Neutrino Mass Pattern Research Articles

Stable lepton mass matrices

We study natural lepton mass matrices, obtained assuming the stability of physical flavour observables with respect to the variations of individual matrix elements. We identify all four possible stable neutrino textures from algebraic conditions on their entries. Two of them turn out to be uniquely associated to specific neutrino mass patterns. We then concentrate on the semi-degenerate pattern, corresponding to an overall neutrino mass scale within the reach of future experiments. In this context we show that i) the neutrino and charged lepton mixings and mass matrices are largely constrained by the requirement of stability, ii) naturalness considerations give a mild preference for the Majorana phase most relevant for neutrinoless double-beta decay, $\alpha \sim \pi/2$, and iii) SU(5) unification allows to extend the implications of stability to the down quark sector. The above considerations would benefit from an experimental determination of the PMNS ratio $|U_{32}/U_{31}|$, i.e. of the Dirac phase $\delta$.

CP-violating phase in a two Higgs triplet scenario: Some phenomenological implications

We consider a scenario where, along with the usual Higgs doublet, two scalar triplets are present. The extension of the triplet sector is required for the Type~II mechanism for the generation of neutrino masses, if this mechanism has to generate a neutrino mass matrix with two-zero texture. One CP-violating phase has been retained in the scalar potential of the model, and all parameters have been chosen consistently with the observed neutrino mass and mixing patterns. We find that a large phase ($\gtrsim 60^{\circ}$) splits the two doubly-charged scalar mass eigenstates wider apart, so that the decay $H_1^{++} \rightarrow H_2^{++} h$ is dominant (with h being the $125$ GeV scalar). We identify a set of benchmark points where this decay dominates. This is complementary to the situation, reported in our earlier work, where the heavier doubly-charged scalar decays as $H_1^{++} \rightarrow H_2^+ W^+$. We point out the rather spectacular signal, ensuing from $H_1^{++} \rightarrow H_2^{++} h$, in the form of Higgs plus same-sign dilepton peak, which can be observed at the Large Hadron Collider.

LUMINEU: a search for neutrinoless double beta decay based on ZnMoO4 scintillating bolometers

The LUMINEU is designed to investigate the possibility to search for neutrinoless double beta decay in 100 Mo by means of a large array of scintillating bolometers based on ZnMoO4 crystals enriched in 100 Mo. High energy resolution and relatively fast detectors, which are able to measure both the light and the heat generated upon the interaction of a particle in a crystal, are very promising for the recognition and rejection of background events. We present the LUMINEU concepts and the experimental results achieved aboveground and underground with large-mass natural and enriched crystals. The measured energy resolution, the α/β discrimination power and the radioactive internal contamination are all within the specifications for the projected final LUMINEU sensitivity. Simulations and preliminary results confirm that the LUMINEU technology can reach zero background in the region of interest (around 3 MeV) with exposures of the order of hundreds kgxyears, setting the bases for a next generation 0v2β decay experiment capable to explore the inverted hierarchy region of the neutrino mass pattern.

Scintillating bolometers based on ZnMoO4 and Zn100MoO4 crystals to search for 0ν2β decay of 100Mo (LUMINEU project): first tests at the Modane Underground Laboratory

The technology of scintillating bolometers based on zinc molybdate (ZnMoO4) crystals is under development within the LUMINEU project to search for 0ν2β decay of 100Mo with the goal to set the basis for large scale experiments capable to explore the inverted hierarchy region of the neutrino mass pattern. Advanced ZnMoO4 crystal scintillators with mass of ∼0.3 kg were developed and Zn100MoO4 crystal from enriched 100Mo was produced for the first time by using the low-thermal-gradient Czochralski technique. One ZnMoO4 scintillator and two samples (59 g and 63 g) cut from the enriched boule were tested aboveground at milli-Kelvin temperature as scintillating bolometers showing a high detection performance. The first results of the low background measurements with three ZnMoO4 and two enriched detectors installed in the EDELWEISS set-up at the Modane Underground Laboratory (France) are presented.

