Non-unitary Mixing Research Articles

Effect of non-unitary mixing on the mass hierarchy and CP violation determination at the Protvino to ORCA experiment

In this paper, we have estimated the neutrino mass ordering and the CP violation sensitivity of the proposed Protvino to ORCA (P2O) experiment after 6 years of data-taking. Both unitary and non-unitary 3×3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3\ imes 3$$\\end{document} neutrino mass mixing have been considered in the simulations. A forecast analysis deriving possible future constraints on non-unitary parameters at P2O have been performed.

Non-unitary three-neutrino mixing in the early Universe

Deviations from unitarity in the three-neutrino mixing canonical picture are expected in many physics scenarios beyond the Standard Model. The mixing of new heavy neutral leptons with the three light neutrinos would in principle modify the strength and flavour structure of charged-current and neutral-current interactions with matter. Non-unitarity effects would therefore have an impact on the neutrino decoupling processes in the early Universe and on the value of the effective number of neutrinos, N eff. We calculate the cosmological energy density in the form of radiation with a non-unitary neutrino mixing matrix, addressing the possible interplay between parameters. Highly accurate measurements of N eff fromforthcoming cosmological observations can provide independent and complementary limits on thedepartures from unitarity. For completeness, we relate the scenario of small deviations fromunitarity to non-standard neutrino interactions and compare the forecasted constraints to otherexisting limits in the literature.

Non-Unitary Neutrino Mixing in the NOνA Near Detector Data

The νμ→νe oscillation probability over a short baseline (≲1 km) would be negligible for the case when the mixing matrix for three active neutrinos is unitary. However, in the case of a non-unitary mixing of three neutrinos, this probability would be non-negligible due to the so-called “zero distance” effect. Hence, the near detector of accelerator experiments such as NOνA can provide strong constraints on the parameters of the non-unitary mixing with very large statistics. By analyzing the NOνA near-detector data, we find that the non-unitary mixing does not improve fits to the νe or νμ events over the standard unitary mixing. This leads to constraints on the non-unitary parameters: α00>0.911, |α10|<0.020, and α11>0.952 at 90% C.L. A combined analysis with the near- and far-detector data does not change these constraints significantly.

Precision measurements and tau neutrino physics in a future accelerator neutrino experiment

We investigate prospects of building a future accelerator-based neutrino oscillation experiment in China, including site selection, beam optimization and tau neutrino physics aspects. CP violation, non-unitary mixing and non-standard neutrino interactions are discussed. We simulate neutrino beam setups based on muon and beta decay techniques and compare Chinese laboratory sites by their expected sensitivities. A case study on the Super Proton–Proton Collider and the China JinPing Laboratory is also presented. It is shown that the muon-decay-based beam setup can measure the Dirac CP phase by about 14.2° precision at 1σ CL, whereas non-unitarity can be probed down to ∣α ij ∣ ≲ 0.37 (i ≠ j = 1, 2, 3) and non-standard interactions to 0.11 (, μ, τ) at 90% CL, respectively.

A Review of the Tension between the T2K and NOνA Appearance Data and Hints to New Physics

In this article, we review the status of the tension between the long-baseline accelerator neutrino experiments T2K and NOνA. The tension arises mostly due to the mismatch in the apappearance data of the two experiments. We explain how this tension arises based on νμ→νe and ν¯μ→ν¯e oscillation probabilities. We define the reference point of vacuum oscillation, maximal θ23 and δCP and compute the νe/ν¯e appearance events for each experiment. We then study the effects of deviating the unknown parameters from the reference point and the compatibility of any given set of values of unknown parameters with the data from T2K and NOνA. T2K observes a large excess in the νe appearance event sample compared to the expected νe events at the reference point, whereas NOνA observes a moderate excess. The large excess in T2K dictates that δCP be anchored at −90° and that θ23 > π/4 with a preference for normal hierarchy. The moderate excess at NOνA leads to two degenerate solutions: (a) NH, 0 < δCP < 180°, and θ23 > π/4; (b) IH, −180° < δCP < 0, and θ23 > π/4. This is the main cause of tension between the two experiments. We review the status of three beyond standard model (BSM) physics scenarios, (a) non-unitary mixing, (b) Lorentz invariance violation, and (c) non-standard neutrino interactions, to resolve the tension.

