Narrow Results

The gauged $B\text{–}L$ symmetry is one of the simplest and well-studied extension of Standard Model. In the conventional case, addition of three singlet right-handed neutrinos each transforming as $-1$ under the $B\text{–}L$ symmetry renders it anomaly-free. It is usually assumed that the $B\text{–}L$ symmetry is spontaneously broken by a singlet scalar having two units of $B\text{–}L$ charge, resulting in a natural implementation of Majorana seesaw mechanism for neutrinos. However, as we discuss here, there is another simple anomaly-free solution which leads to Dirac or inverse seesaw masses for neutrinos. These new possibilities are explored along with an application to neutrino mixing with $S_3$ flavor symmetry.

In this brief review we discuss the generation of Majorana neutrino masses through the seesaw mechanism, the theory of neutrinoless double-beta decay, the implications of neutrino oscillation data for the effective Majorana mass, taking into account the recent Daya Bay measurement of ϑ₁₃, and the interpretation of the results of neutrinoless double-beta decay experiments.

The discrete subgroup Δ(27) of SU(3) has some interesting properties which may be useful for understanding charged-lepton and neutrino mass matrices. Assigning leptons to the 3 and representations of Δ(27), a simple form of the Majorana neutrino mass matrix is obtained and compared to present data.

In this paper, we study the phenomenological implications of enforcing one texture zero on the inverse matrix of the neutrino mass matrix $M_{\nu }^{\mathrm{\text{TM}}}$ that can lead to the trimaximal mixing. It is found that the condition of $M_{12}^{-1}=0$, $M_{13}^{-1}=0$, $M_{22}^{-1}=0$, $M_{23}^{-1}=0$ or $M_{33}^{-1}=0$ can hold in the case of normal neutrino mass ordering, while the condition of $M_{11}^{-1}=0$, $M_{12}^{-1}=0$, $M_{13}^{-1}=0$, $M_{22}^{-1}=0$ or $M_{33}^{-1}=0$ can hold in the case of inverted neutrino mass ordering, where $M_{ij}^{-1}$ (for $i$, $j=1,2,3$) denotes the $ij$ entry of the inverse matrix of $M_{\nu }^{\mathrm{\text{TM}}}$. The ranges of the absolute neutrino mass scale and three CP phases for these cases to hold are studied in some detail. The implications of these cases for the effective neutrino mass $M_{ee}^{}$ which controls the rates of the neutrino-less double beta decay processes are also given.

In the recent successful applications of the non-Abelian discrete symmetry A₄ to the tribimaximal mixing of neutrinos, lepton doublets and singlets do not transform in the same way. It thus appears to be unsuitable as a family symmetry in grand unification. A simple resolution of this dilemma is proposed.

It is argued that four-Fermi interactions induced by non-perturbative effects due to scattering of stringy matter from D-particles, D-instantons and more generally bulk gauge fields in brane models with large extra dimensions have, in specific situations, condensate structure described by flavor vacua.

In the context of quantum field theory (QFT), we compute the amplitudes of weak interaction processes such as W⁺ →e⁺ + ν_e and W⁺ →e⁺ + ν_μ by using different representations of flavor states for mixed neutrinos. Analyzing the short time limit of the above amplitudes, we find that the neutrino states defined in QFT as eigenstates of the flavor charges lead to results consistent with lepton charge conservation. On the contrary, the Pontecorvo flavor states produce a violation of lepton charge in the vertex, which is in contrast with what expected at tree level in the Standard Model.

Recently, several reactor oscillation experiments have successively measured a nonzero value for the neutrino mixing angle θ₁₃, which is greater than five standard deviations. Within framework of the μ from ν Supersymmetric Standard Model (μνSSM), three tiny neutrino masses are generated at the tree level through TeV scale seesaw mechanism. In this work, we analyze the neutrino masses and mixing in the μνSSM with a "top-down" method, assuming neutrino mass spectrum with normal ordering or inverted ordering.