Lepton mass and mixing in a neutrino mass model based onS4flavor symmetry

We study a neutrino mass model based on [Formula: see text] flavor symmetry which accommodates lepton mass, mixing with nonzero [Formula: see text] and CP violation phase. The spontaneous symmetry breaking in the model is imposed to obtain the realistic neutrino mass and mixing pattern at the tree-level with renormalizable interactions. Indeed, the neutrinos get small masses from one [Formula: see text] doublet and two [Formula: see text] singlets in which one being in [Formula: see text] and the two others in [Formula: see text] under [Formula: see text] with both the breakings [Formula: see text] and [Formula: see text] are taken place in charged lepton sector and [Formula: see text] in neutrino sector. The model also gives a remarkable prediction of Dirac CP violation [Formula: see text] or [Formula: see text] in both the normal and inverted spectrum which is still missing in the neutrino mixing matrix. The relation between lepton mixing angles is also represented.

Successful N2 leptogenesis with flavour coupling effects in realistic unified models

In realistic unified models involving so-called SO(10)-inspired patterns of Dirac and heavy right-handed (RH) neutrino masses, the lightest right-handed neutrino N1 is too light to yield successful thermal leptogenesis, barring highly fine tuned solutions, while the second heaviest right-handed neutrino N2 is typically in the correct mass range. We show that flavour coupling effects in the Boltzmann equations may be crucial to the success of such N2 dominated leptogenesis, by helping to ensure that the flavour asymmetries produced at the N2 scale survive N1 washout. To illustrate these effects we focus on N2 dominated leptogenesis in an existing model, the A to Z of flavour with Pati-Salam, where the neutrino Dirac mass matrix may be equal to an up-type quark mass matrix and has a particular constrained structure. The numerical results, supported by analytical insight, show that in order to achieve successful N2 leptogenesis, consistent with neutrino phenomenology, requires a ``flavour swap scenario'' together with a less hierarchical pattern of RH neutrino masses than naively expected, at the expense of some mild fine-tuning. In the considered A to Z model neutrino masses are predicted to be normal ordered, with an atmospheric neutrino mixing angle well into the second octant and the Dirac phase δ≃ 20̂, a set of predictions that will be tested in the next years in neutrino oscillation experiments. Flavour coupling effects may be relevant for other SO(10)-inspired unified models where N2 leptogenesis is necessary.

New mechanism for Type-II seesaw dominance in SO(10) with low-mass $$Z^{\prime }$$ Z ′ , RH neutrinos, and verifiable LFV, LNV and proton decay

Dominance of type-II seesaw mechanism for neutrino masses has attracted considerable attention because of a number of advantages. We show a novel approach to achieve Type-II seesaw dominance in non-supersymmetric $SO(10)$ grand unification where a low mass $Z^{\prime}$ boson and specific patterns of right-handed neutrino masses are predicted within the accessible energy range of the Large Hadron Collider. In spite of the high value of the seesaw scale, $M_{\Delta_L} \simeq 10^8-10^9$ GeV, the model predicts new dominant contributions to neutrino-less double beta decay in the $W_L-W_L$ channel close to the current experimental limits via exchanges of heavier singlet fermions used as essential ingredients of this model even when the light active neutrino masses are normally hierarchical or invertedly hierarchical. We obtain upper bounds on the lightest sterile neutrino mass $m_s\lesssim 3.0$ GeV, $2.0$ GeV, and $0.7$ GeV for normally hierarchical, invertedly hierarchical, and quasi-degenerate patterns of light neutrino masses, respectively. The underlying non-unitarity effects lead to lepton flavor violating decay branching ratios within the reach of ongoing or planned experiments and the leptonic CP-violation parameter nearly two orders larger than the quark sector. Some of the predicted values on proton lifetime for $p\to e^+\pi^0$ are found to be within the currently accessible search limits. Other aspects of model applications including leptogenesis etc. are briefly indicated.