Probing nonunitary neutrino mixing via long-baseline neutrino oscillation experiments based at J-PARC

This paper investigates the capability of long-baseline experiments, which are making use of neutrinos that are coming from Japan Proton Accelerator Research Complex (J-PARC), in establishing the unitarity of active-neutrino mixing by ruling out the non-unitary mixing scheme as a function of true values of CP-violating phase $\delta_{\mathrm{CP}}$. It is found that T2HK can establish unitarity of active neutrino mixing at above 2$\sigma$ C.L. irrespective of neutrino mass hierarchy and true value of $\delta_{CP}$, if non-unitary (NU) parameter $\alpha_{21}$ is of the order of $10^{-2}$. Further, this paper is also discuss the bound on NU parameter in 21 sector and sensitivity limit of these experiments in determining NU parameter. It is found that the bounds on $\left(\alpha_{21}/2\right)$ are 0.028, 0.0026, 0.005 at 2$\sigma$ C.L. respectively for T2K, T2HK, and T2HKK. Moreover, it is also found that the sensitivity limit of T2HK on NU parameter is far better than that of both T2HKK and T2K.

Non-unitary leptonic flavor mixing and CP violation in neutrino-antineutrino oscillations

If massive neutrinos are Majorana particles, then the lepton number should be violated in nature and neutrino-antineutrino oscillations να↔ν‾β (for α,β=e,μ,τ) will definitely take place. In the present paper, we study the properties of CP violation in neutrino-antineutrino oscillations with the non-unitary leptonic flavor mixing matrix, which is actually a natural prediction in the canonical seesaw model due to the mixing between light and heavy Majorana neutrinos. The oscillation probabilities P(να→ν‾β) and P(ν‾α→νβ) are derived, and the CP asymmetries Aαβ≡[P(να→ν‾β)−P(ν‾α→νβ)]/[P(να→ν‾β)+P(ν‾α→νβ)] are also calculated. Taking into account current experimental bounds on the leptonic unitarity violation, we show that the CP asymmetries induced by the non-unitary mixing parameters can significantly deviate from those in the limit of a unitary leptonic flavor mixing.

Searching for non-unitary neutrino oscillations in the present T2K and NOnu A data

The mixing of three active neutrino flavors is parameterized by the unitary PMNS matrix. If there are more than three neutrino flavors and if the extra generations are heavy iso-singlets, the effective 3times 3 mixing matrix for the three active neutrinos will be non-unitary. We have analyzed the latest T2K and NOnu A data with the hypothesis of non-unitary mixing of the active neutrinos. We found that the 2019 NOnu A data slightly (at sim 1, sigma CL) prefer the non-unitary mixing over unitary mixing. In fact, allowing the non-unitary mixing brings the NOnu A best-fit point in the sin ^2{theta _{23}}-delta _{mathrm {CP}} plane closer to the T2K best-fit point. The 2019 T2K data, on the other hand, cannot rule out any of the two mixing schemes. A combined analysis of the NOnu A and T2K 2019 data prefers the non-unitary mixing at 1, sigma CL. We derive constraints on the non-unitary mixing parameters using the best-fit to the combined NOnu A and T2K data. These constraints are weaker than previously found. The latest 2020 data from both the experiments prefer non-unitarity over unitary mixing at 1, sigma CL. The combined analysis prefers non-unitarity at 2, sigma CL. The stronger tension, which exists between the latest 2020 data of the two experiments, also gets reduced with non-unitary analysis.

