The KEK to Kamioka long-baseline neutrino experiment (K2K) has begun its in
vestigation of neutrino oscillations suggested by atmospheric neutrino obse
rvations. Twenty-eight neutrino events have been detected in coincidence wi
th 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 3
7.8(-3.8)(+3.5), derived using measurements of neutrino interactions in a n
ear detector and extrapolation using a beam simulation validated by a measu
rement of pion kinematics after production and focusing. The background is
of order 10(-3) events. (C) 2001 Published by Elsevier Science B.V.