On nonlinear description of neutrino flavour evolution in solar matter

We formulate a formalism of the neutrino state evolution as an open system in the Sun, obtained by solving a quasilinear extension of the von Neumann equation. We obtain similar predictions for the averaged neutrino survival probability measured on Earth as in the Mikheev–Smirnov–Wolfenstein (MSW) approach, differing by details inside the Sun where the quasilinear evolution predicts suppression of oscillations. We discuss the issue of energy transfer between the neutrino and the Sun and the speed of the neutrino state evolution.