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 (μ/e)DATA/(μ/e)MC was measured to be 0.66±0.06(stat.)±0.08(sys.). In addition, a strong distortion in the shape of the μ-like event zenith angle distribution was observed. The ratio of the number of upward to downward μ-like events was found to be 0.52+0.07−0.06(stat.)±0.01(sys.), with an expected value of 0.98±0.03(stat.)±0.02(sys.), while the same ratio for the e-like events was consistent with unity.