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 $[1.74\pm 0.07(\mathrm{stat})\pm 0.02(\mathrm{sys}\left)\right]\times {10}^{-13\mathrm{}}{\mathrm{cm}}^{-2\mathrm{}}{\mathrm{s}}^{-1\mathrm{}}{\mathrm{sr}}^{-1\mathrm{}}$ compared to an expected flux of $[1.97\pm 0.44(\mathrm{theor}\left)\right]\times {10}^{-13\mathrm{}}{\mathrm{cm}}^{-2\mathrm{}}{\mathrm{s}}^{-1\mathrm{}}{\mathrm{sr}}^{-1\mathrm{}}$. 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 ${\nu }_{\mu }\leftrightarrow {\nu }_{\tau }$ oscillation hypothesis with ${sin}^{2}2\theta >0.4\mathrm{}$ and $1\times {10}^{-3\mathrm{}}<\Delta m^2<1\mathrm{}\times 10^{-1\mathrm{}}{\mathrm{eV}}^2$ at 90% confidence level.