Sterile neutrino production via active sterile oscillations: the quantum Zeno effect

We study several aspects of the kinetic approach to sterile neutrino production via active-sterile mixing. We obtain the neutrino propagator in the medium including self-energy correction sup to O(G(F)(2)), from which we extract the dispersion relations and damping rates of the propagating modes. The dispersion relations are the usual ones in terms of the index of refraction in the medium, and the damping rates are Gamma(1)(k)=Gamma(aa)(k)cos(2) theta(m)(k);Gamma(2)(k)=Gamma(aa)(k)sin(2) theta(m)(k)where Gamma(aa)(k) alpha G(F)(2)kT(4) is the active neutrino scattered ingrate and theta(m)(k)is the mixing angle in the medium. We provide a generalization of the transition probability in the medium from expectation values in the density matrix: [GRAPHICS] and study the conditions for its quantum Zeno suppression directly in real time. We find the general conditions for quantum Zeno suppression,which for m(s) similar to keV sterile neutrinos with sin 2 theta less than or similar to 10(-3) may only be fulfilled near an MSW resonance. We discuss the implications for sterile neutrino production and argue that in the early Universe the wide separation of relaxation scales far away from MSW resonances suggest the breakdown of the current kinetic approach.