NEUTRINOS WITH MAGNETIC-MOMENT - DEPOLARIZATION RATE IN PLASMA

Neutrinos with a magnetic moment mu change their helicity when interac ting with an electromagnetic field. Various aspects of this effect hav e been described as spin precession, spin-flip scattering, and magneti c Cherenkov radiation, These perspectives are unified in an expression for the nu(L)-->nu(R) transition rate which involves the correlators of the electromagnetic field distribution, Our general formula correct s a previous result and generalizes it to the case where the fields ca nnot be viewed as classical and where the momentum transfers need not be small, We evaluate our result explicitly for a relativistic QED pla sma and determine the depolarization rate to leading order in the fine structure constant, Assuming that big-bang nucleosynthesis constraint s do not allow a right-handed neutrino in equilibrium we derive the li mit mu < 6.2 x 10(-11) mu(B) on the neutrino magnetic moment. Bounds o n mu from a possible large scale magnetic fields are found to be more stringent even for very weak fields. (C) 1997 Elsevier Science B.V. PA CS: 98.80; 13.15.