Majorana Neutrino Mass Research Articles

BINGO innovative assembly for background reduction in bolometric [formula omitted] experiments

BINGO is a project aiming to set the grounds for large-scale bolometric neutrinoless double-beta-decay experiments capable of investigating the effective Majorana neutrino mass at a few meV level. It focuses on developing innovative technologies (a detector assembly, cryogenic photodetectors and active veto) to achieve a very low background index, of the order of 10−5 counts/(keV kg yr) in the region of interest. The BINGO demonstrator, called MINI-BINGO, is designed to investigate the promising double-beta-decay isotopes 100Mo and 130Te and it will be composed of Li2MoO4 and TeO2 crystals coupled to bolometric light detectors and surrounded by a Bi4Ge3O12-based veto. This will allow us to reject a significant background in bolometers caused by surface contamination from α-active radionuclides by means of light yield selection and to mitigate other sources of background, such as surface contamination from β-active radionuclides, external γ radioactivity, and pile-up due to random coincidence of background events. This paper describes an R&D program towards the BINGO goals, particularly focusing on the development of an innovative assembly designed to reduce the passive materials within the line of sight of the detectors, which is expected to be a dominant source of background in next-generation bolometric experiments. We present the performance of two prototype modules – housing four cubic (4.5-cm side) Li2MoO4 crystals in total – operated in the Canfranc underground laboratory in Spain within a facility developed for the CROSS double-beta-decay experiment.

Neutrinoless double beta decay rates in the presence of light sterile neutrinos

We investigate neutrinoless double-beta decay (0νββ) in minimal extensions of the Standard Model of particle physics where gauge-singlet right-handed neutrinos give rise to Dirac and Majorana neutrino mass terms. We argue that the standard treatment of these scenarios, based on mass-dependent nuclear matrix elements, is missing important contributions to the 0νββ amplitude. First, new effects arise from the exchange of neutrinos with very small (ultrasoft) momenta, for which we compute the associated nuclear matrix elements for the decays of 76Ge and 136Xe. These contributions can dominate the 0νββ rate in cases with light sterile neutrinos. The ultrasoft terms are also relevant in the more standard scenario of just three light Majorana neutrinos where they lead to a 10% reduction of the total 0νββ amplitude. Secondly, we highlight the importance of short-range terms associated with medium-heavy sterile neutrinos and provide explicit formulae that can be used in phenomenological analyses. As examples we discuss impact of these new effects in several explicit scenarios, including a realistic 3 + 2 model with two right-handed gauge-singlet neutrinos.

Simultaneous Measurement of the Half-Life and Spectral Shape of ^{115}In β Decay with an Indium Iodide Cryogenic Calorimeter.

Current bounds on the neutrino Majorana mass are affected by significant uncertainties in the nuclear calculations for neutrinoless double-beta decay. A key issue for a data-driven improvement of the nuclear theory is the actual value of the axial coupling constant g_{A}, which can be investigated through forbidden β decays. We present the first measurement of the 4th-forbidden β decay of ^{115}In with a cryogenic calorimeter based on indium iodide. Exploiting the enhanced spectrum-shape method for the first time to this isotope, our study accurately determines simultaneously spectral shape, g_{A}, and half-life. The interacting shell model, which best fits our data, indicates a half-life for this decay at T_{1/2}=(5.26±0.06)×10^{14} yr.

Neutrino mixing from a fresh perspective

We propose a neutrino mass matrix texture bearing a suitable correlation m22=−2m13 and study its phenomenological implications. In light of both normal and inverted hierarchies, the texture imposes specific bounds on some observational parameters. As a potential application, the prediction of effective Majorana neutrino mass mββ is visualized for both hierarchies. To understand the proposed texture from first principle, we incorporate type-I+II seesaw mechanism in association with A4×Z10×Z2 group.

Neutrino Masses from Generalized Symmetry Breaking

We explore generalized global symmetries in theories of physics beyond the standard model. Theories of Z′ bosons generically contain “noninvertible” chiral symmetries, whose presence indicates a natural paradigm to break this symmetry by an exponentially small amount in an ultraviolet completion. For example, in models of gauged lepton family difference such as the phenomenologically well motivated U(1)Lμ−Lτ, there is a noninvertible lepton number symmetry which protects neutrino masses. We embed these theories in gauged non-Abelian horizontal lepton symmetries, e.g., U(1)Lμ−Lτ⊂SU(3)H, where the generalized symmetries are broken nonperturbatively by the existence of lepton family magnetic monopoles. In such theories, either Majorana or Dirac neutrino masses may be generated through quantum gauge theory effects from the charged lepton Yukawas, e.g., yν∼yτexp(−Sinst). These theories require no bevy of new fields nor additional global symmetries but are instead simple, natural, and predictive: The discovery of a lepton family Z′ at low energies will reveal the scale at which Lμ−Lτ emerges from a larger gauge symmetry. Published by the American Physical Society 2024