LUMINEU: A Pilot Scintillating Bolometer Experiment for Neutrinoless Double Beta Decay Search

The Luminescent Underground Molybdenum Investigation for NEUtrino mass and nature (LUMINEU) aims at preparing the ground for a next-generation neutrinoless double beta decay experiment employing scintillating bolometers: these devices are in fact very promising tools in rare events search, in terms of e ciency, energy resolution and background control. In particular, they can tag alpha events, which are the dominant residual background for double beta decay candidates with a transition energy higher than 2615 keV. LUMINEU's goal is the operation of a pilot detector, consisting of four 400g ZnMoO4 scintillating bolometers, probing an active 100Mo mass of about 0.7kg, the energy transition of this isotope being 3034 keV. The enriched material for this setup is available and the experiment is fully funded by ANR in France. This preliminary investigation intends to be feasibility test for a next-generation neutrinoless double beta decay experiment aiming at probing the inverted hierarchy region of the neutrino mass pattern. LUMINEU will help to fix the detailed structure of the single module of this future large-scale experiment. The ZnMoO4 crystals will be grown at the Nikolaev Institute for Inorganic Chemistry in Novosibirsk, Russia. LUMINEU foresees a systematic optimization of the crystal growth parameters, in order to optimize the bolometric performance, the light yield, the particle rejection factor and the radiopurity of the scintillating bolometers. On this purpose, an aboveground facility was set up at the Centre de Sciences Nucleaireś et de Sciences de la Matiere’ (CSNSM), in Orsay, France. In this contribution, we will describe the LUMINEU program, we will discuss its sensitivity and that one of a future large search based on this technology. We will also present preliminary experimental results achieved in Orsay with scintillating bolometers fabricated employing the first LUMINEU ZnMoO4 crystals, which have been delivered in June 2013.

Enriched Zn $$^{100}$$ 100 MoO $$_4$$ 4 scintillating bolometers to search for $$0\nu 2\beta $$ 0 ν 2 β decay of $$^{100}$$ 100 Mo with the LUMINEU experiment

The LUMINEU project aims at performing a demonstrator underground experiment searching for the neutrinoless double beta decay of the isotope $^{100}$Mo embedded in zinc molybdate (ZnMoO$_4$) scintillating bolometers. In this context, a zinc molybdate crystal boule enriched in $^{100}$Mo to 99.5\% with a mass of 171 g was grown for the first time by the low-thermal-gradient Czochralski technique. The production cycle provided a high yield (the crystal boule mass was 84\% of initial charge) and an acceptable level -- around 4\% -- of irrecoverable losses of the costy enriched material. Two crystals of 59 g and 63 g, obtained from the enriched boule, were tested aboveground at milli-Kelvin temperature as scintillating bolometers. They showed a high detection performance, equivalent to that of previously developed natural ZnMoO$_4$ detectors. These results pave the way to future sensitive searches based on the LUMINEU technology, capable to approach and explore the inverted hierarchy region of the neutrino mass pattern.

Neutrino masses and leptogenesis in type I and type II seesaw models

The baryon to photon ratio in the present Universe is very accurately measured to be $(6.065\ifmmode\pm\else\textpm\fi{}0.090)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$. We study the possible origin of this baryon asymmetry in the neutrino sector through the generic mechanism of baryogenesis through leptogenesis. We consider both the type I and type II seesaw origin of neutrino masses within the framework of left-right symmetric models (LRSM). Using the latest best-fit global neutrino oscillation data of mass squared differences, mixing angles and Dirac $CP$ phase, we compute the predictions for baryon to photon ratio keeping the Majorana $CP$ phases as free parameters for two different choices of lightest neutrino mass eigenvalue for both normal and inverted hierarchical patterns of neutrino masses. We do our calculation with and without lepton flavor effects being taken into account. We choose different diagonal Dirac neutrino mass matrix for different flavor effects in such a way that the lightest right-handed neutrino mass is in the appropriate range. We also study the predictions for baryon asymmetry when the neutrino masses arise from a combination of both type I and type II seesaw (with dominating type I term) and discriminate between several combinations of Dirac and Majorana $CP$ phases by demanding successful predictions for baryon asymmetry.

Radiative generation of non-zero [formula omitted] in MSSM with broken [formula omitted] flavor symmetry

We study the renormalization group effects on neutrino masses and mixing in Minimal Supersymmetric Standard Model (MSSM) by considering a μ–τ symmetric mass matrix at high energy scale giving rise to Tri-Bi-Maximal (TBM) type mixing. We outline a flavor symmetry model based on A4 symmetry giving rise to the desired neutrino mass matrix at high energy scale. We take the three neutrino mass eigenvalues at high energy scale as input parameters and compute the neutrino parameters at low energy by taking into account of renormalization group effects. We observe that the correct output values of neutrino parameters at low energy are obtained only when the input mass eigenvalues are large |m1,2,3|=0.08–0.12 eV with a very mild hierarchy of either inverted or normal type. A large inverted or normal hierarchical pattern of neutrino masses is disfavored within our framework. We also find a preference towards higher values of tanβ, the ratio of vacuum expectation values (vev) of two Higgs doublets in MSSM in order to arrive at the correct low energy output. Such a model predicting large neutrino mass eigenvalues with very mild hierarchy and large tanβ could have tantalizing signatures at oscillation, neutrino-less double beta decay as well as collider experiments.

