Measurement of the Electron Neutrino Charged-current Interaction Rate on Water with the T2K ND280 pi-zero Detector
The first direct observation of the appearance of electron neutrinos in a muon neutrino beam through neutrino oscillation was recently reported by the T2K experiment. The main background in this observation was the presence of the electron neutrino component of the beam, which accounts for 1.2% of the beam below the 1.2 GeV threshold. This paper presents a measurement of the charged current interaction rate of the electron neutrino beam component using the large fiducial mass of the T2K π0 detector. The measured ratio of the observed beam interaction rate to the predicted rate in the detector with water targets filled is 0.89 ±0.08 (stat.) ±0.11 (sys.), and with the water targets emptied is 0.90 ±0.09 (stat.) ±0.13 (sys.). The ratio obtained for the interactions only on-water only an event subtraction method is 0.87 ±0.33 (stat.) ±0.21 (sys.). These are pioneering measurements of the νe interaction rate, particularly on water, which will become crucial in future CP-violation searches in the lepton sector.