Signatures of Type-I Seesaw in Neutrino Oscillation Phenomenology

Abstract

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 $μ\to eγ$.

Figures (10)

• Figure 1: Viable seesaw parameter space in the sterile mass--mixing plane, obtained from a Monte Carlo scan of the 21-dimensional sterile parameter space subject to the oscillation and phenomenological constraints of Table \ref{['tab:constraints']}. Each panel shows the lightest sterile neutrino mass $M_4$ versus the active--sterile mixing element $|\Theta_{\alpha 4}|^2$ for $\alpha = e,\mu,\tau$, respectively. The characteristic inverse-square scaling $M \propto \Theta^{-2}$ from the seesaw relation (Eq. \ref{['eq:Mass_vs_mixing']}) is evident as a linear correlation on the logarithmic scale. Points are colour-coded by their sterile mass scale (eV, keV, MeV, GeV).
• Figure 2: Non-unitarity of the active neutrino mixing matrix as a function of the lightest sterile neutrino mass $M_4$. The vertical axis shows $|NN^\dagger - \mathcal{I}|$, the Frobenius norm of the deviation of the effective $3\times 3$ mixing matrix from unitarity. The monotonic decrease reflects the seesaw-suppressed active-sterile mixing at higher mass scales (Eq. \ref{['eq:non_unitarity_elements']}).
• Figure 3: Neutrino oscillation probabilities in the $6\nu$ framework: (a) NO$\nu$A $\nu_\mu \to \nu_e$ appearance, (b) NO$\nu$A $\nu_\mu \to \nu_\mu$ disappearance, (c) DUNE $\nu_\mu \to \nu_e$ appearance, (d) DUNE $\nu_\mu \to \nu_\mu$ disappearance. The standard three-flavor prediction (black) is compared against seesaw scenarios with sterile neutrinos at the eV (blue), keV (orange), MeV (green), and GeV (red) mass scales, using the benchmark parameters of Table \ref{['tab:parameters']}. The eV-scale sterile neutrinos produce visible high-frequency oscillatory distortions, while heavier sterile states progressively decouple as the rapid oscillation phases average out.
• Figure 4: Simulated event rates: (a) NO$\nu$A $\nu_e$ appearance, (b) NO$\nu$A $\nu_\mu$ disappearance, (c) DUNE $\nu_e$ appearance, (d) DUNE $\nu_\mu$ disappearance, including detector efficiencies and exposure. Each curve corresponds to a different sterile mass scale as in Fig. \ref{['fig:Probability']}. The eV-scale sterile neutrinos induce measurable distortions in the event spectra, whereas keV--GeV-scale scenarios produce rates nearly indistinguishable from the standard three-flavor expectation due to the seesaw-suppressed mixing.
• Figure 5: Effect of the CP-violating phase on sterile neutrino signatures at NO$\nu$A. (a) $\nu_\mu \to \nu_e$ appearance probability; (b) corresponding $\nu_e$ appearance event rates, both evaluated at $\delta_{\rm CP} = 0.87$ rad (upper half-plane). Compared with the lower half-plane value used in Fig. \ref{['fig:Probability']}, the constructive CP-dependent interference enhances the appearance signal. The eV-scale sterile neutrino distortions (blue) remain clearly distinguishable from the three-flavor prediction (black) in both CP-phase configurations.