Development of a low-mass and high-efficiency charged-particle detector

We have developed a low-mass and high-efficiency charged-particle detector for an experimental study of the rare decay <f>$K_L \\rightarrow \\pi ^0 \\nu \\bar {\\nu }$</f>. The detector is important for suppressing the background with charged particles to the level below the signal branching ratio predicted by the Standard Model <f>$\\left (O\\left (10^{-11}\\right )\\right )$</f>. The detector consists of two layers of 3 mm thick plastic scintillators with embedded wavelength-shifting fibers and multi-pixel photon counters for the readout. We manufactured the counter and evaluated the performance in terms of light yield, timing resolution, and efficiency. With this design, we achieved an inefficiency per layer against penetrating charged particles of less than <f>$1.5 \\times 10^{-5}$</f>, which satisfies the requirement of the KOTO experiment determined from simulation studies.