We consider the ν _μ → ν _τ appearance channel in the future Deep Underground Neutrino Experiment (DUNE) which offers a good statistics of the ν _τ sample. In order to measure its impact on constraining the oscillation parameters, we consider several assumptions on the efficiency for ν _τ charged-current signal events (with subsequent τ → e decay) and the related backgrounds and study the effects of various systematic uncertainties. Two different neutrino fluxes have been considered, namely a CP-violation optimized flux and a ν _τ optimized flux. Our results show that the addition of the ν _μ → ν _τ appearance channel does not reduce the current uncertainties on the standard 3-ν oscillation parameters while it can improve in a significant way the sensitivity to the Non-Standard Interaction parameter |∈ _μτ | and to the new mixing angle θ ₃₄ of a sterile neutrino model of the 3 + 1 type.

Next generation neutrino oscillation experiments are expected to measure the remaining oscillation parameters with very good precision. They will have unprecedented capabilities to search for new physics that modify oscillations. DUNE, with its broad band beam, good particle identification, and relatively high energies will provide an excellent environment to search for new physics. If deviations from the standard three-flavor oscillation picture are seen however, it is crucial to know which new physics scenario is found so that it can be verified elsewhere and theoretically understood. We investigate several benchmark new physics scenarios by looking at existing long-baseline accelerator neutrino data from NOvA and T2K and determine at what sensitivity DUNE can differentiate among them. We consider sterile neutrinos and both vector and scalar non-standard neutrino interactions, all with new complex phases, the latter of which could conceivably provide absolute neutrino mass scale information. We find that, in many interesting cases, DUNE will have good model discrimination. We also perform a new fit to NOvA and T2K data with scalar NSI.

We present minimalist constructions for lepton masses and mixing based on flavour symmetry under the modular group Γ_N of lowest level N = 2. As opposed to the only existing model of Γ₂ ≅ S₃ formulated in a SUSY framework, the only non-SM field is the modulus τ, and a generalised CP symmetry is implemented. Charged-leptons masses are reproduced through symmetry arguments, without requiring fine-tuning of the free parameters. As a result, all lepton observables (masses and mixing) are reproduced within 1σ experimental range using a minimum of nine free real parameters (including the real and imaginary parts of the modulus). A normal ordering for the neutrino masses is predicted. We also obtain predictions for the CP violating phases: the Dirac CP phase is predicted around 1.6π, the Majorana phases lie in narrow regions near ±π. The sum of neutrino masses is within the current bound at ∼ 0.09 eV. Furthermore, we provide predictions for the neutrinoless double beta decay and tritium decay effective masses, around 20 meV. Given the reduced number of free input parameters as compared to the existing literature on modular S₃, this work renews interest for a unified predictive model of quark-lepton sectors based on Γ₂ ≅ S₃.

Our knowledge on the active 3νmixing angles (θ₁₂,θ₁₃, andθ₂₃) and the CP phaseδ_CPis becoming accurate day-by-day enabling us to test the unitarity of the leptonic mixing matrix with utmost precision. Future high-precision long-baseline experiments are going to play an important role in this direction. In this work, we study the impact of possible non-unitary neutrino mixing (NUNM) in the context of next-generation long-baseline experiments DUNE and T2HKK/JD+KD having one detector in Japan (T2HK/JD) and a second detector in Korea (KD). We estimate the sensitivities of these setups to place direct, model-independent, and competitive constraints on various NUNM parameters. We demonstrate the possible correlations between the NUNM parameters,θ₂₃, andδ_CP. Our numerical results obtained using only far detector data and supported by simple approximate analytical expressions of the oscillation probabilities in matter, reveal that JD+KD has better sensitivities for|α₂₁|andα₂₂as compared to DUNE, due to its larger statistics in the appearance channel and less systematic uncertainties in the disappearance channel, respectively. For|α₃₁|,|α₃₂|, andα₃₃, DUNE gives better constraints as compared to JD+KD, due to its larger matter effect and wider neutrino energy spectrum. Forα₁₁, both DUNE and JD+KD give similar bounds. We also show how much the bounds on the NUNM parameters can be improved by combining the prospective data from DUNE and JD+KD setups. We find that due to zero-distance effects, the near detectors alone can also constrainα₁₁,|α₂₁|, andα₂₂in both these setups. Finally, we observe that theν_τappearance sample in DUNE can improve the constraints on|α₃₂|andα₃₃.

The hypothesis of the decay of neutrino mass eigenstates leads to a substantial modification of the appearance and disappearance probabilities of flavor eigenstates. We investigate the impact on the standard oscillation scenario caused by the decay of the heaviest mass eigenstate ν ₃ (with a mass m ₃ and a mean life τ ₃ ) to a sterile state in deep underground neutrino experiment. We find that the lower bound of 5.1 × 10 ⁻¹¹ s eV ⁻¹ at 90% confidence level on the decay parameter τ ₃ / m ₃ can be set if the neutral current data are included in the analysis, thus providing the best long-baseline expected limit so far. We also show that the ν _τ appearance channel would give only a negligible contribution to the decay parameter constraints. Our numerical results are corroborated by analytical formulae for the appearance and disappearance probabilities in vacuum (which is a useful approximation for the study of the invisible decay model) that we have developed up to the second order in the solar mass splitting and to all orders in the decay factor t / τ ₃ .

In the recent years experiments have established the existence of neutrino oscillations and most of the oscillation parameters have been measured with a good accuracy. However, in spite of many interesting ideas, no real illumination was sparked on the problem of flavor in the lepton sector. In this review, we discuss the state of the art of models for neutrino masses and mixings formulated in the context of flavor symmetries, with particular emphasis on the role played by grand unified gauge groups.