyen University under grant number T2023-45CBTÐ

We constructed a gauge model with symmetry to explain the quark and lepton mass hierarchies and their mixings with realistic CP phases via the type-I seesaw mechanism. Six quark mases, three quark mixing angles, and the CP phase in the quark sector take the central values whereas Yukawa couplings in the quark sector are diluted in a range of difference of three orders of magnitude by the perturbation theory at the first order. Concerning the neutrino sector, a small neutrino mass is achieved by the type-I seesaw mechanism. Both inverted and normal neutrino mass hierarchies are consistent with the experimental data. The predicted sum of neutrino masses for normal and inverted hierarchies, the effective neutrino masses, and the Dirac CP phase are also consistent with recently reported limits.

Low scale leptogenesis in singlet-triplet scotogenic model

The scotogenic model presents an elegant and succinct framework for elucidating the origin of tiny neutrino masses within the framework of the Standard Model, employing radiative corrections within the domain of the dark sector. We investigate the possibility of achieving low-scale leptogenesis in the singlet-triplet scotogenic model (STSM), where dark matter mediates neutrino mass generation. We initially considered a scenario involving two moderately hierarchical heavy fermions, N and Σ, wherein the lepton asymmetry is generated by the out-of-equilibrium decay of both particles. Our analysis indicates that the scale of leptogenesis in this scenario is similar to that of standard thermal leptogenesis and is approximately M N,Σ ∼ 109 GeV, which is comparable to the Type-I seesaw case. Further, we consider the case with three heavy fermions (N 2, N 2, and Σ) with the hierarchy M N 1 < M Σ ≪ MM N 2 , which yields the lower bound on heavy fermions up to 3.1 TeV, therefore significantly reduce the scale of the leptogenesis up to TeV scale. The only prerequisite is suppression in the N 1 and Σ Yukawa couplings, which causes suppressed washout effects and a small active neutrino mass of about 10-5 eV. This brings about the fascinating insight that experiments aiming to measure the absolute neutrino mass scale can test low-scale leptogenesis in the scotogenic model. Further, the hyperchargeless scalar triplet Ω provides an additional contribution to mass of the W-boson explaining CDF-II result.

Naturally small neutrino mass with asymptotic safety and gravitational-wave signatures

We revisit the dynamical generation of an arbitrarily small neutrino Yukawa coupling in the Standard Model with trans-Planckian asymptotic safety and apply the same mechanism to the gauged B − L model. We show that thanks to the presence of additional irrelevant couplings, the described neutrino-mass generation in the B − L model is potentially more in line with existing theoretical calculations in quantum gravity. Interestingly, the model can accommodate, in full naturalness and without extensions, the possibility of purely Dirac, pseudo-Dirac, and Majorana neutrinos with any see-saw scale. We investigate eventual distinctive signatures of these cases in the detection of gravitational waves from first-order phase transitions. We find that, while it is easy to produce a signal observable in new-generation space interferometers, its discriminating features are washed out by the strong dependence of the gravitational-wave spectrum on the relevant parameters of the scalar potential.

Towers and hierarchies in the Standard Model from Emergence in Quantum Gravity

Based on Quantum Gravity arguments, it has been suggested that all kinetic terms of light particles below the UV cut-off could arise in the IR via quantum (loop) corrections. These loop corrections involve infinite towers of states becoming light (e.g. Kaluza-Klein or string towers). We study implications of this Emergence Proposal for fundamental scales in the Standard Model (SM). In this scheme all Yukawa couplings are of order one in the UV and small Yukawas for lighter generations appear via large anomalous dimensions induced by the towers of states. Thus, the observed hierarchies of quark and lepton masses are a reflection of the structure of towers of states that lie below the Quantum Gravity scale, ΛQG. Small Dirac neutrino masses consistent with experimental observation appear due to the existence of a tower of SM singlet states of mass m0 ≃ Yν3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {Y}_{\ u_3} $$\\end{document}Mp ≃ 7 × 105 GeV, opening up a new extra dimension, while the UV cut-off occurs at ΛQG ≲ 1014 GeV. Additional constraints relating the Electro-Weak (EW) and cosmological constant (c.c.) scales (denoted MEW and V0) appear if the Swampland condition mν1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {m}_{\ u_1} $$\\end{document} ≲ V01/4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {V}_0^{1/4} $$\\end{document} is imposed (with ν1 denoting the lightest neutrino), which itself arises upon applying the AdS non-SUSY Conjecture or the AdS/dS Distance Conjecture to the 3d vacua from circle compactifications of the SM. In particular, the EW scale and that of the extra dimension fulfill m0MEW ≲ 102V01/4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {V}_0^{1/4} $$\\end{document}Mp, thus relating the EW hierarchy problem to that of the c.c. Hence, all fundamental scales may be written as powers of the c.c., i.e. m• ∼ V0δMp1−4δ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {V}_0^{\\delta }{M}_p^{1-4\\delta } $$\\end{document}. The scale of SUSY breaking is m3/2 ≲ 7 × 105 GeV, which favours a Mini-Split scenario that could be possibly tested at LHC and/or FCC.