Doubly charged scalar decays in a type II seesaw scenario with two Higgs triplets

The type II seesaw mechanism for neutrino mass generation usually makes use of one complex scalar triplet. The collider signature of the doubly-charged scalar, the most striking feature of this scenario,consists mostly in decays into same-sign dileptons or same-sign $W$ boson pairs. However, certain scenarios of neutrino mass generation, such as those imposing texture zeros by a symmetry mechanism, require at least two triplets in order to be consistent with type-II the seesaw mechanism. We develop a model with two such complex triplets and show that, in such a case, mixing between the triplets can cause the heavier doubly-charged scalar mass eigenstate to decay into a singly-charged scalar and a $W$ boson of the same sign. Considering a large number of benchmark points with different orders of magnitude of the $\Delta L =2$ Yukawa couplings, chosen in agreement with the observed neutrino mass and mixing pattern, we demonstrate that $H^{++}_1 \rightarrow H^+_2 W^+$ can have more than 99% branching fraction in the cases where the vacuum expectation values of the triplets are small. The implications of this for the LHC are pointed out.

A model of quark and lepton mixing

We propose a model of quark and lepton mixing based on the tetrahedral A(4) family symmetry with quark-lepton unification via the tetra-colour Pati-Salam gauge group SU(4)_{PS}, together with SU(2)_L x U(1)_R. The "tetra-model" solves many of the flavour puzzles and remarkably gives ten predictions at leading order, including all six PMNS parameters. The Cabibbo angle is approximately given by 1/4, due to the tetra-vacuum alignment (1,4,2), providing the Cabibbo connection between quark and lepton mixing. Higher order corrections are responsible for the smaller quark mixing angles and CP violation and provide corrections to the Cabibbo and lepton mixing angles and phases. The tetra-model involves an SO(10)-like pattern of Dirac and heavy right-handed neutrino masses, with the strong up-type quark mass hierarchy cancelling in the see-saw mechanism, leading to a normal hierarchy of neutrino masses with an atmospheric angle in the first octant of 40 +/- 1 degree, a solar angle of 34 +/- 1 degree, a reactor angle of 9.0 +/- 0.5 degree, depending on the ratio of neutrino masses m_2/m_3, and a Dirac CP violating oscillation phase of 260 +/- 5 degrees.

Nuclear matrix elements for neutrinoless double-beta decay and double-electron capture

A new generation of neutrinoless double-beta decay (0νββ-decay) experiments with improved sensitivity is currently being designed and under construction. They will probe the inverted hierarchy region of the neutrino mass pattern. There is also a revived interest in the resonant neutrinoless double-electron capture (0νECEC), which also has the potential to probe lepton number conservation and to investigate the neutrino nature and mass scale. The primary concerns are the nuclear matrix elements. Clearly, the accuracy of the determination of the effective Majorana neutrino mass from the measured 0νββ-decay half-life is mainly determined by our knowledge of the nuclear matrix elements. We review recent progress achieved in the calculation of 0νββ and 0νECEC nuclear matrix elements within the quasiparticle random phase approximation. A considered self-consistent approach allows us to derive the pairing, residual interactions and the two-nucleon short-range correlations from the same modern realistic nucleon–nucleon potentials. The effect of nuclear deformation is taken into account. The possibility of evaluating 0νββ-decay matrix elements phenomenologically is discussed.

Random coincidence of 2ν2β decay events as a background source in bolometric 0ν2β decay experiments

Two-neutrino double β decay can create an irremovable background even in high energy resolution detectors searching for neutrinoless double β decay due to random coincidence of 2ν2β events in the case of poor time resolution. Some possibilities for suppressing this background in cryogenic scintillating bolometers are discussed. It is shown that the present bolometric detector technologies enable one to control this form of background at the level required to explore the inverted hierarchy of the neutrino mass pattern, including the case of bolometers searching for the neutrinoless double β decay of 100Mo, which is characterized by a relatively short two-neutrino double β decay half-life.