Physics of parameter correlations around the solar-scale enhancement in neutrino theory with unitarity violation

Abstract We discuss the physics of the three neutrino flavor transformation with non-unitary mixing matrix, with particular attention to the correlation between the $\nu$SM- and the $\alpha$ parameters which represent the effect of unitarity-violating (UV) new physics. Towards this goal, a new perturbative framework is created to illuminate the effect of non-unitarity in the region of the solar-scale enhanced oscillations. We refute the skepticism about the physical reality of the $\nu$Standard Model CP phase $\delta$–$\alpha$ parameter phase correlation by analysis with the SOL convention of $U_{{\tiny MNS}}$, in which $e^{\pm i \delta}$ is attached to $s_{12}$. Then, a comparative study between the solar- and atmospheric-scale oscillation regions allowed by the framework reveals a dynamical $\delta$–(blobs of the $\alpha$ parameters) correlation in the solar oscillation region, in sharp contrast to the “chiral”-type phase correlation $[e^{- i \delta} \bar{\alpha}_{\mu e},\ e^{- i \delta} \bar{\alpha}_{\tau e},\ \bar{\alpha}_{\tau \mu}]$ in the Particle Data Group convention seen in the atmospheric oscillation region. An explicit perturbative calculation to the first order in the $\nu_{\mu} \rightarrow \nu_{e}$ channel allows us to decompose the UV related part of the probability into the unitary evolution part and the genuine non-unitary part. We observe that the effect of non-unitarity tends to cancel between these two parts, as well as between the different $\alpha_{\beta \gamma}$ parameters.

Standard versus non-standard CP phases in neutrino oscillation in matter with non-unitarity

Abstract We formulate a perturbative framework for the flavor transformation of the standard active three neutrinos but with a non-unitary flavor mixing matrix, a system which may be relevant for the leptonic unitarity test. We use the $\alpha$ parametrization of the non-unitary matrix and take its elements $\alpha_{\beta \gamma}$ ($\beta,\gamma = e,\mu,\tau$) and the ratio $\epsilon \simeq \Delta m^2_{21} / \Delta m^2_{31}$ as the small expansion parameters. Two qualitatively new features that hold in all the oscillation channels are uncovered in the probability formula obtained to first order in the expansion: (1) The phases of the complex $\alpha$ elements always come into the observable in the particular combination with the $\nu$SM CP phase $\delta$ in the form $[e^{- i \delta } \bar{\alpha}_{\mu e}, ~e^{ - i \delta} \bar{\alpha}_{\tau e}, ~\bar{\alpha}_{\tau \mu}]$ under the Particle Data Group convention of a unitary $\nu$SM mixing matrix. (2) The diagonal $\alpha$ parameters appear in particular combinations $\left( a/b - 1 \right) \alpha_{ee} + \alpha_{\mu \mu}$ and $\alpha_{\mu \mu} - \alpha_{\tau \tau}$, where $a$ and $b$ denote, respectively, the matter potential due to charged current and neutral current reactions. This property holds only in the unitary evolution part of the probability, and there is no such feature in the genuine non-unitary part, while the $\delta$–$\alpha$ parameter phase correlation exists for both. The reason for such remarkable stability of the phase correlation is discussed.

Short-baseline neutrino oscillations with 3 + 1 non-unitary mixing

We consider a scenario with unitary mixing of the three light standard neutrinos and a non-unitary mixing contribution of a heavier massive neutrino that can generate short-baseline neutrino oscillations. We show that this scenario predicts constant flavor-changing probabilities at short-baseline distances. Therefore, it cannot explain the spectral distortions observed in the LSND and MiniBooNE appearance experiments. On the other hand, the survival probabilities oscillate as functions of L/E and could explain oscillations in short-baseline disappearance experiments. We also derive the bounds on the mixing parameters from the existing short-baseline neutrino oscillation data.