Phenomenology of the simplest linear seesaw mechanism

The linear seesaw mechanism provides a simple way to generate neutrino masses. In addition to Standard Model particles, it includes quasi-Dirac leptons as neutrino mass mediators, and a leptophilic scalar doublet seeding small neutrino masses. Here we review its associated physics, including restrictions from theory and phenomenology. The model yields potentially detectable μ → eγ rates as well as distinctive signatures in the production and decay of heavy neutrinos (Ni) and the charged Higgs boson (H±) arising from the second scalar doublet. We have found that production processes such as e+e−→ NN, e−γ → NH− and e+e−→ H+H− followed by the decay chain {H}^{pm}to {ell}_i^{pm }N , Nto {ell}_j^{pm }{W}^{mp } leads to striking lepton number violation signatures at high energies which may probe the Majorana nature of neutrinos.

Enhanced neutrino polarizability

We point out that neutrinos can have enhanced couplings to photons, if light (pseudo)scalar mediators are present, resulting in a potentially measurable neutrino polarizability. We show that the expected suppression from small neutrino masses can be compensated by the light mediator mass, generating dimension 7 Rayleigh operators at low scales. We explore the rich phenomenology of such models, computing in detail the constraints on the viable parameter space, spanned by the couplings of the mediator to neutrinos and photons. Finally, we build several explicit models that lead to an enhanced neutrino polarizability by modifying the inverse see-saw majoron, i.e., the pseudo-Nambu-Goldstone boson of the U(1)L global lepton number responsible for generating small neutrino masses.

Heavy neutral leptons at muon colliders

The future high-energy muon colliders, featuring both high energy and low background, could play a critical role in our searches for new physics. The smallness of neutrino mass is a puzzle of particle physics. Broad classes of solutions to the neutrino puzzles can be best tested by seeking the partners of SM light neutrinos, dubbed as heavy neutral leptons (HNLs), at muon colliders. We can parametrize HNLs in terms of the mass mN and the mixing angle with ℓ-flavor Uℓ. In this work, we focus on the regime mN> O(100) GeV and study the projected sensitivities on the |Uℓ|2 − mN plane with the full-reconstructable HNL decay into a hadronic W and a charged lepton. The projected reach in |Uℓ|2 leads to the best sensitivities in the TeV realm.

Search for heavy Majorana neutrinos at future lepton colliders* *Supported in part by the National Natural Science Foundation of China (12235008, 11875179), and the Natural Science Foundation of Shandong Province, China (ZR2021QA040)

A nonzero neutrino mass may be a sign of new physics beyond the standard model (SM). To explain the small neutrino mass, we can extend the SM using right-handed Majorana neutrinos in a low-scale seesaw mechanism, and the CP violation effect can be induced due to the CP phase in the interference of heavy Majorana neutrinos. The existence of heavy Majorana neutrinos may lead to lepton number violation processes, which can be used to search for the signals of heavy Majorana neutrinos. In this paper, we focus on the CP violation effect related to two generations of heavy Majorana neutrinos at GeV GeV in the pair production of W bosons and rare decays. It is valuable to investigate Majorana neutrino production signals and the related CP violation effects in rare W boson decays at future lepton colliders.

Dark matter and radiative neutrino masses in conversion-driven scotogenesis

The scotogenic model generates Majorana neutrino masses radiatively, with dark matter particles running in the loop. We explore the parameter space in which the relic density of fermionic dark matter is generated via a conversion-driven freeze-out mechanism. The necessity for small Yukawa couplings to initiate chemical decoupling for conversion processes naturally reproduces small neutrino masses as long as the active neutrinos are hierarchical. The model can also resolve the recently reported deviation in the $W$-boson mass while satisfying constraints from direct detection, charged lepton flavor violation as well as collider bounds. Parts of the parameter space lead to long-lived particle signatures to be probed at the LHC.

