Observation of the East-West Anisotropy of the Atmospheric Neutrino Flux

The east-west anisotropy, caused by the deflection of primary cosmic rays in the Earth's magnetic field, is observed for the first time in the flux of atmospheric neutrinos. Using a 45 kt yr exposure of the Super-Kamiokande detector, 552 $e$-like and 633 $\ensuremath{\mu}$-like horizontally going events are selected in the momentum range between 400 and $3000\mathrm{MeV}/c$. The azimuthal distributions of $e$-like and $\ensuremath{\mu}$-like events agree with the expectation from atmospheric neutrino flux calculations, verifying that the flux of atmospheric neutrinos in the GeV energy range is reasonably well modeled by calculations that account for the geomagnetic field.