Neutrino Mass Spectrum Research Articles

Probing new physics scenarios in accelerator and reactor neutrino experiments

We perform a detailed combined fit to the disappearence data of the Daya Bay experiment and the appearance and disappearance data of the Tokai to Kamioka (T2K) one in the presence of two models of new physics affecting neutrino oscillations, namely a model where sterile neutrinos can propagate in a large compactified extra dimension and a model where non-standard interactions (NSI) affect the neutrino production and detection. We find that the Daya Bay ⨁ T2K data combination constrains the largest radius of the compactified extra dimensions to be μm at 2σ C.L. (for the inverted ordering of the neutrino mass spectrum) and the relevant NSI parameters in the range , for particular choices of the charge parity violating phases.

The double mass hierarchy pattern: Simultaneously understanding quark and lepton mixing

The charged fermion masses of the three generations exhibit the two strong hierarchies m3≫m2≫m1. We assume that also neutrino masses satisfy mν3>mν2>mν1 and derive the consequences of the hierarchical spectra on the fermionic mixing patterns. The quark and lepton mixing matrices are built in a general framework with their matrix elements expressed in terms of the four fermion mass ratios, mu/mc, mc/mt, md/ms and ms/mb, and me/mμ, mμ/mτ, mν1/mν2 and mν2/mν3, for the quark and lepton sector, respectively. In this framework, we show that the resulting mixing matrices are consistent with data for both quarks and leptons, despite the large leptonic mixing angles. The minimal assumption we take is the one of hierarchical masses and minimal flavor symmetry breaking that strongly follows from phenomenology. No special structure of the mass matrices has to be assumed that cannot be motivated by this minimal assumption. This analysis allows us to predict the neutrino mass spectrum and set the mass of the lightest neutrino well below 0.01 eV. The method also gives the 1σ allowed ranges for the leptonic mixing matrix elements. Contrary to the common expectation, leptonic mixing angles are found to be determined solely by the four leptonic mass ratios without any relation to symmetry considerations as commonly used in flavor model building. Still, our formulae can be used to build up a flavor model that predicts the observed hierarchies in the masses — the mixing follows then from the procedure which is developed in this work.

Theoretical and Phenomenological Status of Neutrino Physics: A Brief Review

We present an overview of recent progress in the theoretical and phenomenological studies of neutrino masses, lepton avor mixing, and CP violation. Firstly, We discuss the status of neutrino mass with in the Standard Model (SM) of particle physics. Then the possible ways in which neutrino mass terms can be included in the SM are discussed. The inclusion of new physics beyond the SM inevitably brings new parameters which are not constrained by the present experimental data on neutrino masses and mixing angles and, thus, are free parameters of the theory. We, also, discuss various theoretically motivated phenomenological approaches which can be used to reduce the number of free parameters and, thus, provide an excellent tool to understand the underlying physics of neutrino masses and mixings. Current experimental constraints on the neutrino mass spectrum and the lepton avor mixing parameters, including the recent observation of nonzeroθ13, have been summarized. Finally, We discuss the renewed interest in the possible existence of one or more sterile neutrinos and their phenomenology.

Neutrino Mass Hierarchy and Neutrino Oscillation Parameters with One Hundred Thousand Reactor Events

High-statistics reactor neutrino experiments at medium baselines will probe mass-mixing parameters governing neutrino oscillations at long wavelength, driven by the (δm2, θ12) and at short wavelength, driven by (Δm2, θ13).The interference between these two oscillations will allow to probe the mass hierarchy. The determination of the neutrino mass spectrum hierarchy, however, will require an unprecedented level of detector performance and collected statistics, and the control of several systematics at (sub)percent level. In this work we perform accurate theoretical calculations of reactor event spectra and refined statistical analyses to show that with O(105) reactor events, a typical sensitivity of ∼ 2σ could be achieved by an experiment such as JUNO. We also show the impact of the energy scale and spectrum shape systematics on the determination of the hierarchy.

New textures for the lepton mass matrices

We study predictive textures for the lepton mass matrices in which the charged-lepton mass matrix has either four or five zero matrix elements while the neutrino Majorana mass matrix has, respectively, either four or three zero matrix elements. We find that all the viable textures of these two kinds share many predictions: the neutrino mass spectrum is inverted, the sum of the light-neutrino masses is close to 0.1 eV, the Dirac phase δ in the lepton mixing matrix is close to either 0 or π, and the mass term responsible for neutrinoless double-beta decay lies in between 12 and 22 meV.