Neutrino mixing, interval matrices and singular values

We study the properties of singular values of mixing matrices embedded within an experimentally determined interval matrix. We argue that any physically admissible mixing matrix needs to have the property of being a contraction. This condition constrains the interval matrix, by imposing correlations on its elements and leaving behind only physical mixings that may unveil signs of new physics in terms of extra neutrino species. We propose a description of the admissible three-dimensional mixing space as a convex hull over experimentally determined unitary mixing matrices parametrized by Euler angles which allows us to select either unitary or nonunitary mixing matrices. The unitarity-breaking cases are found through singular values and we construct unitary extensions yielding a complete theory of minimal dimensionality larger than three through the theory of unitary matrix dilations. We discuss further applications to the quark sector.

Non-unitary lepton mixing in an inverse seesaw and its impact on the physics potential of long-baseline experiments

In this paper, we consider the low energy scale inverse seesaw mechanism in which the observed neutrino mass and lepton mixing are explained by introducing right-handed neutrinos and the gauge-singlet fermions with experimentally testable energy scale. Moreover, the presence of such new fermions leads to unitarity violation in lepton mixing due to significantly large mixing between active neutrinos and the heavy fermions. In addition to this, such large lepton mixing also gives rise to potentially large lepton flavor violation, which allows one to constrain the non-unitarity parameters via lepton flavor violating decays (li → lj γ). We make use of these constraints on non-unitarity parameters and investigate their effects on the determination of current unknown oscillation parameters at long-baseline experiments. We find that non-unitarity parameters are sensitive to the NOνA experiment. However, it is observed that the NOνA experiment is not expected to improve the current knowledge of non-unitarity parameter η21. We also find that the sensitivities to current unknowns are deteriorated significantly in the presence of non-unitary lepton mixing and these sensitivities crucially depend upon the new CP-violating phase in the non-unitary mixing. Further, we find that the degeneracy resolution capability of the NOνA experiment is reduced in the presence of non-unitarity parameters. However, the synergy between the currently running experiments T2K and NOνA can improve the parameter degeneracy resolution and hence there is enhancement in the sensitivities of unknowns.

Probing CP violation with non-unitary mixing in long-baseline neutrino oscillation experiments: DUNE as a case study

When neutrino masses arise from the exchange of neutral heavy leptons, as in most seesaw schemes, the effective lepton mixing matrix N describing neutrino propagation is non-unitary, hence neutrinos are not exactly orthonormal. New CP violation phases appear in N that could be confused with the standard phase characterizing the three neutrino paradigm. We study the potential of the long-baseline neutrino experiment DUNE in probing CP violation induced by the standard CP phase in the presence of non-unitarity. In order to accomplish this we develop our previous formalism, so as to take into account the neutrino interactions with the medium, important in long baseline experiments such as DUNE. We find that the expected CP sensitivity of DUNE is somewhat degraded with respect to that characterizing the standard unitary case. However the effect is weaker than might have been expected thanks mainly to the wide neutrino beam. We also investigate the sensitivity of DUNE to the parameters characterizing non-unitarity. In this case we find that there is no improvement expected with respect to the current situation, unless the near detector setup is revamped.

Measuring the leptonic CP phase in neutrino oscillations with nonunitary mixing

Non-unitary neutrino mixing implies an extra CP violating phase that can fake the leptonic Dirac CP phase $\delta_{CP}$ of the simplest three-neutrino mixing benchmark scheme. This would hinder the possibility of probing for CP violation in accelerator-type experiments. We take T2K and T2HK as examples to demonstrate the degeneracy between the "standard" (or "unitary") and "non-unitary" CP phases. We find, under the assumption of non-unitary mixing, that their CP sensitivities severely deteriorate. Fortunately, the TNT2K proposal of supplementing T2(H)K with a $\mu$DAR source for better measurement of $\delta_{CP}$ can partially break the CP degeneracy by probing both $\cos \delta_{CP}$ and $\sin \delta_{CP}$ dependences in the wide spectrum of the $\mu$DAR flux. We also show that the further addition of a near detector to the $\mu$DAR setup can eliminate the degeneracy completely.

