Neutron multiplicity measurement in muon capture on oxygen nuclei in the Gd-loaded Super-Kamiokande detector

In recent neutrino detectors, neutrons produced in neutrino reactions play an important role. Muon capture on oxygen nuclei is one of the processes that produce neutrons in water Cherenkov detectors. We measured neutron multiplicity in the process using cosmic ray muons that stop in the gadolinium-loaded Super-Kamiokande detector. For this measurement, neutron detection efficiency is obtained with the muon capture events followed by gamma rays to be $50.2^{+2.0}{-2.1}%$. By fitting the observed multiplicity considering the detection efficiency, we measure neutron multiplicity in muon capture as $P(0)=24\pm3%$, $P(1)=70^{+3}{-2}%$, $P(2)=6.1\pm0.5%$, $P(3)=0.38\pm0.09%$. This is the first measurement of the multiplicity of neutrons associated with muon capture on oxygen without neutron energy threshold.

💡 Research Summary

The paper reports the first direct, threshold‑free measurement of neutron multiplicity accompanying muon capture on natural oxygen nuclei using the gadolinium‑loaded Super‑Kamiokande (SK) water Cherenkov detector. Cosmic‑ray muons that stop inside the inner detector (ID) are selected as a source of muon‑capture events. Stopping muons are identified by a single hit cluster in the outer detector (OD) and by reconstructing their entrance point, direction, and momentum from the ID photomultiplier tube (PMT) charge and timing information. Assuming standard muon energy loss in water, the stopping point is reconstructed with a resolution of about 10 cm; only events whose stopping point lies more than 300 cm from the ID wall are retained to ensure that any neutrons emitted are captured within the fiducial volume.

To determine the neutron‑tagging efficiency, the authors exploit a “control sample” consisting of muon‑capture events that are followed by high‑energy de‑excitation gamma rays (≥5 MeV, corresponding to ≥30 PMT hits). These gamma‑ray rich events are dominated by the ¹⁶N* → ¹⁵N + n channel (≈67 % of captures) with a small contribution from the ¹⁴N* → ¹³N + 2n channel (≈0.8 %). By comparing the observed gamma‑ray energy spectrum to Monte‑Carlo (MC) simulations, a +2 % energy scale correction is applied, consistent with other SK analyses. The resulting neutron detection efficiency, derived from the control sample, is 50.2 % with asymmetric uncertainties (+2.0 %/‑2.1 %). This efficiency reflects the combined probability that a neutron thermalizes, is captured on gadolinium (producing an 8 MeV gamma cascade) or on a free proton (producing a 2.2 MeV gamma), and that the resulting gamma signal is successfully identified.

The observed neutron multiplicity distribution is then unfolded using a Bayesian inference method that incorporates the measured efficiency. The final probabilities are:

• P(0 neutrons) = 24 % ± 3 %,
• P(1 neutron) = 70 % (+3 %/‑2 %),
• P(2 neutrons) = 6.1 % ± 0.5 %,
• P(3 neutrons) = 0.38 % ± 0.09 %, with higher multiplicities consistent with zero within statistical uncertainties. These results differ from earlier indirect measurements that relied on coincident gamma‑ray detection and reported lower two‑neutron yields; the enhanced two‑neutron fraction observed here is attributed to the high neutron‑capture efficiency afforded by gadolinium loading.

The paper provides a thorough description of the data set (562.4 live days from 2020–2022, yielding 1,986,465 stopping muons, ≈3,532 events per day), the event selection criteria, background suppression (including radioactive decay, spallation products, and accidental coincidences), and systematic uncertainties (energy scale, timing windows, PMT hit thresholds). It also discusses the relevance of the measurement for neutrino physics: neutron tagging improves background rejection in diffuse supernova neutrino searches, aids in nucleon decay studies, and helps distinguish ν from ν̅ in oscillation analyses. Moreover, the direct multiplicity data provide valuable input for nuclear‑physics models of muon capture on light nuclei, where pre‑equilibrium and equilibrium statistical models are insufficient.

In summary, by leveraging the 4π coverage and low‑energy sensitivity of the Gd‑loaded SK detector, the authors have achieved an unbiased measurement of neutron emission following muon capture on oxygen, establishing a benchmark for future water‑Cherenkov experiments and for refining theoretical descriptions of weak nuclear capture processes.