Neutrino–neutrino interactions and flavour mixing in dense matter

An algebraic approach to the neutrino propagation in dense media is presented. The Hamiltonian describing a gas of neutrinos interacting with each other and with background fermions is written in terms of the appropriate SU ( N ) operators, where N is the number of neutrino flavours. The evolution of the resulting many-body problem is formulated as a coherent-state path integral. Some commonly used approximations are shown to represent the saddle-point solution of the path integral for the full many-body system.