1 M. Yokoyama (Kyoto University) for K2K collaboration June 10 th, U. 2 K2K experiment since 1999 First accelerator-based long baseline (250km) neutrino experiment. 12GeV PS beamline Beam monitor Near detector 50 kton Water Cherenkov detector Search for disappearance and e appearance 250km Pure beam (99%) w/ ~1.3GeV 1. Introduction 3 Flavor mixing in lepton sector Flavor eigenstates mass eigenstates Parameters: 3 mixing angle ( 12, 23, 13 ) 1 complex phase ( ) S ij :sin ij, C ij ;cos ij e m1m1 m2m2 m3m3 4 Neutrino Oscillation Time evolution of neutrino Consider mixing b/w two flavors If there is neutrino oscillation m 2 0 m0! If m 2 m 3, is in mixed state after time evolution! Consider neutrino generated as pure cos sin 5 Super-Kamiokande atmospheric 1.3x10 -3 30 ) where one track is 25 A hint of K2K forward deficit. SciBar non-QE Events K2K observed forward deficit. A source is non-QE events. For CC-1, Suppression of ~q 2 /0.1[GeV 2 ] at q 2