Sterile Mixing Angles Research Articles

Study of neutrinoless double beta decay in the Standard Model extended with sterile neutrinos

We study a model where the Standard Model is augmented with three sterile neutrinos. By adopting a particular parameterization of a (6×6)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(6\ imes 6)$$\\end{document} unitary matrix—in this context, light neutrino masses being generated via a type-I seesaw mechanism—we analytically derive the masses of the sterile states using an exact seesaw relation. The masses of the sterile states are derived in terms of the lightest mass of active neutrinos and active–active and active–sterile mixing angles and phases; they can be all light, all heavy, or a mixture of light and heavy compared to the active states. This can be attributed to the interplay of the CP-violating (CPV) phases of the mixing matrix. As both active and sterile states can mediate the neutrinoless double beta decay (0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0\ u \\beta \\beta $$\\end{document}) process, their contributions to the effective mass of the electron neutrino, |mee|\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$|m_{ee}|$$\\end{document}, become a function of the mass of the lightest active state and active–active and active–sterile mixing angles and phases. We explore the parameter space of |mee|\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$|m_{ee}|$$\\end{document}, keeping in mind the present and future sensitivity of 0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0\ u \\beta \\beta $$\\end{document} decay searches. By making use of constraints from charged lepton flavor-violating (cLFV) processes and non-unitarity, we explore the role of additional CPV phases and active–sterile mixing angle values. The numerical values thus obtained for |mee|\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$|m_{ee}|$$\\end{document} can vary from as low as O(10-4)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathscr {O}(10^{-4})$$\\end{document} to saturating the present experimental limit. We also check the reliability of our result by calculating the branching ratio of μ→eγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu \\rightarrow e \\gamma $$\\end{document}, a prominent cLFV process, and non-unitarity in this framework.

Constraining super-light sterile neutrinos at Borexino and KamLAND

The presence of a super-light sterile neutrino can lead to a dip in the survival probability of solar neutrinos, and explain the suppression of the upturn in the low energy solar neutrino data. In this work, we systematically study the survival probabilities in the 3+1 framework by taking into account of the non-adiabatic transitions and the coherence effect. We obtain an analytic equation that can predict the position of the dip. We also place constraints on the parameter space of sterile neutrinos by using the latest Borexino and KamLAND data. We find that the low and high energy neutrino data at Borexino are sensitive to different regions in the sterile neutrino parameter space. In the case with only θ01 being nonzero, the 8B data sets the strongest bounds at Delta {m}_{01}^2 ≈ (1.1 ∼ 2.2) Delta {m}_{21}^2 , while the low energy neutrino data is more sensitive to other mass-squared regions. The lowest bounds on Delta {m}_{01}^2 from the pp data can reach 10−12 eV2 because of the coherence effect. Also, due to the presence of non-adiabatic transitions, the bounds in the range of 10−9 eV2 ≲ Delta {m}_{01}^2 ≲ 10−5 eV2 become weaker as Delta {m}_{01}^2 or sin2 2θ01 decreases. We also find that in the case with only θ02 or θ03 being nonzero, the low energy solar neutrino data set similar but weaker bounds as compared to the case with only θ01 being nonzero. However, the bounds from the high energy solar data and the KamLAND data are largely affected by the sterile mixing angles.

New physics at nuSTORM

In this work we investigate the usefulness of nuSTORM as a probe of two new-physics scenarios which are sterile neutrinos and non-unitarity of the neutrino mixing matrix. For the sterile neutrino we show the importance of the neutral current events when combined with the charged current events to constrain the effective mixing angle, $\theta_{\mu \mu}$, and the sterile mixing angles $\theta_{14}$ and $\theta_{24}$. We also study the role nuSTORM will play in the study of neutrino oscillation physics if the three generation neutrino mixing matrix is non-unitary. In this context we elucidate the role of nuSTORM, considering both charged current and neutral current events, in constraining the various non-unitarity parameters such as $\alpha_{11}$, $|\alpha_{21}|$ and $\alpha_{22}$.

