Sterile neutrinos and supernova nucleosynthesis

TL;DR

This work addresses the neutron deficit problem in neutrino-driven supernova outflows, which hampers robust heavy $r$-process nucleosynthesis due to the alpha effect driven by the $ u_e$ flux. It proposes a four-neutrino mass-mixing framework with a light sterile neutrino, enabling a two-step, matter-enhanced flavor transformation: first $ u_ ext{mu}, u_ au ightarrow u_s$, then $ u_ ext{mu}, u_ au ightarrow u_e$, effectively removing the bulk of the $ u_e$ flux from the neutrino-heated ejecta region below the weak freeze-out radius. This suppression of the $ u_e$ flux prevents neutron destruction, yields an electron fraction in the desirable range $1/3<Y_e< ilde{Y}_e oughly 0.4$, and thereby stabilizes the neutron-to-seed ratio $R>100$ needed for a successful $r$-process. The scenario ties together solar, atmospheric, and LSND neutrino data, proposes testable predictions for late-time supernova neutrino spectra, and offers a plausible astrophysical application for sterile neutrinos with masses and mixings consistent with existing constraints.

Abstract

A light sterile neutrino species has been introduced to explain simultaneously the solar and atmospheric neutrino puzzles and the results of the LSND experiment, while providing for a hot component of dark matter. Employing this scheme of neutrino masses and mixings, we show how matter-enhanced active-sterile neutrino transformation followed by active-active neutrino transformation can solve robustly the neutron deficit problem encountered by models of r-process nucleosynthesis associated with neutrino-heated supernova ejecta.

Figures (3)

• Figure 1: The neutrino mass scheme discussed in this paper. A doublet of (near)-maximally-mixed $\nu_\mu$ and $\nu_\tau$ neutrinos with mass-squared difference $\delta m_{\mu\tau}^2 \sim {10}^{-2}$ eV$^2$ is split from a doublet of lower-mass $\nu_e$ and $\nu_s$ by a mass-squared difference $\delta m_{\rm doublets}^2$.
• Figure 2: Cartoon of the instantaneous neutrino mass levels (effective mass-squared $m^2_{\rm eff}$) as functions of matter density $\rho$ for $Y_e > 1/3$.
• Figure 3: Example $\nu_e$ and $\nu_{\mu,\tau}$ energy distribution functions. Here we take these functions to be of the form in Eq. (\ref{['eqn:fd']}) with $T_{\nu_e}=2.75$ MeV, $\eta_{\nu_e}=3$ (corresponding to $\langle E_{\nu_e}\rangle = 11\,{\rm MeV}$) and $T_{\nu_{\mu,\tau}}=6.76$ MeV, $\eta_{\nu_{\mu,\tau}}=3$ (corresponding to $\langle E_{\nu_{\mu,\tau}}\rangle = 27\,{\rm MeV}$). The resonance energy $E_{\nu}^{\rm RES}$ for neutrino flavor/type transformation sweeps from low to high energy through the neutrino energy distribution functions as a fluid element moves away from the neutron star.