The T2K off-axis near detector, ND280, is used to make the first differential cross section measurements of muon neutrino charged current single positive pion production on a water target at energies ${sim}0.8$ GeV. The differential measurements are presented as a function of muon and pion kinematics, in the restricted phase-space defined by $p_{pi^+}>200$MeV/c, $p_{mu^-}>200$MeV/c, $cos theta_{pi^+}>0.3$ and $cos theta_{mu^-}>0.3$. The total flux integrated $ u_mu$ charged current single positive pion production cross section on water in the restricted phase-space is measured to be $langlesigmarangle_phi=4.25pm0.48 (mathrm{stat})pm1.56 (mathrm{syst})times10^{-40} mathrm{cm}^{2}/mathrm{nucleon}$. The total cross section is consistent with the NEUT prediction ($5.03times10^{-40} mathrm{cm}^{2}/mathrm{nucleon}$) and 2$sigma$ lower than the GENIE prediction ($7.68times10^{-40} mathrm{cm}^{2}/mathrm{nucleon}$). The differential cross sections are in good agreement with the NEUT generator. The GENIE simulation reproduces well the shapes of the distributions, but over-estimates the overall cross section normalization.
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