A framework for testing leptonic unitarity by neutrino oscillation experiments

If leptonic unitarity is violated by new physics at an energy scale much lower than the electroweak scale, which we call low-scale unitarity violation, it has different characteristic features from those expected in unitarity violation at high-energy scales. They include maintaining flavor universality and absence of zero-distance flavor transition. We present a framework for testing such unitarity violation at low energies by neutrino oscillation experiments. Starting from the unitary 3 active plus N (arbitrary positive integer) sterile neutrino model we show that by restricting the active-sterile and sterile-sterile neutrino mass squared differences to ≳ 0.1 eV2 the oscillation probability in the (3 + N) model becomes insensitive to details of the sterile sector, providing a nearly model-independent framework for testing low-scale unitarity violation. Yet, the presence of the sterile sector leaves trace as a constant probability leaking term, which distinguishes low-scale unitarity violation from the high-scale one. The non-unitary mixing matrix in the active neutrino subspace is common for the both cases. We analyze how severely the unitarity violation can be constrained in νe-row by taking a JUNO-like setting to simulate medium baseline reactor experiments. Possible modification of the features of the (3 + N) model due to matter effect is discussed to first order in the matter potential.

Probing CP violation with T2K, NOνA and DUNE in the presence of non-unitarity

The presence of non-unitary neutrino mixing can affect the measurement of the three-neutrino leptonic Dirac CP phase and hamper efforts to probe CP violation due to degeneracies of the extra non-unitary CP phase with the standard CP phase. We study the effect of including non-unitarity on probing CP violation with the long-baseline experiments NO$\nu$A, T2K and DUNE. We analyze the effect of non-unitary mixing at the level of oscillation probabilities associated with the relevant baselines, and present the CP violation sensitivity for the individual experiments and their combination. Our results show that there is an improvement in the CP violation sensitivity of the combination compared to the individual experiments.

19pAL-4 ニュートリノのCP非対称性に関する研究

19pAL-4 ニュートリノのCP非対称性に関する研究

Light sterile neutrinos and short baseline neutrino oscillation anomalies

We study two possible explanations for short baseline neutrino oscillation anomalies, such as the LSND and MiniBooNE anti-neutrino data, and for the reactor anomaly. The first scenario is the mini-seesaw mechanism with two eV-scale sterile neutrinos. We present both analytic formulas and numerical results showing that this scenario could account for the short baseline and reactor anomalies and is consistent with the observed masses and mixings of the three active neutrinos. We also show that this scenario could arise naturally from an effective theory containing a TeV-scale VEV, which could be related to other TeV-scale physics. The minimal version of the mini-seesaw relates the active-sterile mixings to five real parameters and favors an inverted hierarchy. It has the interesting property that the effective Majorana mass for neutrinoless double beta decay vanishes, while the effective masses relevant to tritium beta decay and to cosmology are respectively around 0.2 and 2.4 eV. The second scenario contains only one eV-scale sterile neutrino but with an effective non-unitary mixing matrix between the light sterile and active neutrinos. We find that though this may explain the anomalies, if the non-unitarity originates from a heavy sterile neutrino with a large (fine-tuned) mixing angle, this scenario is highly constrained by cosmological and laboratory observations.

Flavor changing neutral currents in the 3-3-1 model with right-handed neutrinos

Flavor changing neutral currents coming from a new non-universal neutral Gauge-Boson and from the non-unitary quark mixing matrix for the $SU(3)_c\otimes SU(3)_L\otimes U(1)_X$ model with right handed neutrinos are studied. By imposing as experimental constraints the measured values of the 3x3 quark mixing matrix, the neutral meson mixing, and the bounds measured values for direct flavor changing neutral current processes, the largest mixing of the known quarks with the exotic ones can be established, with new sources of flavor changing neutral currents being identified. Our main result is that for a $|V_{tb}|$ value smaller than one, large rates of rare top decays such as $t\to c\gamma$, $t\to cZ$, and $t\to cg$ (where g stands for the gluon field) are obtained; but if $|V_{tb}|\approx 1$ the model can survive present experimental limits only if the mass of the new neutral Gauge Boson becomes larger that 10 TeV.