Extending the sensitivity of heavy sterile neutrino searches with solar neutrino experiments

TL;DR

This work addresses MeV-scale heavy sterile neutrinos that mix with electron neutrinos and are produced in the Sun via $^8$B decay. It proposes two complementary search channels in solar neutrino detectors: (i) detecting $e^+e^-$ pairs from in-detector decays and (ii) detecting $\nu_e$ from decays occurring outside the detector. The authors derive the production flux $\Phi_{\nu_H}(E) = |U_{eH}|^2 \sqrt{1 - (m_{\nu_H}/E)^2} \Phi_{^8B}(E)$ and the primary decay widths, $\Gamma_{3\nu_e} = \frac{G_F^2}{192\pi^3} m_{\nu_H}^5 |U_{eH}|^2$ and a comparable $\Gamma_{e^+e^-\nu_e}$ with a ratio near 0.6, to obtain lifetimes as functions of $m_{\nu_H}$ and $|U_{eH}|^2$. By computing expected signal yields for a 500-ton detector over one year, the study maps sensitivity across the $(m_{\nu_H}, |U_{eH}|^2)$ plane, showing crest-like regions for the $e^+e^-$ channel and a separate domain where the $\nu_e$ channel dominates. The results indicate that combining both methods enables substantial coverage of the target parameter space and inform discriminants based on energy and angle to suppress solar backgrounds, with implications for Borexino-like experiments and future facilities such as Jinping.

Abstract

A sensitivity study of the search for heavy sterile neutrinos ($ν_H$) in the MeV mass range using solar neutrino experiments is presented. $ν_H$, with masses ranging from a few MeV up to around 15 MeV, can be produced in the Sun through $^8$B decay and subsequently decay into $ν_e e^+ e^-$. Its flux and lifetime strongly depend on the mixing parameter $|U_{eH}|^2$ and mass $m_{ν_H}$. The $ν_H$ signal can be detected via its decay products, either the $e^+e^-$ pair or $ν_e$, depending on whether $ν_H$ decays inside or outside the detector. Expected signal yields for both detection methods (detecting $e^+e^-$ or $ν_e$ signal) are presented across the full $|U_{eH}|^2$ and $m_{ν_H}$ parameter space. These two methods are found to be complementary in different regions of the $|U_{eH}|^2$ and $m_{ν_H}$ phase space. By combining both approaches, we anticipate observing at least a handful of signal events across most of the parameter space of $10^{-6} < |U_{eH}|^2 < 1$ and 2 MeV $< m_{ν_H} < $ 14 MeV, assuming a 500-ton solar neutrino experiment operating for one year. Key variables, such as the energy spectra and opening angle of $ν_e$ or $e^+e^-$ and the solar angle of $ν_e$ and its scattered electron, are also discussed to help distinguish signal from major backgrounds, such as solar neutrino events.

Figures (11)

• Figure 1: Energy spectra of $\nu_H$ with different masses emitted from $^8$B decay in the Sun. The spectra are based on the $^8 \mathrm{B}$ left-handed solar neutrino spectrum ($\nu_e$ in the plot, taken from BLA+StandardNeutrinoSpectrum1996) and are suppressed by the mixing parameter $|U_{eH}|^2$ and the phase-space factor according to Eq. \ref{['eq:nuHSpectrum']}, where $|U_{eH}|^2 = 1.0$ in the spectra shown in this plot.
• Figure 2: Feynman diagrams of $\nu_H$ production from solar $^8 \mathrm{B}$ decay and its decay $\nu_H \rightarrow e^+e^-\nu_e$ via $W^{\pm}$ or Z boson exchange.
• Figure 3: Proper lifetime ($\tau_{c.m.}=1/\Gamma_{\mathrm{total}}$) of $\nu_H$ as a function of mass $m_{\nu_H}$ and mixing parameter $|U_{eH}|^2$.
• Figure 4: Two detection Methods for heavy sterile neutrinos: (1) $e^+e^-$ pair detection from $\nu_H$ decays occurring inside the detector, and (2) $\nu_e$ detection from $\nu_H$ decays occurring outside the detector.
• Figure 5: Top: The count rate (per year) of $e^{+}e^{-}$ signal from $\nu_H$ decay inside a 500-ton detector on Earth, as a function of $m_{\nu_H}$ and $|U_{eH}|^2$. Bottom: The energy spectrum of $e^{+}e^{-}$ signal from $\nu_H$ decay, along with the energy spectrum of the major background (scattered electron in the detector from solar $^8\mathrm{B}$ neutrino). Different values of $\nu_H$ mass are shown, with $|U_{eH}|^2 = 10^{-5}$.