Collisional Triggering of Fast Flavor Conversions of Supernova Neutrinos

Fast flavor conversions of supernova neutrinos, possible near the neutrinosphere, depends on an interesting interplay of collisions and neutrino oscillations. Contrary to naïve expectations, the rate of self-induced neutrino oscillations, due to neutrino-neutrino forward scattering, comfortably exceeds the rate of collisions even deep inside the supernova core. Consistently accounting for collisions and oscillations, we present the first calculations to show that collisions can create the conditions for fast flavor conversions of neutrinos, but oscillations can continue without significant damping thereafter. This may have interesting consequences for supernova explosions and the nature of its associated neutrino emission.

Figure 1

Properties of an $11M_{\odot }$ supernova derived from the simulation in Ref. [17]. Radial profiles of neutrino potential $\mu$, matter potential $\lambda$, and scattering rates ${\mathrm{\Gamma }}_{{\nu }_e}$ and ${\mathrm{\Gamma }}_{{\overline{\nu }}_e}$, are shown for a snapshot at a post-bounce time $t=0.170\mathrm{s}$.

Figure 2

Equilibrium abundances and relative collision rates for ${\nu }_e$ and ${\overline{\nu }}_e$. Left panel: Equilibrium values of the occupation numbers, ${\varrho }_{ee}^{\mathrm{eq}}$, for ${\nu }_e$ and ${\overline{\nu }}_e$ in the forward and backward directions. Right panel: Collision rates ${\mathrm{\Gamma }}_{{\nu }_e}$ and ${\mathrm{\Gamma }}_{{\overline{\nu }}_e}$.

Figure 3

Two beam model in the no-oscillation limit, with ${\mathrm{\Gamma }}_{{\nu }_e}(0)={\mathrm{\Gamma }}_{{\overline{\nu }}_e}(0)=0.1$ and $\mu =0$. Evolution of forward and backward going mode occupations for ${\nu }_e$ and ${\overline{\nu }}_e$, as a function of $z$ for different representative times. Note the approach to equilibrium, followed by free streaming in the right-most zone at late times.

Figure 4

Two beam model with ${\mathrm{\Gamma }}_{{\nu }_e}(0)={\mathrm{\Gamma }}_{{\overline{\nu }}_e}(0)=0.1$ and $\mu =1$. Evolution of forward and backward going mode occupations for ${\nu }_e$ and ${\overline{\nu }}_e$ as a function of $z$ for different representative times. Note the instability in the $t=200$ snapshot and that, except for a leading transient, ${\rho }_{ee}$ approaches approximately the same equilibrium as in Fig. 3.

Figure 5

Two beam model with ${\mathrm{\Gamma }}_{{\nu }_e}(0)={\mathrm{\Gamma }}_{{\overline{\nu }}_e}(0)={10}^{-4}$ and $\mu =1$. Evolution of forward and backward going mode occupations for ${\nu }_e$ and ${\overline{\nu }}_e$ as a function of $z$ for different representative times. Note that the instability in the $t=1600$ snapshot is not suppressed even at later times and a different equilibrium is reached after a longer transient.