A next-generation neutrinoless double beta decay experiment based on ZnMoO4 scintillating bolometers

The search for neutrinoless double β decay probes lepton number conservation with high sensitivity and investigates the neutrino nature and mass scale. Experiments presently in preparation will cover the quasi-degeneracy region of the neutrino mass pattern. Probing the inverted hierarchy region requires improved sensitivities and next-generation experiments, based either on large expansions of the present searches or on new ideas. We examine here a novel technology relying on ZnMoO4 scintillating bolometers, which can provide an experiment with background close to zero in the ton × year exposure scale. The promising performance of a pilot detector is presented, both in terms of energy resolution and background control. A preliminary study of the sensitivities of future experiments shows that the inverted hierarchy region is within the reach of the technique here proposed. A realistic phased approach program towards a next-generation search is presented and briefly discussed.

Neutrino Physics with Low-Temperature Detectors

In the last years, neutrino physics has provided exciting discoveries that for the first time have cracked the solid building of the Standard Model. However, many mysteries remain, and prospects are even more appealing. Low temperature detectors can give fundamental contributions to this field. They already play a major role in the study of neutrinoless double beta decay, a rare nuclear process that can ascertain if neutrino is a self-conjugate elementary fermion and fix its mass scale. Cuoricino, a project based on macro-bolometers, is the most sensitive double-beta-decay search in the world together with the much debated Heidelberg–Moscow experiment. CUORE, its natural continuation, is one of the most promising experiments under construction or commissioning, capable to start to attack the so-called inverted hierarchy region of the neutrino mass pattern. Several ideas based on low-temperature detectors (among which the simultaneous detection of phonon and scintillation light) are among the most promising approaches for next-generation experiments, capable to cover fully the inverted hierarchy region. In other sectors of neutrino physics, like the direct measurement of the neutrino mass in the MARE and ECHO projects or the detection of coherent neutrino-nucleus elastic scattering, low temperature detectors look less mature scientifically. However, they remain extremely promising devices to address these very challenging searches.

Models for neutrino mass with discrete symmetries

Discrete non-abelian flavor symmetries give in a natural way tri-bimaximal (TBM) mixing as showed in a prototype model. However neutrino mass matrix pattern may be very different from the tri-bimaximal one if small deviations of TBM will be observed. We give the result of a model independent analysis for TBM neutrino mass pattern.

Broken [formula omitted] symmetry in the neutrino mass matrix

We explore the feasibility of the discrete flavor symmetry S3 to explain the pattern of neutrino masses and mixings. In the flavor basis, the neutrino mass matrix is taken to be invariant under S3 symmetry at the zeroth order. The effects of breaking S3 symmetry by matrices invariant under different S2 subgroups of S3 are studied. The resulting retrocirculant perturbation matrix leads to the perturbed S3 neutrino mass matrix having a trimaximal eigenvector as suggested by the solar neutrino data. It is found that after the third order perturbation the neutrino mixings only depend on the perturbation parameter and are consistent with the current experimental data leading to very small deviations from tribimaximal mixing. These perturbations in the S3 invariant neutrino mass matrix result in interesting interplay between the solar and the reactor neutrino mixing angles. We also get a strongly suppressed range of effective Majorana mass which lies well within the reach of the ongoing experiments.

Neutrino masses and mixing fromS4flavor twisting

We discuss a neutrino mass model based on the ${S}_{4}$ discrete symmetry where the symmetry breaking is triggered by the boundary conditions of the bulk right-handed neutrino in the fifth spacial dimension. The three generations of the left-handed lepton doublets and the right-handed neutrinos are assigned to be the triplets of ${S}_{4}$. The magnitudes of the lepton mixing angles, especially the reactor angle, are related to the neutrino mass patterns, and the model will be tested in future neutrino experiments, e.g., an early discovery of the reactor angle favors the normal hierarchy. For the inverted hierarchy, the lepton mixing is predicted to be almost the tribimaximal mixing. The size of the extra dimension has a connection to the possible mass spectrum; a small (large) volume corresponds to the normal (inverted) mass hierarchy.