Toward Powerful Probes of Neutrino Self-Interactions in Supernovae

Neutrinos remain mysterious. As an example, enhanced self-interactions ($\nu \mathrm{SI}$), which would have broad implications, are allowed. At the high neutrino densities within core-collapse supernovae, $\nu \mathrm{SI}$ should be important, but robust observables have been lacking. We show that $\nu \mathrm{SI}$ make neutrinos form a tightly coupled fluid that expands under relativistic hydrodynamics. The outflow becomes either a burst or a steady-state wind; which occurs here is uncertain. Though the diffusive environment where neutrinos are produced may make a wind more likely, further work is needed to determine when each case is realized. In the burst-outflow case, $\nu \mathrm{SI}$ increase the duration of the neutrino signal, and even a simple analysis of SN 1987A data has powerful sensitivity. For the wind-outflow case, we outline several promising ideas that may lead to new observables. Combined, these results are important steps toward solving the 35-year-old puzzle of how $\nu \mathrm{SI}$ affect supernovae.

Figure 1

Potential constraints on $\nu \mathrm{SI}$ from SN 1987A (assuming the burst-outflow case), previous limits, and relevant scales [12, 36, 86, 87]. $K$-decay bounds apply only to ${\nu }_e$ and ${\nu }_{\mu }$. Strong $\nu \mathrm{SI}$ would change the time profile of the SN 1987A neutrino signal; we show a conservative analysis (30-s duration), and an estimated sensitivity (3-s smearing).

Figure 2

Macroscopic evolution of a neutrino outflow from a supernova (lengths not to scale). Without $\mathit{\nu }\mathrm{SI}$, the final width of the neutrino shell is ${\ell }_0\sim c·10\mathrm{s}$, much larger than the PNS and set by neutrino diffusion therein. With strong $\mathit{\nu }\mathrm{SI}$, neutrinos diffuse in the expanding neutrino ball. In the burst-outflow case, the size of the ball when neutrinos start decoupling from each other, ${\ell }_{\mathrm{FS}}$, sets the final width of the neutrino shell. The duration of the observed neutrino signal will thus be significantly extended when ${\ell }_{\mathrm{FS}}>{\ell }_0$.

Figure 3

Microscopic evolution of neutrino scattering due to $\nu \mathrm{SI}$ at different times for the burst-outflow case (lengths not to scale). Neutrinos move in all directions until the $\nu \mathrm{SI}$ optical depth becomes small. After then, neutrinos are no longer significantly deflected and the ball becomes a shell.

Figure 4

Observed time profile of SN 1987A neutrinos compared to schematic predictions with $\nu \mathrm{SI}$ for the burst-outflow case. The main figure is for our conservative analysis and the inset for our estimated sensitivity. $\nu \mathrm{SI}$ corresponding to the conservative analysis are clearly incompatible with observations, while those for the estimated sensitivity could be probed with a dedicated analysis.