Study of the atmospheric neutrino flux in the multi-GeV energy range

The flavor ratio of the atmospheric neutrino flux and its zenith angle dependence have been studied in the multi-GeV energy range using an exposure of 25.5 kiloton-years of the Super-Kamiokande detector. By comparing the data to a detailed Monte Carlo simulation, the ratio $(\mu/e)_{DATA}/(\mu/e)_{MC}$ was measured to be $0.66\pm0.06(stat.)\pm0.08(sys.)$. In addition, a strong distortion in the shape of the $\mu$-like event zenith angle distribution was observed. The ratio of the number of upward to downward $\mu$-like events was found to be 0.52$^{+0.07}_{-0.06}(stat.)\pm0.01(sys.)$, with an expected value of $0.98\pm0.03(stat.)\pm0.02(sys.)$, while the same ratio for the $e$-like events was consistent with unity.