Density-Dependent Matter-Induced Dephasing in Neutrino Oscillations with Preserved Vacuum Unitarity

Version 2 revision sharpens the model by explicitly confining all non-Hermitian effects to matter through a density-dependent dephasing law, while enforcing exact PMNS unitarity in vacuum. Compared to the previous version, the phenomenology is now more tightly constrained, numerically consistent, and framed around concrete, falsifiable experimental signatures rather than framework exposition. _______________ We present a density-dependent, matter-only dephasing model for three-flavor neutrino oscillations that preserves exact vacuum unitarity. Neutrinos are represented as coupled damped modes, with damping acting diagonally in the flavor basis and frequencies diagonal in the mass basis. A smooth density scaling law for the damping matrix enforces Γ_f(x) → 0 in vacuum, so standard PMNS oscillations are recovered identically along vacuum segments, while all non-Hermitian effects are confined to propagation through matter. The model predicts small but structured deviations from standard oscillations, including an energy-dependent spectral tilt, zenith-angle-dependent coherence loss for atmospheric trajectories, and exceptional-point shifts near MSW resonance. These signatures can be tested through differential measurements in JUNO, DUNE, Hyper-Kamiokande, and global oscillation fits.