Non-holomorphic modular flavor symmetry

The formalism of non-holomorphic modular flavor symmetry is developed, and the Yukawa couplings are level N polyharmonic Maaß forms satisfying the Laplacian condition. We find that the integer (even) weight polyharmonic Maaß forms of level N can be decomposed into multiplets of the finite modular group ΓN′\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\Gamma}_N^{\\prime } $$\\end{document} (ΓN). The original modular invariance approach is extended by the presence of negative weight polyharmonic Maaß forms. The non-holomorphic modular flavor symmetry can be consistently combined with the generalized CP symmetry. We present three example models for lepton sector based on the Γ3 ≅ A4 modular symmetry, the charged lepton masses and the neutrino oscillation data can be accommodated very well, and the predictions for the leptonic CP violation phases and the effective Majorana neutrino mass are studied.

Neutrino masses from new seesaw models: low-scale variants and phenomenological implications

With just the Standard Model Higgs doublet, there are only three types of seesaw models that generate light Majorana neutrino masses at tree level after electroweak spontaneous symmetry breaking. However, if there exist additional TeV scalars acquiring vacuum expectation values, coupled with heavier fermionic multiplets, several new seesaw models become possible. These new seesaws are the primary focus of this study and correspond to the tree-level ultraviolet completions of the effective operators studied in a companion publication. We are interested in the genuine cases, in which the standard seesaw contributions are absent. In addition to the tree-level generation of neutrino masses, we also consider the one-loop contributions. Furthermore, we construct low-energy versions that exhibit a very rich phenomenology. Specifically, we scrutinise the generation of dimension-6 operators and explore their implications, including non-unitarity of the leptonic mixing matrix, non-universal Z-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Z-$$\\end{document}boson interactions, and lepton flavor violation. Finally, we provide (Generalised) Scotogenic-like variants that incorporate viable dark matter candidates.

Search for heavy Majorana neutrinos in e±e± and e±μ± final states via WW scattering in pp collisions at [formula omitted] TeV with the ATLAS detector

A search for heavy Majorana neutrinos in scattering of same-sign W boson pairs in proton–proton collisions at s=13 TeV at the LHC is reported. The dataset used corresponds to an integrated luminosity of 140 fb−1, collected with the ATLAS detector during 2015–2018. The search is performed in final states including a same-sign ee or eμ pair and at least two jets with large invariant mass and a large rapidity difference. No significant excess of events with respect to the Standard Model background predictions is observed. The results are interpreted in a benchmark scenario of the Phenomenological Type-I Seesaw model. New constraints are set on the values of the |VeN|2 and |VeNVμN⁎| parameters for heavy Majorana neutrino masses between 50 GeV and 20 TeV, where VℓN is the matrix element describing the mixing of the heavy Majorana neutrino mass eigenstate with the Standard Model neutrino of flavour ℓ=e,μ. The sensitivity to the Weinberg operator is investigated and constraints on the effective ee and eμ Majorana neutrino masses are reported. The statistical combination of the ee and eμ channels with the previously published μμ channel is performed.

A realistic neutrino mixing scheme arising from A4 symmetry

We propose a unique lepton mixing scheme and its association with an exact hierarchy-philic neutrino mass matrix texture in the light of Type-I+Type-II seesaw mechanism under the framework of discrete flavour symmetry. The proposed model successfully predicts the normal ordering of neutrino masses and the two Majorana phases. Additionally, the analysis extends to the effective Majorana neutrino mass, in the context of neutrinoless double beta .

Searching for 76Ge neutrinoless double beta decay with the CDEX-1B experiment* *Supported by the National Key Research and Development Program of China (2017YFA0402201, 2022YFA1604701, 2022YFA1605000) and the National Natural Science Foundation of China (12322511, 12175112, 12005111, 11725522)

We operated a p-type point contact high purity germanium (PPCGe) detector (CDEX-1B, 1.008 kg) in the China Jinping Underground Laboratory (CJPL) for 500.3 days to search for neutrinoless double beta ( ) decay of 76Ge. A total of 504.3 kg day effective exposure data was accumulated. The anti-coincidence and the multi/single-site event (MSE/SSE) discrimination methods were used to suppress the background in the energy region of interest (ROI, 1989–2089 keV for this work) with a factor of 23. A background level of 0.33 counts/ (keV kg yr) was realized. The lower limit on the half life of 76Ge decay was constrained as , corresponding to the upper limits on the effective Majorana neutrino mass: 3.2–7.5 .

