Measurement of the branching ratio of $\mathrm{^{16}N}$, $\mathrm{^{15}C}$, $\mathrm{^{12}B}$, and $\mathrm{^{13}B}$ isotopes through the nuclear muon capture reaction in the Super-Kamiokande detector

TL;DR

This work measures the branching ratios of unstable isotopes produced by nuclear muon capture on oxygen in water at Super-Kamiokande, addressing backgrounds that limit solar-neutrino analyses. By tagging pairs of stopping muons and delayed isotope decays and employing detailed MC simulations plus a chi-squared fit to time and energy distributions, the study extracts production rates for $^{16}$N, $^{15}$C, $^{12}$B, and $^{13}$B. The combined results yield Br($^{16}$N$)=9.0or 1 ext{stat. + syst.}$%, Br($^{15}$C$)=0.6or 1 ext{stat. + syst.}$%, Br($^{12}$B$)=0.98or 1 ext{stat. + syst.}$%, Br($^{13}$B$)=0.14or 1 ext{stat. + syst.}$% (with 13B 90% CL upper limits). These measurements, including the first determinations for $^{15}$C, $^{12}$B, and $^{13}$B, provide world-leading precision for $^{16}$N and crucial benchmarks for MC codes (GEANT4, PHITS, Fluka) that simulate muon-induced backgrounds. The results improve background modeling for solar neutrinos and other low-energy signals, guide future detector simulations, and support more accurate explorations of weak interactions in light nuclei.

Abstract

The Super-Kamiokande detector has measured solar neutrinos for more than $25$ years. The sensitivity for solar neutrino measurement is limited by the uncertainties of energy scale and background modeling. Decays of unstable isotopes with relatively long half-lives through nuclear muon capture, such as $\mathrm{^{16}N}$, $\mathrm{^{15}C}$, $\mathrm{^{12}B}$ and $\mathrm{^{13}B}$, are detected as background events for solar neutrino observations. In this study, we developed a method to form a pair of stopping muon and decay candidate events and evaluated the production rates of such unstable isotopes. We then measured their branching ratios considering both their production rates and the estimated number of nuclear muon capture processes as $Br(\mathrm{^{16}N})=(9.0 \pm 0.1)\%$, $Br(\mathrm{^{15}C})=(0.6\pm0.1)\%$, $Br(\mathrm{^{12}B})=(0.98 \pm 0.18)\%$, $Br(\mathrm{^{13}B})=(0.14 \pm 0.12)\%$, respectively. The result for $\mathrm{^{16}N}$ has world-leading precision at present and the results for $\mathrm{^{15}C}$, $\mathrm{^{12}B}$, and $\mathrm{^{13}B}$ are the first branching ratio measurements for those isotopes.

Figures (27)

• Figure 1: The software trigger thresholds with the QBEE electronics as a function of time from October 2008 to June 2022. Red circles, light-green squares, blue upper triangles, and pink lower triangles are the SLE, LE, HE, and SHE triggers, respectively Nishino:2009zu. The black left slanting region shows the period of the refurbishment work and the light-gray filled region shows the period of first and second Gd-loading Super-Kamiokande:2021theSuper-Kamiokande:2024kcb.
• Figure 2: Examples of the decay curves of $\mathrm{^{16}N}$ and $\mathrm{^{15}C}$. The red solid line and green dashed lines show the decay curve of $\mathrm{^{16}N}$ and $\mathrm{^{15}C}$ decays, respectively. For comparison purposes, $10^{6}$ decays are simulated for each of the two decays.
• Figure 3: Kinetic energies of emitted electrons and $\gamma$-rays from decays of $\mathrm{^{16}N}$ and $\mathrm{^{15}C}$. The red dashed and green dotted lines show the energies of electrons from $\mathrm{^{16}N}$ and $\mathrm{^{15}C}$ decays, respectively. The red left-slanting-line histogram and green filled histogram show the energies of $\gamma$-rays from $\mathrm{^{16}N}$ and $\mathrm{^{15}C}$ decays, respectively. For comparison purposes, $10^{6}$ decays are simulated for each of the two decays with Geant4 as detailed in Sec. \ref{['sec:mc-intro']}.
• Figure 4: Examples of the decay curve of $\mathrm{^{12}B}$ and $\mathrm{^{13}B}$. The blue dotted line and magenta dot-dashed lines show the decay curve of $\mathrm{^{12}B}$ and $\mathrm{^{13}B}$ decays. Comparing with the decay curves of $\mathrm{^{16}N}$ and $\mathrm{^{15}C}$ described in Fig. \ref{['fig:mc_expect-decay']}, the decay of $\mathrm{^{12}B}$ quickly occurs and fully completes within $0.3$ s. For comparison purposes, $10^{6}$ decays are simulated for each of the two decays.
• Figure 5: Kinetic energies of emitted electrons and $\gamma$-rays from decays of $\mathrm{^{12}B}$ and $\mathrm{^{13}B}$. The blue dashed and magenta dot-dashed lines show the energies of electrons from $\mathrm{^{12}B}$ and $\mathrm{^{13}B}$ decays, respectively. The blue right-slanting-line histogram and magenta checkered histogram show the energies of $\gamma$-rays from $\mathrm{^{12}B}$ and $\mathrm{^{13}B}$ decays, respectively. For comparison purposes, $10^{6}$ decays are simulated for each of the two decays with Geant4 as detailed in Sec. \ref{['sec:mc-intro']}.