GPS survey in long baseline neutrino-oscillation measurement

We made a series of surveys to fund the neutrino beam line direction toward SuperKamiokande (SK) at a distance of 250 km for the long-baseline neutrino oscillation experiment at KEK. We found the beam line is directed to SK within 0.03 mr and 0.09 mr (in sigma) in horizontal and vertical directions, respectively. During the beam operation, we monitored the muon distribution from secondary pions produced at the target and collected by the magnetic horn system. We could determine the muon centroid within 6mm and 12mm 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 horizontal and vertical directions, respectively. We found the muon centroid is sensitively moved with respect to the primary beam position at the target, while the centroid position is almost stable against the change of the incident angle of the primary beam. This response can be understood that the horn system realizes a point-to-parallel ion optical characteristics with the magnification of approximately -100. We confirmed the beam direction with the neutrino distribution reconstructed at the near detector in KEK.