First Measurement of the Muon Neutrino Charged Current Single Pion Production Cross Section on Water with the T2K Near Detector

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 $\nu_\mu$ charged current single positive pion production cross section on water in the restricted phase-space is measured to be $\langle\sigma\rangle_\phi=4.25\pm0.48 (\mathrm{stat})\pm1.56 (\mathrm{syst})\times10^{-40} \mathrm{cm}^{2}/\mathrm{nucleon}$. The total cross section is consistent with the NEUT prediction ($5.03\times10^{-40} \mathrm{cm}^{2}/\mathrm{nucleon}$) and 2$\sigma$ lower than the GENIE prediction ($7.68\times10^{-40} \mathrm{cm}^{2}/\mathrm{nucleon}$). 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

Measurement of muon antineutrino oscillations with an accelerator-produced off-axis beam

T2K reports its first measurements of the parameters governing the disappearance of ν-μ in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic ν-μ beam, produced with a peak energy of 0.6 GeV at J-PARC, is observed at the far detector Super-Kamiokande, 295 km away, where the ν-μ survival probability is expected to be minimal. Using a data set corresponding to 4.01×1020 protons on target, 34 fully contained μ-like events were observed. The best-fit oscillation parameters are sin2(θ-23)=0.45 and |Δm-322|=2.51×10-3 eV2 with 68% confidence intervals of 0.38-0.64 and 2.26-2.80×10-3 eV2, respectively. These results are in agreement with existing antineutrino parameter measurements and also with the νμ disappearance parameters measured by T2K

Measurement of the single pi(0) production rate in neutral current neutrino interactions on water

The single π0 production rate in neutral current neutrino interactions on water in a neutrino beam with a peak neutrino energy of 0.6 GeV has been measured using the PØD, one of the subdetectors of the T2K near detector. The production rate was measured for data taking periods when the PØD contained water (2.64×10(20) protons-on-target) and also periods without water (3.49×10(20) protons-on-target). A measurement of the neutral current single π0 production rate on water is made using appropriate subtraction of the production rate with water in from the rate with water out of the target region. The subtraction analysis yields 106 ± 41 ± 69 signal events where the uncertainties are statistical (stat.) and systematic (sys.) respectively. This is consistent with the prediction of 157 events from the nominal simulation. The measured to expected ratio is 0.68 ± 0.26 (stat) ± 0.44 (sys) ± 0.12 (flux). The nominal simulation uses a flux integrated cross section of 7.63×10(−39)cm(2) per nucleon with an average neutrino interaction energy of 1.3 GeV

Search for proton decay via p -> mu(+) K-0 in 0.37 megaton-years exposure of Super-Kamiokande

We searched for proton decay via p→μ+K0 in 0.37 Mton·years of data collected between 1996 and 2018 from the Super-Kamiokande water Cherenkov experiment. The selection criteria were defined separately for KS0 and KL0 channels. No significant event excess has been observed. As a result of this analysis, which extends the previous search by an additional 0.2 Mton·years of exposure and uses an improved event reconstruction, we set a lower limit of 3.6×1033 years on the proton lifetime