The flavor-dependent U(1)F\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$U(1)_F$$\\end{document} model

A flavor-dependent model (FDM) is proposed in this work. The model extends the Standard Model by an extra U(1)F\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$U(1)_F$$\\end{document} local gauge group, two scalar doublets, one scalar singlet and two right-handed neutrinos, where the additional U(1)F\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$U(1)_F$$\\end{document} charges are related to the particles’ flavor. The new fermion sector in the FDM can explain the flavor mixings puzzle and the mass hierarchy puzzle simultaneously, and the nonzero Majorana neutrino masses can be obtained naturally by the Type I see-saw mechanism. In addition, the B meson rare decay processes B¯→Xsγ,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\bar{B}} \\rightarrow X_s\\gamma ,$$\\end{document}Bs0→μ+μ-,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$B_s^0 \\rightarrow \\mu ^+\\mu ^-,$$\\end{document} the top quark rare decay processes t→ch,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$t\\rightarrow ch,$$\\end{document}t→uh\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$t\\rightarrow uh$$\\end{document} and the τ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au $$\\end{document} lepton flavor violation processes τ→3e,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au \\rightarrow 3e,$$\\end{document}τ→3μ,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au \\rightarrow 3\\mu ,$$\\end{document}μ→3e\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu \\rightarrow 3e$$\\end{document} predicted in the FDM are analyzed.

νe → νe scattering with massive Dirac or Majorana neutrinos and general interactions

We calculate the neutrino-electron elastic scattering cross section, extending the results previously obtained in ref. [1], in the presence of generic new interactions that take into account all the effects caused by finite neutrino masses. We address the potential significance of a heavy neutrino sector during precision measurements, particularly for tau neutrinos scattering with masses in the MeV range, for which the existing upper bounds on |Uτ4|2 would result in conceivably measurable contributions. Finally, we comment on the possibility to distinguish between Dirac and Majorana neutrinos, including the analysis of the new emerging parameters and its application to illustrative model-dependent scenarios.

Towards a systematic study of non-thermal leptogenesis from inflaton decays

This paper investigates non-thermal leptogenesis from inflaton decays in the minimal extension of the canonical type-I seesaw model, where a complex singlet scalar ϕ is introduced to generate the Majorana masses of right-handed neutrinos (RHNs) and to play the role of inflaton. First, we systematically study non-thermal leptogenesis with the least model dependence. We give a general classification of the parameter space and find four characteristic limits by carefully examining the interplay between inflaton decay into RHNs and the decay of RHNs into the standard-model particles. Three of the four limits are truly non-thermal, with a final efficiency larger than that of thermal leptogenesis. Two analytic estimates for these three limits are provided with working conditions to examine the validity. In particular, we find that the strongly non-thermal RHNs scenario occupies a large parameter space, including the oscillation-preferred K range, and works well for a relatively-low reheating temperature TRH ≥ 103 GeV, extending the lower bound on the RHN mass to 2 × 107 GeV. The lepton flavor effects are discussed. Second, we demonstrate that such a unified picture for inflation, neutrino masses, and baryon number asymmetry can be realized by either a Coleman-Weinberg potential (for the real part of ϕ) or a natural inflation potential (for the imaginary part of ϕ). The allowed parameter ranges for successful inflation and non-thermal leptogenesis are much more constrained than those without inflationary observations. We find that non-thermal leptogenesis from inflaton decay offers a testable framework for the early Universe. It can be further tested with upcoming cosmological and neutrino data. The model-independent investigation of non-thermal leptogenesis should be useful in exploring this direction.

Neutrino masses from new Weinberg-like operators: phenomenology of TeV scalar multiplets

The unique dimension-5 effective operator, LLHH, known as the Weinberg operator, generates tiny Majorana masses for neutrinos after electroweak spontaneous symmetry breaking. If there are new scalar multiplets that take vacuum expectation values (VEVs), they should not be far from the electroweak scale. Consequently, they may generate new dimension-5 Weinberg-like operators which in turn also contribute to Majorana neutrino masses. In this study, we consider scenarios with one or two new scalars up to quintuplet SU(2) representations. We analyse the scalar potentials, studying whether the new VEVs can be induced and therefore are naturally suppressed, as well as the potential existence of pseudo-Nambu-Goldstone bosons. Additionally, we also obtain general limits on the new scalar multiplets from direct searches at colliders, loop corrections to electroweak precision tests and the W-boson mass.

