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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We thank the KEK and ICRR Directorates for their strong support and encouragement. K2K is made possible by the inventiveness and the diligent efforts of the KEK-PS machine and beam channel groups. We thankfully appreciate discussions on the statistical treatment with Dr. Louis Lyons. We gratefully acknowledge the cooperation of the Kamioka Mining and Smelting Company. This work has been supported by the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan and its grants for Scientific Research, the Japan Society for Promotion of Science, the U.S. Department of Energy, the Korea Research Foundation, the Korea Science and Engineering Foundation, the CHEP in Korea, and Polish KBN Grant No. 5P03B06531.