Measurement of K^+ \to \pi^0 \mu^+ \nu \gamma decay using stopped kaons

The K^+ \to \pi^0 \mu^+ \nu \gamma ($K_{\mu 3 \gamma}$) decay has been measured with stopped positive kaons at the KEK 12 GeV proton synchrotron. A $K_{\mu 3 \gamma}$ sample containing 125 events was obtained. The partial branching ratio $Br(K_{\mu 3 \gamma}, E_{\gamma}>30 {\rm MeV}, \theta_{\mu^+ \gamma}>20^{\circ})$ was found to be $[2.4 \pm 0.5(stat) \pm 0.6(syst)]\times 10^{-5}$, which is in good agreement with theoretical predictions.Comment: 12 pages, 3 figures, to be published in Physics Letters

Measurement of direct photon emission in K+→π+π0γK^+ \to \pi^+ \pi^0 \gamma decay using stopped positive kaons

The radiative decay $K^+ \to \pi^+ \pi^0 \gamma$ ($K_{\pi 2 \gamma}$) has been measured with stopped positive kaons. A $K_{\pi 2 \gamma}$ sample containing 4k events was analyzed, and the $K_{\pi 2 \gamma}$ branching ratio of the direct photon emission process was determined to be $[6.1\pm2.5({\rm stat})\pm1.9({\rm syst})]\times 10^{-6}$. No interference pattern with internal bremsstrahlung was observed.Comment: 12 pages, 6 figures, 2 tables, to be published in Phys. Lett.

Apparatus for a Search for T-violating Muon Polarization in Stopped-Kaon Decays

The detector built at KEK to search for T-violating transverse muon polarization in K+ --> pi0 mu+ nu (Kmu3) decay of stopped kaons is described. Sensitivity to the transverse polarization component is obtained from reconstruction of the decay plane by tracking the mu+ through a toroidal spectrometer and detecting the pi0 in a segmented CsI(Tl) photon calorimeter. The muon polarization was obtained from the decay positron asymmetry of muons stopped in a polarimeter. The detector included features which minimized systematic errors while maintaining high acceptance.Comment: 56 pages, 30 figures, submitted to NI

Search for the decay K+ to pi+ gamma gamma in the pi+ momentum region P>213 MeV/c

We have searched for the K+ to pi+ gamma gamma decay in the kinematic region with pi+ momentum close to the end point. No events were observed, and the 90% confidence-level upper limit on the partial branching ratio was obtained, B(K+ to pi+ gamma gamma, P>213 MeV/c) < 8.3 x 10-9 under the assumption of chiral perturbation theory including next-to-leading order ``unitarity'' corrections. The same data were used to determine an upper limit on the K+ to pi+ gamma branching ratio of 2.3 x 10-9 at the 90% confidence level.Comment: 15 pages, 3 figures; no change in the results, accepted for publication in Physics Letters

Measurement of Γ(Kμ3)/Γ(Ke3)\Gamma(K_{\mu 3})/\Gamma(K_{e3}) ratio using stopped positive kaons

The ratio of the $K^{+}\to \pi^{0} \mu^{+} \nu$ ($K_{\mu3}^+$) and $K^{+}\to \pi^{0} e^{+} \nu$ ($K_{e3}^+$) decay widths, $\Gamma(K_{\mu 3})/\Gamma(K_{e3})$, has been measured with stopped positive kaons. $K_{\mu3}^+$ and $K_{e3}^+$ samples containing 2.4$\times 10^4$ and 4.0$\times 10^4$ events, respectively, were analyzed. The $\Gamma(K_{\mu3})/\Gamma(K_{e3})$ ratio was obtained to be 0.671$\pm$0.007(stat.)$\pm$0.008(syst.) calculating the detector acceptance by a Monte Carlo simulation with the assumption of $\mu$-$e$ universality in $K_{l3}^+$ decay. The coefficient of the $q^2$ dependent term of the $f_0$ form factor was also determined to be $\lambda_0$=0.022$\pm$0.005(stat.)$\pm$0.004(syst.).Comment: 12 pages, 6 figure

Development of electronics and data acquisition system for the J-PARC T59 (WAGASCI) experiment

International audienceWAGASCI is a new experiment at J-PARC neutrino beamline, to measure the cross section ratio of charged current neutrino interaction on nucleus between water target and plastic target with the accuracy of a few percent. The detector adopts three-dimensional grid structure of 3-mm- thick scintillator bars around water and plastic targets, to obtain large angular acceptance. As a photodetector a 32-channel arrayed MPPC has been developed for the WAGASCI detector. The WAGASCI detector has 1280 MPPC channels in total. Our front-end electronics is SPIROC2D, which is an auto-triggered, bi-gain, 36-channel ASIC, allowing to measure the charge from one to 2000 photoelectron and the time with 100ps step. It contains a 16-deep analog memory array, which allows to store 16 hits in an acquisition gate. The back-end boards control the data output from the front-end ASIC and reception of trigger signals for neutrino beam. The module con- struction has been completed and the data acquisition system has almost been completed, to be ready for the neutrino beam measurement from October 2017