Estimating JUNO's Sensitivity to Solar Neutrino-to-antineutrino Conversion and Neutrino Magnetic Moments

TL;DR

This paper evaluates JUNO's sensitivity to solar neutrino–to–antineutrino conversion via spin-flavor precession and to neutrino magnetic moments using a sensitivity-based, counting-experiment framework. It derives ensemble-average 90% C.L. upper limits on the solar antineutrino flux $\phi_{\mathrm{lim}}$, the conversion probability $P_{\nu_e\rightarrow\bar{\nu}_e}$, and $\mu_\nu$ in two energy windows (1.8–16.8 MeV and 8–16.8 MeV), then translates these into magnetic-moment constraints with assumed solar transverse magnetic fields $B_{\perp}$. The high-energy window (8–16.8 MeV) yields $P_{\nu_e\rightarrow\bar{\nu}_e}\le 2.07\times10^{-5}$ and, for plausible $B_{\perp}$, $\mu_\nu$ limits around $\mathcal{O}(10^{-11})\,\mu_B$, approaching the strongest astrophysical bounds, while the full window is background-limited by reactor antineutrinos. These results demonstrate JUNO's potential to impose competitive limits on exotic solar neutrino processes and provide guidance for future spectral analyses to further enhance sensitivity.

Abstract

We investigated JUNO's sensitivity to a possible conversion of solar neutrinos into antineutrinos via the spin-flavor precession (SFP) mechanism, and assessed the implications for constraining the neutrino-magnetic moment (NMM). Using a sensitivity-based framework appropriate for counting experiments with no prior observations, we derive 90\% C.L.\ ensemble-average sensitivities on the solar antineutrino flux for 1.8--16.8 MeV and 8.0--16.8 MeV. For the entire energy window, the results do not improve the restrictions of other experiments; the relevance occurs in the highes-energy window. In this window, we report a flux of $φ_{\mathrm{lim}}\le 4.01\times10^{1}\ \mathrm{cm^{-2}\,s^{-1}}$ and a probability of $P_{ν_e\rightarrow\barν_e}\le 2.07\times10^{-5}$, the latter normalized to the ${}^8$B flux above threshold, $Φ_{\rm SSM}(E>8~\mathrm{MeV})$. Assuming transverse solar magnetic fields of $B_\perp=50$ and $100$~kG, the corresponding magnetic-moment sensitivities are $μ_ν\le 7.27\times10^{-11}\,μ_B$ and $3.64\times10^{-11}\,μ_B$ in the high-energy window. These results highlight that JUNO has the potential to achieve sensitivities comparable to the most stringent astrophysical limits; in particular, the high-energy selection (8.0--16.8~MeV) provides a sensitivity that is competitive with current results, while the full-energy window remains primarily limited by near-reactor backgrounds.

Figures (1)

• Figure 1: Reference comparison of published upper limits on the conversion probability $P(\nu_e\!\to\!\bar{\nu}_e)$ and their illustrative mapping to magnetic moment sensitivities. Important: the experimental (and JUNO projected) limits on $P$ are field-independent; the abscissa $B_\perp$ is used here only to visualize how a given probability would translate into $\mu_\nu$ under the scaling $\mu_\nu = 7.4\times10^{-7}\,B_\perp^{-1}\sqrt{P/\sin^22\theta_{12}}$ with $\sin^22\theta_{12}=0.86$. The coloured band encodes the corresponding $\mu_\nu$ (in $\mu_B$) for any pair $(P,B_\perp)$, and should not be interpreted as implying a dependence of $P$ on $B_\perp$. Horizontal dashed lines show the 90% C.L. limits on $P$ from Borexino (2010), KamLAND (2012, 2022), Super-Kamiokande, and the JUNO ensemble-average sensitivities for the two prompt-energy windows ($1.8-16.8$ MeV and $8.0-16.8$ MeV). Vertical dotted lines mark benchmark field strengths associated with different solar regions (convective zone, tachocline, radiative zone, core). Plotted ranges: $600~\mathrm{G}\le B_\perp\le 7~\mathrm{MG}$ and $5\times10^{-6}\le P\le 6\times10^{-4}$.