Supernova neutrinos: Flavor conversion independent of their mass

Abstract

In extremely dense neutrino environments like in supernova core, the neutrino-neutrino refraction may give rise to self-induced flavor conversion. These neutrino flavor oscillations are well understood from the idea of the exponentially growing modes of the interacting oscillators in the flavor space. Until recently, the growth rates of these modes were found to be of the order of the vacuum oscillation frequency $\Delta m^2/2E$ [$\mathcal {O}(1~\mathrm{km}^{-1})$] and were considered slow growing. However, in the last couple of years it was found that if the system was allowed to have different zenith-angle distributions for the emitted $\nu _e$ and $\bar{\nu }_e$ beams then the fastest growing modes of the interacting oscillators grew at the order of $\mu =\sqrt{2} G_\mathrm{F}n_{\nu }$, a typical $\nu$–$\nu$ interaction energy [$\mathcal {O}(10^5~\mathrm{km}^{-1})$]. Thus the growth rates are very large in comparison to the so-called ‘slow oscillations’ and can result in neutrino flavor conversion on a much faster scale. In fact, the point that the growth rates are no longer dependent on the vacuum oscillation frequency $\Delta m^2/2E$, makes these ‘fast flavor conversions’ independent of $\Delta m^2$ (thus mass) and energy. This is a surprising result as neutrino flavor conversions are considered to be the ultimate proof of massive neutrinos. However, the importance of this effect in the realistic astrophysical scenarios still remains to be understood.