Beyond the MSW effect: Neutrinos in a dense medium

We present a theory of neutrino oscillations in dense medium which goes beyond the effective matter potential used description MSW effect. show how purity state is degraded by interactions with environment and neutrino--matter can be source decoherence. new oscillation formulae for neutrinos interacting leptons carry out numerical analysis exhibits deviations from propagation through Earth ultra-high energy neutrinos. In particular, we that at high density and/or energy, vanishing transition probabilities derived effect, are non zero when scattering taken into account.