We present a model of neutrino mixing based on the flavor group Δ(27) in order to account for the observation of a nonzero reactor mixing angle (θ₁₃). The model provides a common flavor structure for the charged-lepton and the neutrino sectors, giving their mass matrices a "circulant-plus-diagonal" form. Mass matrices of this form readily lead to mixing patterns with realistic deviations from tribimaximal mixing, including nonzero θ₁₃. With the parameters constrained by existing measurements, our model predicts an inverted neutrino mass hierarchy. We obtain two distinct sets of solutions in which the atmospheric mixing angle lies in the first and the second octants. The first (second) octant solution predicts the lightest neutrino mass, m₃~29 meV(m₃~65 meV) and the CP phase, , offering the possibility of large observable CP violating effects in future experiments.

The pp interactions taking place in the cosmos around us are a source of the astrophysical neutrinos of all the three flavors. In these interactions, the electron and the muon neutrinos mainly come from the production and the decay of the π^± mesons, whereas the tau neutrinos mainly come from the production and the decay of the mesons. We estimate the three intrinsic neutrino flavor ratios for 1 GeV ≤E≤10¹² GeV in the pp interactions and found them to be 1:2:3×10^-5. We study the effects of neutrino oscillations on these intrinsic ratios. We point out that the three ratios become 1:1:1 if L(pc)/E(GeV)≥10^-10 in the presence of neutrino oscillations, where L is the distance to the astrophysical neutrino source in units of parsecs.

In an improved application of the tetrahedral symmetry A₄ first introduced by Ma and Rajasekaran, supplemented by the discrete symmetry Z₃ as well as supersymmetry, a two-parameter form of the neutrino mass matrix is derived which exhibits the tribimaximal mixing of Harrison, Perkins and Scott. This form is the same as the one obtained previously by Altarelli and Feruglio, and the inverse of that obtained by Babu and He. If only A₄ is used, then corrections appear, making tan² θ₁₂ different from 0.5, without changing significantly sin²2θ₂₃ from one or θ₁₃ from zero.

Neutrino tribimaximal mixing is obtained from the breaking of A₄ to Z₃ in the charged-lepton sector and to Z₂ in the neutrino sector. To enforce this conflicting pattern, extra particles and symmetries are usually invoked, often accompanied by nonrenormalizable interactions and even extra dimensions. It is shown here in a specific renormalizable model how A₄ alone will accomplish this, with only the help of lepton number.

Assuming that neutrinos acquire radiative seesaw Majorana masses through their interactions with dark matter, i.e. scotogenic from the Greek "scotos" meaning darkness, and using the non-Abelian discrete symmetry A₄, we propose a model of neutrino masses and mixing with nonzero θ₁₃ and necessarily large leptonic CP violation, allowing both the normal and inverted hierarchies of neutrino masses, as well as quasi-degenerate solutions.

In this paper, we study all possible neutrino mass matrices with one zero element and two equal nonzero elements, known as hybrid texture neutrino mass matrices. In the diagonal charged lepton basis, we consider 39 such possible cases which are consistent with the latest neutrino data. Using the two constraints on neutrino mass matrix elements imposed by hybrid textures, we numerically evaluate the neutrino parameters like the lightest neutrino mass $m_{\mathrm{\text{lightest}}}$, one Dirac CP phase $\delta$ and two Majorana CP phases $\alpha$, $\beta$ by using the global fit $3\sigma$ values of three mixing angles and two mass squared differences. We then constrain this parameter space by using the cosmological upper bound on the sum of absolute neutrino masses given by Planck experiment. We also calculate the effective neutrino mass $m_{\nu }^{\mathrm{\text{eff}}}$ for this region of parameter space which can have relevance in future neutrinoless double beta decay experiments. We finally discriminate between these hybrid texture mass matrices from the requirement of producing correct baryon asymmetry through type I seesaw leptogenesis. We also constrain the light neutrino parameter space as well as the lightest right-handed neutrino mass from the constraint on baryon asymmetry of the Universe from Planck experiment.

Assuming the existence of one light sterile neutrino, we investigate the neutrino flavor mixing matrix in matter. Sum rules between the mixing parameters in vacuum and their counterparts in matter are derived. By using these new sum rules, we obtain the simple but exact expressions of the effective flavor mixing matrix in matter in terms of neutrino masses and the mixing parameters in vacuum. The rephasing invariants, sides of unitarity quadrangles and oscillation probabilities in matter are also achieved. Our model-independent results will be very helpful for analyzing flavor mixing and CP violation in the future long-baseline neutrino oscillation experiments.

