Expected flavor composition of supernova neutrinos

TL;DR

This work addresses predicting the flavor composition of core-collapse supernova neutrinos at Earth when inner-core collective effects are poorly understood. By assuming decoherence at the boundary with the MSW-dominated outer layers and applying standard matter effects, the authors derive robust bounds on the electron-flavor fraction: for normal ordering $f_{\nu_e}^{\rm NO} \lesssim 0.5$ and for inverted ordering $f_{\nu_e}^{\rm IO} \simeq 1/3$, with shock-induced nonadiabaticity potentially pushing NO toward equipartition. The analysis ties decoherence and MSW physics to concrete predictions for SN neutrino signals, and suggests that deviations in future observations could signal either a breakdown of the decoherence assumption or new physics in flavor conversion. Antineutrino fractions receive somewhat weaker constraints, but conservative bounds place $f_{\nu_e}, f_{\bar\nu_e} \lesssim 0.7$ in general. Overall, high-statistics Galactic SN data could test these robust predictions and probe outer-layer flavor dynamics and possible new phenomena.

Abstract

We revisit the flavor composition of neutrinos from core-collapse supernovae (SN), focusing on robust predictions that are insensitive to the poorly known dynamics of collective flavor conversion in the inner core. Assuming that the many different trajectories and microscopic histories of neutrinos lead to decoherence of the ensemble at the boundary between the region of collective effects and the Mikheyev-Smirnov-Wolfenstein (MSW) dominated layers, we show that standard matter effects alone strongly constrain the electron-flavor fraction at Earth. For normal mass ordering (NO) we obtain $f_{ν_e}^{\rm NO}\lesssim 0.5$ at all times and energies, while for inverted ordering (IO), we predict $f_{ν_e}^{\rm IO}\simeq 1/3$, i.e.\ near flavor equipartition. Shock-wave propagation through the high (H) MSW resonance drives the system toward equipartition also in NO. In this way our framework links simple assumptions about decoherence and standard matter effects to robust expectations for the flavor evolution inside core-collapse supernovae. This contribution summarizes the main results of arXiv:2403.14762.

Figures (2)

• Figure 1: Schematic view of flavor evolution zones in a core-collapse supernova. Neutrinos are produced near the $\nu$-sphere, traverse regions where fast and slow collective effects can occur, and eventually enter the MSW-dominated outer layers. Different trajectories experience different histories in the inner regions, which motivates treating the ensemble as decohered when it reaches the boundary with the MSW region.
• Figure 2: Allowed flavor compositions of supernova neutrinos at Earth for normal and inverted mass ordering. Red regions correspond to adiabatic propagation; maroon regions include possible nonadiabatic effects at the H resonance caused by shock waves; pink regions illustrate the larger space of possibilities if the ensemble emerges as an incoherent mixture of matter eigenstates. Figure adapted from Ref. Capanema:2025