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 $3\sigma$. For the viable parameter points, we simulate the modified neutrino oscillation probabilities and event rates at the long-baseline experiments DUNE and NO$\nu$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 $m_\beta$ from beta decay, the effective Majorana mass $|m_{\beta\beta}|$ from neutrinoless double beta decay ($0\nu\beta\beta$), and the charged-lepton-flavor-violating branching ratio $\text{BR}(\mu \to e\gamma)$. 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 $\mu \to e\gamma$.