The gauged B – L symmetry is one of the simplest and well studied extension of standard model. In the conventional case, addition of three singlet right-handed neutrinos each transforming as −1 under the B – L symmetry renders it anomaly free. It is usually assumed that the B – L symmetry is spontaneously broken by a singlet scalar having two units of B – L charge, resulting in a natural implementation of Majorana seesaw mechanism for neutrinos. However, as we discuss in this proceeding, there is another simple anomaly free solution which leads to Dirac or inverse seesaw masses for neutrinos. These new possibilities are explored along with an application to neutrino mixing with S₃ flavour symmetry.

It is shown that Lorentz invariance implies that in general flavor neutrinos in oscillation experiments are superpositions of massive neutrinos with different energies and different momenta. It is also shown that for each process in which neutrinos are produced, there is either a Lorentz frame in which all massive neutrinos have the same energy or a Lorentz frame in which all massive neutrinos have the same momentum. In the case of neutrinos produced in two-body decay processes, there is a Lorentz frame in which all massive neutrinos have the same energy.

We study an effective gauge theory whose gauge group is a semidirect product G = G_c ⋊ Γ with G_c and Γ being a connected Lie group and a finite group, respectively. The semidirect product is defined through a projective homomorphism (i.e. homomorphism up to the center of G_c) from Γ into G_c. To be specific, we take SU(3)_L as G_c and ℤ₃ × ℤ₃ as Γ. We notice that the irreducible representations of the gauge group G necessarily contain three G_c-multiplets in spite of the Abelian nature of Γ = ℤ₃ × ℤ₃. This triplication phenomenon is due to the semidirect product structure of G. We suggest that the appearance of three families is attributable to this triplication. We give a toy model on the lepton mixing and show that under a particular vacuum alignment the tri-bimaximal mixing matrix is reproduced.

One of the possible ways to explain the observed flavor structure of fundamental particles is to include flavor symmetries in the theories. In this work, we investigate the rare charged lepton flavor violating (cLFV) decay process $(\mu \to e\gamma )$ in two of the low-scale ($\sim$TeV) seesaw models: (i) the Inverse seesaw (ISS) and (ii) Linear seesaw (LSS) models within the framework of $A_4$ flavor symmetry. Apart from the $A_4$ flavor symmetry, some other symmetries like $U{(1)}_X$, $Z_4$ and $Z_5$ are included to construct the Lagrangian. We use results from our previous work [M. R. Devi and K. Bora, A comparative study of type-II, inverse and linear seesaw mechanisms with $A_4$ flavor symmetry, presented at DAE HEP Symp., 2020, Jatni, Odisha; M. R. Devi and K. Bora, Mod. Phys. Lett. A 37, 2250073] where we computed unknown neutrino oscillation parameters within $3\sigma$ limits of their global best fit values, and apply those results to compute the branching ratio (BR) of the muon decay for both the seesaw models. Next we compare our results with the current experimental bounds and sensitivity limits of BR$(\mu \to e\gamma )$ as projected by various experiments, and present a comparative analysis that which of the two models is more likely to be tested by which current/future experiment. This is done for various values of currently allowed non-unitarity parameter. This comparative study will help us to pinpoint that which of the low-scale seesaw models and triplet flavon VEV alignments will be more viable and favorable for testing under a common flavor symmetry ($A_4$ here), and hence can help discriminate between the two models.

If the sterile neutrino mass matrix in an otherwise conventional see-saw model has a rank less than the number of flavors, it is possible to produce pseudo-Dirac neutrinos. In a two-flavor, sterile rank 1 case, we demonstrate analytic conditions for large active mixing induced by the existence of (and coupling to) the sterile neutrino components. For the three-flavor, rank 1 case, "3+2" scenarios with large mixing also devolve naturally as we show by numerical examples. We observe that, in this approach, small mass differences can develop naturally without any requirement that masses themselves are small. Additionally, we point out that significant three-channel mixing and limited experimental resolution can combine to produce extracted two-channel mixing parameters at variance with the actual values.