On Fast Neutrino Flavor Conversion Modes in the Nonlinear Regime

A dense neutrino medium could experience self-induced flavor conversions on relatively small scales in the presence of the so-called fast flavor conversion modes. Owing to the fact that fast conversion scales could be much smaller than the ones of the traditional collective neutrino oscillations, it has been speculated that fast modes could lead to some sort of flavor decoherence/equilibrium. We study the evolution of fast modes in the nonlinear regime and we show that not only fast modes are not guaranteed to lead to flavor equilibrium, but also they could lead to some sort of collective neutrino oscillations but on scales determined by neutrino number density. For the $\nu_e$ dominated case, we observe large amplitude collective oscillations, whereas a sort of flavor stabilization is reached for the $\bar{\nu}_e$ dominated case.