A new charged particle detector for the KOTO experiment at J-PARC

We report the performance of a new scintillator array Downstream Charged Veto (DCV) for the J-PARC KOTO experiment to suppress the K L → π + π − π 0 decay background. Since the background originates from undetected charged pions passing through the beam hole of the electromagnetic calorimeter, the DCV detector was installed in vacuum downstream of the calorimeter. The DCV is composed of two plastic scintillator pipes read out by Multi Pixel Photon Counters through wavelength shifting fibers. The light yield was found to be 60 photoelectrons/0.8 MeV in a test bench when cosmic-rays pass through the center of the DCV. Energy calibration was performed using cosmic-rays after installation.