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

TL;DR

This study measures neutron multiplicity from muon capture on oxygen in the Gd-loaded Super-Kamiokande detector, providing the first such measurement without neutron energy threshold.

Contribution

It introduces a novel measurement of neutron multiplicity in muon capture on oxygen using a large water Cherenkov detector with gadolinium.

Findings

Neutron detection efficiency is approximately 50%.

Neutron multiplicity distribution is quantified with specific probabilities.

First measurement of neutron multiplicity in muon capture on oxygen without energy threshold.

Abstract

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.