Suka Sriyansu Pattanaik

arXiv2026

Probing Quasi-Dirac Neutrino Oscillations at Long Baseline Experiments

The Dirac or Majorana nature of neutrinos remains one of the most fundamental open problems in particle physics. A natural intermediate scenario arises when small lepton-number-violating Majorana mass terms break the exact Dirac symmetry. These cause mass eigenstates to form nearly degenerate pairs, each separated by a small mass splitting. We study this quasi-Dirac scenario within a five-neutrino framework, extending the Standard Model by two right-handed neutrinos, which form a nearly degenerate sterile pair alongside the three active states. This extended mixing structure introduces additional phenomenological parameters, including new mixing angles, mass splittings, and CP phases. We use appearance and disappearance data from NO$ν$A and T2K to constrain the active-sterile mixing angles $θ_{sa}$ and the mass splitting within the sterile pair $Δm_{54}^2$ and forecast the sensitivity achievable at DUNE. We analyse how these parameters modify CP-violating observables relative to the standard three-neutrino case, comparing predicted event rates across both mass orderings and as a function of $δ_{CP}$.

hep-ph↗

arXiv2026

Signatures of Type-I Seesaw in Neutrino Oscillation Phenomenology

We investigate the low-energy phenomenology of the Type-I seesaw mechanism within a 3+3 framework containing three active and three sterile neutrinos. Using the exact seesaw relation as a bridge between the high-scale sterile-sector parameters and the standard oscillation observables, we perform a comprehensive Monte Carlo scan of the 21-dimensional sterile parameter space, retaining only those configurations consistent with current neutrino oscillation data within $3σ$. For the viable parameter points, we simulate the modified neutrino oscillation probabilities and event rates at the long-baseline experiments DUNE and NO$ν$A, and the medium-baseline reactor experiment JUNO, quantifying their sensitivity to sterile neutrino effects across the eV--GeV mass range. We find that eV-scale sterile neutrinos produce pronounced spectral distortions, while heavier states decouple progressively from oscillation experiments. In parallel, we confront the seesaw predictions with complementary probes: cosmological bounds on $\sum m_i$, the kinematic mass $m_β$ from beta decay, the effective Majorana mass $|m_{ββ}|$ from neutrinoless double beta decay ($0νββ$), and the charged-lepton-flavor-violating branching ratio $\text{BR}(μ\to eγ)$. The combination of all constraints significantly narrows the allowed parameter space: the predicted sum of neutrino masses clusters at $\sum m_i \sim 0.05$--$0.07$~eV, within reach of next-generation cosmological surveys, and eV-scale sterile neutrinos are found to be under significant tension from the current MEG bound on $μ\to eγ$.

hep-ph↗