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

Abstract 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.