Measurement of the Flux and Zenith-Angle Distribution of Upward Throughgoing Muons by Super-Kamiokande

A total of 614 upward throughgoing muons of minimum energy 1.6 GeV are observed by Super-Kamiokande during 537 detector live days. The measured muon flux is $[1.74±{}0.07(stat)±{}0.02(sys)]×{}{10}^{{-}13}{cm}^{{-}2}{s}^{{-}1}{sr}^{{-}1}$ compared to an expected flux of $[1.97±{}0.44(theor)]×{}{10}^{{-}13}{cm}^{{-}2}{s}^{{-}1}{sr}^{{-}1}$. The absolute measured flux is in agreement with the prediction within the errors. However, the zenith-angle dependence of the observed upward throughgoing muon flux does not agree with no-oscillation predictions. The observed distortion in shape is consistent with the ν_μ↔ν_τ oscillation hypothesis with sin²2θ>0.4 and 1×10^-3<Δm²<1×10^-1eV² at 90% confidence level.