W boson mass shift, dark matter, and (g−2)ℓ in a scotogenic-Zee model

We present a singly charged scalar extension of the scotogenic model, the scotogenic-Zee model, which resolves the recently reported deviations in the $W$ boson mass as well as lepton $g\ensuremath{-}2$. The model admits a scalar or a fermionic dark matter while realizing naturally small radiative neutrino masses. The mass splitting of $\ensuremath{\sim}100\text{ }\text{ }\mathrm{GeV}$, which is required by the shift in $W$ boson mass, among the inert doublet fields can be evaded by its mixing with the singlet scalar, which is also key to resolving the $(g\ensuremath{-}2{)}_{\ensuremath{\ell}}$ anomaly within $1\ensuremath{\sigma}$. We establish the consistency of this framework with dark matter relic abundance while satisfying constraints from charged lepton flavor violation, direct detection, and collider bounds. The model gives predictions for the lepton flavor violating $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\ell}\ensuremath{\gamma}$ processes that will be testable in upcoming experiments.

Limits on the cosmic neutrino background

We present the first comprehensive discussion of constraints on the cosmic neutrino background (CνB) overdensity, including theoretical, experimental and cosmological limits for a wide range of neutrino masses and temperatures. Additionally, we calculate the sensitivities of future direct and indirect relic neutrino detection experiments and compare the results with the existing constraints, extending several previous analyses by taking into account that the CνB reference frame may not be aligned with that of the Earth. The Pauli exclusion principle strongly disfavours overdensities ην ≫ 1 at small neutrino masses, but allows for overdensities ην ≲ 125 at the KATRIN mass bound mν ≃ 0.8 eV. On the other hand, cosmology strongly favours 0.2 ≲ ην ≲ 3.5 in all scenarios. We find that direct detection proposals are capable of observing the CνB without a significant overdensity for neutrino masses mν ≳ 50 meV, but require an overdensity ην ≳ 3 × 105 outside of this range. We also demonstrate that relic neutrino detection proposals are sensitive to the helicity composition of the CνB, whilst some may be able to distinguish between Dirac and Majorana neutrinos.

Type II Dirac seesaw portal to the mirror sector: Connecting neutrino masses and a solution to the strong CP problem

We present a version of the Type II seesaw mechanism for parametrically small Dirac neutrino masses. Our model starts from an $\mathrm{SU}(2{)}_{\mathrm{L}}\ensuremath{\bigotimes}\mathrm{SU}(2{)}^{\ensuremath{'}}\ensuremath{\bigotimes}\mathrm{U}(1{)}_{\mathrm{X}}$ gauge extension of the Standard Model involving a sector of mirror fermions. A bidoublet scalar with a very small vacuum expectation value connects the SM leptons with their mirror counterparts, and we can identify the mirror neutrino with the right-handed neutrino. Similar to the conventional Type II seesaw, our particle spectrum features singly and doubly charged scalars. The strong $CP$ problem is solved by a discrete exchange symmetry between the two sectors that forces the contributions of quarks and mirror quarks to the strong $CP$ phase to cancel each other. We discuss the low-energy phenomenology, comment on the cosmological implications of this scenario, and indicate how to realize successful Dirac leptogenesis.

Gravitational waves-tomography of Low-Scale-Leptogenesis

A long-lived scalar field (Φ) which couples weakly to the right-handed (RH) neutrinos (NRi), generates small RH neutrino masses (Mi) in Low-Scale-Leptogenesis (LSL) mechanisms, despite having a large vacuum expectation value vΦ. In this case, the correlation shared by the Mis and the duration of the non-standard cosmic history driven by the Φ provides an excellent opportunity to study LSL signatures on primordial gravitational waves (GWs). We find it engaging, specifically for the gravitational waves that originate due to the inflationary blue-tilted tensor power spectrum and propagate through the non-standard cosmic epoch. Depending on Mi, broadly, the scenario has two significant consequences. First, if LSL is at play, GWs with a sizeable blue tilt do not contradict the Big-Bang-Nucleosynthesis (BBN) bound even for the post-inflationary models with very high-scale reheating. Second, it opens up a possibility to probe LSLs via a low-frequency and a complementary high-frequency measurement of GW-spectral shapes which are typically double-peaked. For a case study, we consider the recent results on GWs from the Pulsar-Timing-Arrays (PTAs) as a ‘measurement’ at the low frequencies and forecast the signatures of LSL mechanisms at the higher frequencies.