NEUTRINO SPIN-FLIP IN MEDIUM WITH RANDOM MAGNETIC-FIELD

We consider Dirac neutrino propagation in a medium with a randomly flu ctuating-magnetic field by averaging over the fluctuations and derivin g a simple kinetic equation for the neutrino spin rotation. We show th at the rotation frequency depends on the size of the domains inside wh ich the random magnetic field can be replaced by the mean field. When the size L of the domains is test than the neutrino spin oscillation l ength, or if L less than or similar to 10(-2)l(H)(T/MeV)-3 with l(H) t he horizon length, the probability for neutrino helicity change will b e suppressed.