Detection of accelerator-produced neutrinos at a distance of 250 km

Abstract The KEK to Kamioka long-baseline neutrino experiment (K2K) has begun its investigation of neutrino oscillations suggested by atmospheric neutrino observations. Twenty-eight neutrino events have been detected in coincidence with the expected arrival time of the beam in the 22.5 kt fiducial volume of Super-Kamiokande, the far detector at 250 km distance. The expectation is 37.8 +3.5 −3.8 , derived using measurements of neutrino interactions in a near detector and extrapolation using a beam simulation validated by a measurement of pion kinematics after production and focusing. The background is of order 10 −3 events.