A three-loop radiative neutrino mass model with dark matter

We present a model that generates small neutrino masses at three-loop level due to the existence of Majorana fermionic dark matter, which is stabilized by a Z2 symmetry. The model predicts that the lightest neutrino is massless. We show a prototypical parameter choice allowed by relevant experimental data, which favors the case of normal neutrino mass spectrum and the dark matter with m∼50–135 GeV and a sizable Yukawa coupling. It means that new particles can be searched for in future e+e− collisions.

The dark side of θ 13, δ CP , leptogenesis and inflation in type-I seesaw

In the context of the type-I seesaw mechanism, it is known that $\theta_{13}$ is zero and leptogenesis can not be realized if there exists a residual flavor symmetry resulting in the Tri-Bimaximal neutrino mixing pattern. We propose a simple framework where additional particles, odd under a $Z_2$ symmetry, break the residual flavor symmetry and the lightest of the $Z_2$ odd particles is the dark matter candidate. As as result, nonzero $\theta_{13}$, $\delta_{CP}$, leptogenesis and the correct dark matter density can be accommodated. On the other hand, a $Z_2$ odd scalar can play the role of the inflaton with mass of $10^{13}$ GeV motivated by the recent BICEP2 results. Interestingly, the model can "generate" $\delta_{CP} \simeq -\pi/2$, preferred by the T2K experiment in the normal hierarchy neutrino mass spectrum.

Lepton masses and mixing inSU(3)C⊗SU(3)L⊗U(1)Xmodels with aS3flavor symmetry

We propose a model based on the gauge group $SU(3)_{C}\otimes SU(3)_{L}\otimes U(1)_{X}$ with an extra $S_{3}$ flavor symmetry, which accounts for the lepton masses and mixing. The small active neutrino masses are generated via a double seesaw mechanism. In this scenario, the spectrum of neutrinos presents very light, light and very heavy masses. The model predicts a quasidegenerate normal hierarchy active neutrino mass spectrum and the relation $\Delta m_{21}^{2}\ll \Delta m_{31}^{2}$ arises from effective six-dimensional operators. The obtained neutrino mixing parameters are in agreement with the neutrino oscillation experimental data. We find CP violation in neutrino oscillations with a Jarlskog invariant of about $10^{-2}$.

Revisiting discrete dark matter model: θ 13 ≠ 0 and ν R dark matter

We revisit the discrete dark matter model with $A_4$ flavor symmetry originally introduced by M.Hirsch {\it et.al}. We show that radiative corrections can lead to non-zero $\theta_{13}$ and non-zero mass for the lightest neutrino. We find an interesting relation among neutrino mixing parameters and it indicates the sizable deviation of $s_{23}$ from the maximal angle $s_{23}^2=1/2$ and the degenerate mass spectrum for neutrinos. Also we study the possibilities that the right-handed neutrino is a dark matter candidate. Assuming the thermal freeze-out explains observed dark matter abundance, TeV-scale right-handed neutrino and flavored scalar bosons are required. In such a case, flavor symmetry plays an important role for the suppression of lepton flavor violating processes as well as for the stability of dark matter. We show that this scenario can be viable against currently existing constraints from collider, low energy experiments and cosmological observations.

Thermal leptogenesis inf(R)cosmology

The cosmological consequences of $f(R)$ gravity are reviewed in the framework of thermal leptogenesis. In $f(R)$ cosmology the expansion laws of the Universe are modified with respect to the standard cosmology, which also changes the thermal history of particles. In these cosmological scenarios it turns out that the condition for the out-of-equilibrium decay of the lightest heavy neutrino gives rise to a modulated upper bound on the lightest light neutrino mass, i.e. ${\stackrel{\texttildelow{}}{m}}_{1}\ensuremath{\lesssim}{10}^{\ensuremath{-}3}A(T={M}_{1})\text{ }\text{ }\mathrm{eV}$, where $A(T)$ is the ratio between the expansion rates of the Universe evaluated in $f(R)$ cosmology and in standard cosmology, and ${M}_{1}$ is the lightest heavy neutrino mass. This factor can be parametrized in terms of the temperature $T$ as $A(T)=(T/{T}_{*}{)}^{p}$, where ${T}_{*}$ is the transition temperature at which the expansion law in $f(R)$ cosmology deviates from the standard one, and $p$ is related to the parameters characterizing the $f(R)$ model under consideration. Results imply that if $A\ensuremath{\gg}1$ then the thermal leptogenesis scenario can accommodate the case of a degenerate mass spectrum of the light neutrinos (for a low enough transition temperature), which is otherwise impossible with standard cosmology. In our analysis we consider $f(R)$ models of the form $f(R)=R+\ensuremath{\alpha}{R}^{n}$.

