The search for electron neutrino appearance from accelerator-produced muon neutrinos in the K2K 250 km long-baseline neutrino experiment was described. It was observed that one electron event was found in the data corresponding to an exposure of 4.8 × 1019 protons on target. The number of expected background neutrino events in the absence of neutrino oscillations was found to be 2.4±0.6 events and was found to be dominated by misidentification of events from neutral current production. The probability density function of the expected number of electron events was evaluated from the the Poisson distribution, which was convoluted with systematic uncertainty.
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