Evidence of Electron Neutrino Appearance in a Muon Neutrino Beam

Abstract

The T2K collaboration reports evidence for electron neutrino appearance at the atmospheric mass splitting, j m2 32j 2:4 10􀀀3 eV2. An excess of electron neutrino interactions over background is observed from a muon neutrino beam with a peak energy of 0:6 GeV at the Super-Kamiokande (SK) detector 295 km from the beam's origin. Signal and background predictions are constrained by data from near detectors located 280 m from the neutrino production target. We observe 11 electron neutrino candidate events at the SK detector when a background of 3:3 0:4(syst.) events 3 is expected. The background-only hypothesis is rejected with a p-value of 0.0009 (3.1 ), and a t assuming ! e oscillations with sin22 23=1, CP=0 and j m2 32j = 2:4 10􀀀3 eV2 yields sin22 13=0:088+0:049 􀀀0:039(stat.+syst.)