Three-loop inverse scotogenic seesaw models

We propose a class of models providing an explanation of the origin of light neutrino masses, the baryon asymmetry of the Universe via leptogenesis and offering viable dark matter candidates. In these models the Majorana masses of the active neutrino are generated by the inverse seesaw mechanism with the lepton number violating right-handed Majorana neutrino masses μ arising at three loops. The latter is ensured by the preserved discrete symmetries, which also guarantee the stability of the dark matter candidate. We focus on one of these models and perform a detailed analysis of the phenomenology of its leptonic sector. The model can successfully accommodate baryogenesis through leptogenesis in both weak and strong washout regimes. The lightest heavy fermion turns out to be a viable dark matter candidate, provided that the entries of the Majorana submatrix μ are in the keV to MeV range. The solutions are consistent with the experimental constraints, accommodating both mass orderings for active neutrinos, in particular charged-lepton flavor violating decays μ → eγ, μ → eee, and the electron-muon conversion processes get sizable rates within future sensitivity reach.

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.

The physics case for neutrino–neutrino collisions

Addressing the mass origin and properties of neutrinos is of strong interest to particle physics, baryogenesis and cosmology. Popular explanations involve physics beyond the standard model, for example, the dimension-5 Weinberg operator or heavy Majorana neutrinos arising from ‘seesaw’ models. The current best direct limits on the electron neutrino mass, derived from nuclei beta decay or neutrinoless double beta decay processes, are at the sub-electronvolt level. Here we propose a novel neutrino–neutrino collider where the neutrino beam is generated from TeV scale muon decays. Such collisions can happen between either neutrinos and anti-neutrinos, or neutrinos and neutrinos. We find that with a tiny integrated luminosity of about 10−5 fb−1 we can already expect to observe direct neutrino anti-neutrino annihilation, νν¯→Z , which also opens the door to explore neutrino related resonances νν¯→X . The low luminosity requirement can accommodate a relatively large emittance muon beam. Such a device would also allow for probing heavy Majorana neutrino and effective Majorana neutrino mass through ν ν → HH to a competitive level, for both electron and muon types.

Radiative Majorana neutrino masses in a parity solution to the strong CP problem

The strong CP problem is solved in Parity symmetric theories, with the electroweak gauge group containing SU(2)L × SU(2)R broken by the minimal set of Higgs fields. Neutrino masses may be explained by adding the same number of gauge singlet fermions as the number of generations. The neutrino masses vanish at tree-level and are only radiatively generated, leading to larger couplings of right-handed neutrinos to Standard Model particles than with the tree-level seesaw mechanism. We compute these radiative corrections and the mixing angles between left- and right-handed neutrinos. We discuss sensitivities to these right-handed neutrinos from a variety of future experiments that search for heavy neutral leptons with masses from tens of MeV to the multi-TeV scale.

Neutrino phenomenology in a model with generalized CP symmetry within type-I seesaw framework

We investigate the consequences of generalized CP (GCP) symmetry within the context of the two Higgs doublet model (2HDM), specifically focusing on the lepton sector. Utilizing the type-I seesaw framework, we study an intriguing connection between the Dirac Yukawa couplings originating from both Higgs fields, leading to a reduction in the number of independent Yukawa couplings and simplifying the scalar and Yukawa sectors when compared to the general 2HDM. The constraints coming from CP symmetry of class three (CP3) results in two right-handed neutrinos having equal masses and leads to a diagonal right-handed Majorana neutrino mass matrix. Notably, CP symmetry experiences a soft breaking due to the phase associated with the vacuum expectation value of the second Higgs doublet. The model aligns well with observed charged lepton masses and neutrino oscillation data, explaining both masses and mixing angles, and yields distinct predictions for normal and inverted neutrino mass hierarchies. It features a novel interplay between atmospheric mixing angle θ23 and neutrino mass hierarchy: The angle θ23 is below maximal for the normal hierarchy and above maximal for inverted hierarchy. Another interesting feature of the model is inherent CP violation for the inverted hierarchy. Published by the American Physical Society 2024

Soft supersymmetry breaking as the sole origin of neutrino masses and lepton number violation

We discuss a scenario in which the supergravity induced soft terms, conventionally used for breaking supersymmetry, also lead to non-zero Majorana neutrino masses. The soft terms lead to the spontaneous violation of the lepton number at the gravitino mass scale m3/2 which in turn leads to (i) the Majorana masses of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(m3/2) for the right-handed neutrinos and (ii) the R-parity breaking at the same scale. The former contributes to light neutrino masses through the type I seesaw mechanism, while the latter adds to it through neutrino-neutralino mixing. Both contributions can scale inversely with respect to m3/2 given that gaugino and Higgsino masses are also of order m3/2. Together, these two contributions adequately explain observed neutrino masses and mixing. One realization of the scenario also naturally leads to a μ parameter of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(m3/2). Despite the lepton number symmetry breaking close to the weak scale, the Majoron in the model exhibits very weak coupling to leptons, satisfying existing constraints on Majoron-lepton interactions. The right-handed neutrinos in the model have a large coupling to Higgsinos. This coupling and the relatively large heavy-light neutrino mixing induced through the seesaw mechanism may lead to the observable signals at colliders in terms of displaced vertices.