Neutrino Spin Relaxation in Medium With Stochastic Characteristics

Abstract

The helicity evolution of a neutrino propagating in randomly moving and polarized matter is studied. The type of the neutrino interaction with background fermions is arbitrary. We derive the equation for the description of the averaged neutrino helicity evolution. In the particular case of a τ -neutrino interacting with ultrarelativistic electron-positron plasma we obtain the expression for the neutrino helicity relaxation rate in the explicit form. We study the neutrino spin relaxation in the relativistic primordial plasma. Supposing that the conversion of left-handed neutrinos into right-handed ones is suppressed at the early stages of the Universe evolution we get the upper limit on the τ -neutrino mass.