Measurement of the Charged Current $\nu_e$(and $\bar\nu_e$) Interaction Rate on Water with the T2K P0D Detector

T2K is a long baseline neutrino oscillation experiment with the primary goal of measuring neutrino oscillation parameters including CP violation phase, $\delta_{CP}$ using $\nu_\mu$ disappearance and $\nu_e$ appearance from $\nu_\mu$ beams. The largest irreducible background for the $\nu_e$ appearance measurement comes from intrinsic $\nu_e$ component in a $\nu_\mu$ beam. While T2K uses Super-Kamiokande water Cherenkov detector as its far detector, there has been no precise measurement of $\nu_e$ interaction on water. Thus, the interaction rate of $\nu_e$ on water is constrained by the measured rate of $\nu_\mu$ on carbon. Consequently, in current neutrino oscillation analysis, one of the major systematic uncertainties originates from cross-section model uncertainties for C versus for O. To measure $\delta_{CP}$, measurements of both $\nu_e$ appearance and $\bar\nu_e$ appearance are needed. So, measurements of both interactions rate of $\nu_e$ and $\bar\nu_e$ on water are important. The design of P0D, a component of near detectors in T2K, which includes fillable water targets, allows us to measure on-water interaction rate and cross-section. I will present the measurement of the charged current $\nu_e$ interaction rate on water using P0D and a preliminary result with $\bar\nu_e$.