Inert dark matter in type-II seesaw

Weakly interacting massive particle (WIMP) as a dark matter (DM) candidate is further inspired by recent AMS-02 data, which confirm the excess of positron fraction observed earlier by PAMELA and Fermi-LAT experiments. Additionally, the excess of positron+electron flux is still significant in the measurement of Fermi-LAT. For solving the problems of massive neutrinos and observed excess of cosmic-ray, we study the model with an inert Higgs doublet (IHD) in the framework of type-II seesaw model by imposing a Z 2 symmetry on the IHD, where the lightest particle of IHD is the DM candidate and the neutrino masses originate from the Yukawa couplings of Higgs triplet and leptons. We calculate the cosmic-ray production in our model by using three kinds of neutrino mass spectra, which are classified by normal ordering, inverted ordering and quasi-degeneracy. We find that when the constraints of DM relic density and comic-ray antiproton spectrum are taken into account, the observed excess of positron/electron flux could be explained well in normal ordered neutrino mass spectrum. Moreover, excess of comic-ray neutrinos is implied in our model. We find that our results on 〈σv〉 are satisfied with and close to the upper limit of IceCube analysis. More data from comic-ray neutrinos could test our model.

Effect of steriles states on lepton magnetic moments and neutrinoless double beta decay

We address the impact of sterile fermion states on the anomalous magnetic moment of charged leptons, as well as their contribution to neutrinoless double beta decays. We illustrate our results in a minimal, effective extension of the Standard Model by one sterile fermion state, and in a well-motivated framework of neutrino mass generation, embedding the Inverse Seesaw into the Standard Model. The simple “3+1” effective case succeeds in alleviating the tension related to the muon anomalous magnetic moment, albeit only at the 3σ level, and for light sterile states (corresponding to a cosmologically disfavoured regime). Interestingly, our analysis shows that a future 0ν2β observation does not necessarily imply an inverted hierarchy for the active neutrinos in this simple extension. Although the Inverse Seesaw realisation here addressed could indeed ease the tension in (g − 2) μ , bounds from lepton universality in kaon decays mostly preclude this from happening. However, these scenarios can also have a strong impact on the interpretation of a future 0ν2β signal regarding the hierarchy of the active neutrino mass spectrum.

Search for Majorana neutrinos with the first two years of EXO-200 data.

Many extensions of the standard model of particle physics suggest that neutrinos should be Majorana-type fermions-that is, that neutrinos are their own anti-particles-but this assumption is difficult to confirm. Observation of neutrinoless double-β decay (0νββ), a spontaneous transition that may occur in several candidate nuclei, would verify the Majorana nature of the neutrino and constrain the absolute scale of the neutrino mass spectrum. Recent searches carried out with (76)Ge (the GERDA experiment) and (136)Xe (the KamLAND-Zen and EXO (Enriched Xenon Observatory)-200 experiments) have established the lifetime of this decay to be longer than 10(25) years, corresponding to a limit on the neutrino mass of 0.2-0.4 electronvolts. Here we report new results from EXO-200 based on a large (136)Xe exposure that represents an almost fourfold increase from our earlier published data sets. We have improved the detector resolution and revised the data analysis. The half-life sensitivity we obtain is 1.9 × 10(25) years, an improvement by a factor of 2.7 on previous EXO-200 results. We find no statistically significant evidence for 0νββ decay and set a half-life limit of 1.1 × 10(25) years at the 90 per cent confidence level. The high sensitivity holds promise for further running of the EXO-200 detector and future 0νββ decay searches with an improved Xe-based experiment, nEXO.

Neutrino Mixing in the BLMSSM

In recent years, a nonzero value for the neutrino mixing angle θ13 has been successively measured by the international famous reactor oscillation experiments, which is greater than 5 standard deviations. Our study is in the framework of the MSSM, where baryon and lepton numbers are local gauged symmetries (BLMSSM). This model can generate three tiny neutrino masses at the tree level through TeV scale seesaw mechanism. In our paper, we analyze the neutrino masses and their corresponding mixing angles with a “top-down” method, assuming neutrino mass spectrum with normal ordering (NO) and inverted ordering (IO).

Longevity problem of sterile neutrino dark matter

Sterile neutrino dark matter of mass O(1–10) keV decays into an active neutrino and an X-ray photon, and the non-observation of the corresponding X-ray line requires the sterile neutrino to be more long-lived than estimated based on the seesaw formula: the longevity problem. We show that, if one or more of the B–L Higgs fields are charged under a flavor symmetry (or discrete R symmetry), the split mass spectrum for the right-handed neutrinos as well as the required longevity is realized. We provide several examples in which the predicted X-ray flux is just below the current bound.

