Neutrino Spectrum Distortion Due to Oscillations and its BBN Effect
Abstract
We study the distortion of electron neutrino energy spectrum due to oscillations with the sterile neutrino nue <-> nus, for different initial populations of the sterile state delta Ns at the onset of oscillations. The influence of this spectrum distortion on Big Bang Nucleosynthesis is analyzed. Only the case of an initially empty sterile state was studied in previous publications. The primordial abundance of He-4 is calculated for all possible delta Ns: 0<= delta Ns<= 1 in the model of oscillations, effective after electron neutrino decoupling, for which the spectrum distortion effects on the neutron--proton transitions are the strongest. It is found that the spectrum distortion effect may be the dominant one not only in the case of small delta Ns, but also in the case of big initial population of nus. For example, in the resonant case it may play a considerable role even for very big delta Ns ~ 0.8.
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