Measurements of the charge ratio and polarization of cosmic-ray muons with the Super-Kamiokande detector

Abstract

We present the results of the charge ratio ($R$) and polarization ($P^{\hat{I}}{¼}_0$) measurements using the decay electron events collected from 2008 September to 2022 June by the Super-Kamiokande detector. Because of its underground location and long operation, we performed high precision measurements by accumulating cosmic-ray muons. We measured the muon charge ratio to be $R=1.32\pm 0.02$ $(stat.+syst.)$ at $E_{\hat{I}}¼\cos \hat{I}{¸}_{\mathrm{Zenith}}=0.7_{-0.2}^{+0.3}$ $\mathrm{TeV}$, where $E_{\hat{I}}¼$ is the muon energy and $\hat{I}{¸}_{\mathrm{Zenith}}$ is the zenith angle of incoming cosmic-ray muons. This result is consistent with the Honda flux model while this suggests a tension with the $\ddot{I}€K$ model of $1.9\ddot{I}ƒ$. We also measured the muon polarization at the production location to be $P^{\hat{I}}{¼}_0=0.52\pm 0.02$ $(stat.+syst.)$ at the muon momentum of $0.9_{-0.1}^{+0.6}$ $\mathrm{TeV}/c$ at the surface of the mountain; this also suggests a tension with the Honda flux model of $1.5\ddot{I}ƒ$. This is the most precise measurement ever to experimentally determine the cosmic-ray muon polarization near $1\mathrm{TeV}/c$. These measurement results are useful to improve the atmospheric neutrino simulations.

29 pages, 45 figures