An Active-Sterile Neutrino Transformation Solution for r-Process Nucleosynthesis

TL;DR

This paper tackles the challenge of producing robust $r$-process nucleosynthesis in neutrino-driven supernova winds by proposing matter-enhanced active-sterile neutrino transformations in the channels $ν_e↔ν_s$ and $ν̄_e↔ν̄_s$. It develops a self-consistent, outflow-coupled MSW framework within a one-dimensional wind model, coupling electron fraction evolution, neutrino flavor transformation, and NSE/nucleosynthesis to show that a sterile neutrino with $δm^2$ in the range $3$–$70$ eV$^2$ and $sin^2(2θ_{es})$ above about $10^{-3}$ can lower $Y_e$ and suppress the alpha effect, increasing the neutron-to-seed ratio toward $R\gtrsim 100$. The study finds an island of favorable parameters in $(δm^2, sin^2(2θ_{es}))$ that yields robust $r$-process yields across plausible wind-expansion timescales, with nontrivial feedback between $Y_e$ and the potential. These results imply that heavy-element nucleosynthesis could serve as a probe of light sterile neutrinos, while providing a mechanism to alleviate longstanding bottlenecks in $r$-process production within core-collapse supernovae. The work highlights a concrete intersection of particle physics and astrophysical nucleosynthesis, suggesting new avenues to constrain sterile neutrino properties via observed heavy-element abundances.

Abstract

We discuss how matter-enhanced active-sterile neutrino transformation in both neutrino and antineutrino channels could enable the production of the rapid neutron capture (r-process) nuclei in neutrino-heated supernova ejecta. In this scheme the lightest sterile neutrino would be heavier than the electron neutrino and split from it by a vacuum mass-squared difference roughly between 3 and 70 eV$^2$ and vacuum mixing angle given by $\sin^2 2θ_{es} > 10^{-4}$.

Figures (12)

• Figure 1: The geometry in the calculation of the effective neutrino fraction.
• Figure 2: Density is plotted against distance as measured from the center of the neutron star.
• Figure 3: The upper pair of curves shows the actual and equilibrium electron fraction in the absence of any flavor transformation. The lower pair of curves shows the same with neutrino mixing parameters as in Figures \ref{['fig:rates']} and \ref{['fig:energy']}. In each pair, the lower line corresponds to the equilibrium ${\rm Y}_{\rm e}$. Above 11 km we include the effects of feedback. Above ${\rm Y}_{\rm e} = 1/3$, electron neutrinos may undergo flavor transformation, while below ${\rm Y}_{\rm e} = 1/3$ electron antineutrinos may transform. The neutrino driven wind parameters are the same as in Figures \ref{['fig:energy']} and \ref{['fig:rates']}. For this dynamical timescale, the actual ${\rm Y}_{\rm e}$ closely tracks the equilibrium ${\rm Y}_{\rm e}$. The near complete transformation of electron neutrinos drives the electron fraction to very low values in the lower set of curves. In addition, it almost completely suppresses the alpha effect.
• Figure 4: Electron neutrino (lower curve) and electron antineutrino (upper curve) capture rates on neutrons and protons respectively, plotted against $r$, the distance from the center of the protoneutron star, for the same choice of parameters as in Figure \ref{['fig:energy']}. The $1/r^2$ dependence of the neutrino flux has been removed for illustrative purposes only. All variation seen in the capture rates is due to transformation into sterile neutrinos.
• Figure 5: The potential, $V$, is plotted against distance (solid line). For comparison we also show $V$ when feedback effects are not included (dotted line). The nearly vertical line at the left edge of the plot corresponds to the inmost $\bar{\nu}_e$ and $\nu_e$ resonance at the surface of the neutron star.