GPS survey in long baseline neutrino-oscillation measurement

We made a series of surveys to obtain neutrino beam line direction toward SuperKamiokande (SK) at a distance of 250 km for the long-baseline neutrino oscillation experiment at KEK. We found that the beam line is directed to SK within 0.03 mr and 0.09 mr (in sigma) in the horizontal and vertical directions, respectively. During beam operation, we monitored the muon distribution from secondary pions produced at the target and collected by a magnetic horn system. We found that the horn system functions like a lens of a point-to-parallel optics with magnification of a proximately - 100 and the focal length of 2.3 m. Namely, a small displacement of the primary beam position at the target is magnified about a factor - 100 at the muon centroid, while the centroid position is almost stable against a change of the incident angle of the primary beam. Therefore, the muon centroid can be a useful monitor of the neutrino beam direction. We could determine the muon centroid within 6 nun and 12 mm in horizontal and vertical directions, respectively. This means that the neutrino beam direction could be controlled within 0.03 mr and 0.06 mr (in sigma) in the horizontal and vertical directions, respectively. We confirmed the beam direction with the neutrino distribution reconstructed at the near detector in KEK.