Neutrono dispersion at finite temperature and density

Abstract

We investigate the interaction of neutrinos with a thermal background in the framework of the standard model of weak interactions. We calculate the lowest order contribution to the refractive index, including the imaginary part and corrections from the finite gauge boson masses. These corrections are the dominant refractive effect in a CP-symmetric plasma. In the early universe, the occurence of neutrino oscillations, and particularly of magnetically induced left-right transitions, is determined by these effects. In an expanding universe, “level crossing” between left-and right-handed neutrinos occurs near the weak-decoupling temperature ( T weak ≈ 2 MeV), with no such effect for anti-neutrinos.