Consequences of μ−τ reflection symmetry for 3+1 neutrino mixing

We investigate the consequences of $\mu-\tau$ reflection symmetry in presence of a light sterile neutrino for the $3+1$ neutrino mixing scheme. We discuss the implications of total $\mu-\tau$ reflection symmetry as well partial $\mu-\tau$ reflection symmetry. For the total $\mu-\tau$ reflection symmetry we find values of $\theta_{23}$ and $\delta$ remains confined near $\pi/4$ and $\pm \pi/2$ respectively. The current allowed region for $\theta_{23}$ and $\delta$ in case of inverted hierarchy lies outside the area preferred by the total $\mu-\tau$ reflection symmetry. However, interesting predictions on the neutrino mixing angles and Dirac CP violating phases are obtained considering partial $\mu-\tau$ reflection symmetry. We obtain predictive correlations between the neutrino mixing angle $\theta_{23}$ and Dirac CP phase $\delta$ and study the testability of these correlations at the future long baseline experiment DUNE. We find that while the imposition of $\mu-\tau$ reflection symmetry in the first column admit both normal and inverted neutrino mass hierarchy, demanding $\mu-\tau$ reflection symmetry for the second column excludes the inverted hierarchy. Interestingly, the sterile mixing angle $\theta_{34}$ gets tightly constrained considering the $\mu-\tau$ reflection symmetry in the fourth column. We also study consequences of $\mu-\tau$ reflection symmetry for the Majorana phases and neutrinoless double beta decay.

Measuring the sterile neutrino CP phase at DUNE and T2HK

The CP phases associated with the sterile neutrino cannot be measured in the dedicated short-baseline experiments being built to test the sterile neutrino hypothesis. On the other hand, these phases can be measured in long-baseline experiments, even though the main goal of these experiments is not to test or measure sterile neutrino parameters. In particular, the sterile neutrino phase delta _{24} affects the charged-current electron appearance data in long-baseline experiment. In this paper we show how well the sterile neutrino phase delta _{24} can be measured by the next-generation long-baseline experiments DUNE, T2HK (and T2HKK). We also show the expected precision with which this sterile phase can be measured by combining the DUNE data with data from T2HK or T2HKK. The T2HK experiment is seen to be able to measure the sterile phase delta _{24} to a reasonable precision. We also present the sensitivity of these experiments to the sterile mixing angles, both by themselves, as well as when DUNE is combined with T2HK or T2HKK.

Imprints of a light sterile neutrino at DUNE, T2HK, and T2HKK

We evaluate the impact of sterile neutrino oscillations in the so-called 3+1 scenario on the proposed long baseline experiment in USA and Japan. There are two proposals for the Japan experiment which are called T2HK and T2HKK. We show the impact of sterile neutrino oscillation parameters on the expected sensitivity of T2HK and T2HKK to mass hierarchy, CP violation and octant of $\theta_{23}$ and compare it against that expected in the case of standard oscillations. We add the expected ten years data from DUNE and present the combined expected sensitivity of T2HKK+DUNE to the oscillation parameters. We do a full marginalisation over the relevant parameter space and show the effect of the magnitude of the true sterile mixing angles on the physics reach of these experiments. We show that if one assumes that the source of CP violation is the standard CP phase alone in the test case, then it appears that the expected CP violation sensitivity decreases due to sterile neutrinos. However, if we give up this assumption, then the CP sensitivity could go in either direction. The impact on expected octant of $\theta_{23}$ and mass hierarchy sensitivity is shown to depend on the magnitude of the sterile mixing angles in a nontrivial way.

Results from the search for eV-sterile neutrinos with IceCube

The IceCube neutrino telescope at the South Pole has measured the atmospheric muon neutrino spectrum as a function of zenith angle and energy. Using IceCubes full detector configuration we have performed searches for eV-scale sterile neutrinos. Such a sterile neutrino, motivated by the anomalies observed in short-baseline experiments, is expected to have a significant effect on survival probability due to matter-induced resonant effects for energies of order 1 TeV. This effect makes this search unique and sensitive to small sterile mixing angle values. This work comprises results obtained using up-going muon neutrinos taken with one year of full detector configuration.

