Neutrino Oscillation Decoherence over Gigaparsec Baselines: Damping Envelopes and the Density Ceiling

This paper computes coherence lengths for neutrino oscillations under energy-dependent decoherence of the form commonly used in phenomenological searches. For a coefficient of 1e-23 per kilometre at one gigaelectronvolt with quadratic energy dependence, the coherence length is 1.000e+23 kilometres at one gigaelectronvolt, 1.000e+19 kilometres at one hundred gigaelectronvolts, and 1.000e+17 kilometres at one teraelectronvolt. High-energy astrophysical neutrinos therefore test decoherence far more stringently than reactor or accelerator neutrinos, because the quadratic growth shortens the coherence length below the gigaparsec baseline precisely in the IceCube energy range. A density ceiling does not enter the oscillation phase directly; its role is confined to the production environment. The study is simulated and computed from stated assumptions; it is not a measurement.