Abstract:  In certain physical situations, such as in the early universe or near a supernova core, the number density of neutrinos may be comparable to -- or even grater than -- the number density of other elementary particles. In such cases the neutrino flavor evolution is affected by the MSW effect from the neutrino background. The equations describing this phenomenon were proposed by several authors in the early 1990s, however, the physical mechanism by which these equations arise, and the assumptions that go into the derivation, remained somewhat mysterious.  In this talk, I will decribe how these equations can be arrived at by correctly adding up the amplitudes of many elementary scattering events. In the course of the derivation, I will discuss that, although the accepted formalism overcounts the neutrino interaction energy, the correct result differs from it by an overall energy shift, with no effect on the neutrino flavor evolution.

Host: J. Feng