Search for the sterile neutrino mixing with the ICAL detector at INO

The study has been carried out on the prospects of probing the sterile neutrino mixing with the magnetized iron calorimeter (ICAL) at the India-based Neutrino Observatory (INO), using atmospheric neutrinos as a source. The so-called 3 + 1 scenario is considered for active–sterile neutrino mixing and lead to projected exclusion curves in the sterile neutrino mass and mixing angle plane. The analysis is performed using the neutrino event generator NUANCE, modified for ICAL, and folded with the detector resolutions obtained by the INO collaboration from a full GEANT4-based detector simulation. A comparison has been made between the results obtained from the analysis considering only the energy and zenith angle of the muon and combined with the hadron energy due to the neutrino induced event. A small improvement has been observed with the addition of the hadron information to the muon. In the analysis we consider neutrinos coming from all zenith angles and the Earth matter effects are also included. The inclusion of events from all zenith angles improves the sensitivity to sterile neutrino mixing by about 35% over the result obtained using only down-going events. The improvement mainly stems from the impact of Earth matter effects on active–sterile mixing. The expected precision of ICAL on the active–sterile mixing is explored and the allowed confidence level (C.L.) contours presented. At the assumed true value of 10^circ for the sterile mixing angles and marginalization over Delta m^2_{41} and the sterile mixing angles, the upper bound at 90% C.L. (from two-parameter plots) is around 20^circ for theta _{14} and theta _{34}, and about 12^circ for theta _{24}.

Search for sub-eV sterile neutrinos in the precision multiple baselines reactor antineutrino oscillation experiments

According to different effects on neutrino oscillations, the unitarity violation in the MNSP matrix can be classified into the direct unitarity violation and the indirect unitarity violation which are induced by the existence of the light and the heavy sterile neutrinos respectively. Of which sub-eV sterile neutrinos are of most interesting. We study in this paper the possibility of searching for sub-eV sterile neutrinos in the precision reactor antineutrino oscillation experiments with three different baselines at around 500 m, 2 km and 60 km. We find that the antineutrino survival probabilities obtained in the reactor experiments are sensitive only to the direct unitarity violation and offer very concentrated sensitivity to the two parameters θ14 and Δm412. If such light sterile neutrinos do exist, the active–sterile mixing angle θ14 could be acquired by the combined rate analysis at all the three baselines and the mass-squared difference Δm412 could be obtained by taking the Fourier transformation to the L/E spectrum. Of course, for such measurements to succeed, both high energy resolution and large statistics are essentially important.

Implication of a vanishing element in the3+1scenario

In this paper we study the phenomenological implications of the one-zero textures of low energy neutrino mass matrices in the presence of a sterile neutrino. We consider the $3+1$ scheme and use the results from a global fit for short baseline neutrino oscillation data, which provides the bounds on the three additional mixing angles. We find that the mass matrix elements ${m}_{\ensuremath{\alpha}\ensuremath{\beta}}$ ($\ensuremath{\alpha}$, $\ensuremath{\beta}=e$, $\ensuremath{\mu}$, $\ensuremath{\tau}$) involving only the active states can assume vanishing values in the allowed parameter space for all of the mass spectrum. Among the mass matrix elements connecting the active and sterile states, ${m}_{es}$ and ${m}_{\ensuremath{\mu}s}$ can become small only for the quasidegenerate neutrinos. The element ${m}_{\ensuremath{\tau}s}$, on the other hand, can vanish even for lower values of masses since the 3--4 mixing angle only has an upper bound from current data. The mass matrix element (${m}_{ss}$) involving only the sterile state stays $\ensuremath{\sim}\mathcal{O}(1)\text{ }\text{ }\mathrm{eV}$ in the whole parameter region. We study the possible correlations between the sterile mixing angles and the Majorana phases to give a zero element in the mass matrix.

NEUTRINO PHYSICS FROM NEW MEASUREMENTS

Emergence of neutrino physics is fueled by the recent growth in quality and quantity of experimental data, yet, there are still open questions. How much can the determination of mixing parameters be improved? Can we improve bounds on θ13 before experiments designed specifically for this parameter start? How well can we determine or limit sterile fraction of neutrino flux from sun and sterile mixing angle? We examine the impact of outcome from new measurements in the context of providing answers to these questions.

Effects of sterile neutrinos on the ultrahigh-energy cosmic neutrino flux

We investigate the effect of sterile neutrinos that are nearly degenerate with active ones on the flux of ultrahigh-energy cosmic ray neutrinos at earth. This offers a way to probe neutrino oscillations in the mass-squared range 10−16 eV2≲δm2≲10−11 eV2 which maybe hard to detect by any other means. Taking into account the present experimental uncertainties of the active–active mixing angles and by allowing any values for the active–sterile mixing angles we find that the ratio of the electron and muon neutrino fluxes may change by −40% to 70% in comparison with the ratio in the absence of active–sterile mixing.