Generalised geometrical CP violation in a T ′ lepton flavour model

We analyse the interplay of generalised CP transformations and the non-Abelian discrete group T ′ and use the semi-direct product G f = T ′ ⋊H CP, as family symmetry acting in the lepton sector. The family symmetry is shown to be spontaneously broken in a geometrical manner. In the resulting flavour model, naturally small Majorana neutrino masses for the light active neutrinos are obtained through the type I see-saw mechanism. The known masses of the charged leptons, lepton mixing angles and the two neutrino mass squared differences are reproduced by the model with a good accuracy. The model allows for two neutrino mass spectra with normal ordering (NO) and one with inverted ordering (IO). For each of the three spectra the absolute scale of neutrino masses is predicted with relatively small uncertainty. The value of the Dirac CP violation (CPV) phase δ in the lepton mixing matrix is predicted to be δ = π/2 or 3π/2. Thus, the CP violating effects in neutrino oscillations are predicted to be maximal (given the values of the neutrino mixing angles) and experimentally observable. We present also predictions for the sum of the neutrino masses, for the Majorana CPV phases and for the effective Majorana mass in neutrinoless double beta decay. The predictions of the model can be tested in a variety of ongoing and future planned neutrino experiments.

Transition magnetic moment of Majorana neutrinos in the μνSSM

The nonzero vacuum expectative values of sneutrinos induce spontaneously R-parity and lepton number violation, and generate three tiny Majorana neutrino masses through the seesaw mechanism in the $\mu\nu$SSM, which is one of Supersymmetric extensions beyond Standard Model. Applying effective Lagrangian method, we study the transition magnetic moment of Majorana neutrinos in the model here. Under the constraints from neutrino oscillations, we consider the two possibilities on the neutrino mass spectrum with normal or inverted ordering.

Dual models of the neutrino mass spectrum

We show that any model with a homogeneous relationship among elements of the neutrino mass matrix with one mass hierarchy yields predictions for the oscillation parameters and Majorana phases similar to those given by a model with the same homogeneous relationship among cofactors of the neutrino mass matrix with the opposite mass hierarchy, except when the lightest mass is of order 20 meV or less.

A New Aspect of Two-Loop Neutrino Mass Generation

We propose a class of models which the neutrino masses are generated at two-loop level. In this mechanism, only the right-handed leptonic Yukawa interaction is involved without imposing any new symmetry but the Standard Model gauge symmetry. Neutrino mass spectrum is predicted to be normal hierarchy. The novel feature of the direct relation between the neutrino masses and the Yukawa couplings enable us pin down the neutrino parameters such as the absolute scale, the Dirac CP phase via the leptonic processes. The dynamics of lepton number breaking and some collider phenomenology of this model are studied.

A generalized electromagnetic theory for the mass spectrum of neutrinos

In previous papers it was shown that solutions of Weyl equation that are eigenstates of the parity operator describe a coupled pair of a monopole anti-monopole system. These results suggest to seek a solution of the Maxwell equation \(\partial F^\infty = - g\mathcal{J}\) with a current \(\mathcal{J}\) as a source and such that the Lorentz force on the current is null. We first identify a solution where \(J_m = - \gamma ^5 \mathcal{J}\) is a spacelike field. More surprisingly we find that there exists a solution F of the free Maxwell ∂ F = 0 that is equivalent to the inhomogeneous equation for F ∞. Once this result is proved, it suggests by itself to seek more general (subluminal and even superluminal) solutions \(\mathfrak{F}\) of the free Maxwell equation equivalent to an inhomogeneous Maxwell equation for a field \(\mathfrak{F}_0\) with a current term as a source which may be a timelike or spacelike field. We exhibit one such subluminal solution, for which the Dirac-Hestenes spinor field ψ associated with the electromagnetic field \(\mathfrak{F}_0\) satisfies a Dirac-like equation for a bradyonic neutrino under the ansatz that the current is \(ce^{\lambda \gamma ^5 } g\psi \gamma ^0 \tilde \psi\) with g the quantum of magnetic charge and λ a constant to be determined in such a way that the auto-force is zero. Together with Dirac’s quantization condition this gives a quantized mass spectrum (Eq. (46)) for neutrinos, with masses of the different flavor neutrinos being of the same order of magnitude (Eq. (47)), which is in accord with recent experimental findings.