Dynamics of fast neutrino flavor conversions with scattering effects: a detailed analysis

Abstract: We calculate fast conversions of two flavor neutrinos by considering Boltzmann collisions of neutrino scatterings. In an idealized angular distribution of neutrinos with electron-lepton number crossing, we find that the collision terms of the neutrino scattering enhance the transition probability of fast flavor conversions as in the previous study. We analyze the dynamics of fast flavor conversions with collisions in detail based on the motion of polarization vectors in cylindrical coordinate analogous to a pendulum motion. The phase of the all polarization vector synchronizes in the linear evolution, and the phase deviation from the Hamiltonian governs the conversion of neutrino flavor. In the non-linear evolution, a closed orbit in the phase space is observed. The collision terms break the closed orbit and gradually make the phase space smaller. The flavor conversions are enhanced during this limit cycle. After the significant flavor conversion, all of the neutrino polarization vectors start to align with the z-axis owing to the collision effect within the time scale of the collision term irrespective of neutrino scattering angles. We also show the enhancement or suppression of the flavor conversions in various setups of the collision terms and verify consistency with previous studies. Though our analysis does not fully understand the dynamics of fast flavor conversion, the framework gives a new insight into this complicated phenomenon in further study.