Upgrade of In-Beam Charged Particle Detector for the KOTO Experiment

Abstract

The KOTO experiment aims at measuring the branching ratio of the decay. Since the decay is highly suppressed in the Standard Model, it is sensitive to new physics beyond the standard model. In the KOTO experiment, two photons from the π0 in the final state are detected by an electromagnetic calorimeter, and the absence of extra particles except for the escaping neutrinos is ensured by the surrounding veto counters. As one of the veto counters, an in-beam charged particle detector, composed of 3-mm thick plastic scintillators, covered the beam hole of the calorimeter. It was operating in a high-rate environment with a single counting rate of 10 MHz, which caused 10% loss to the signal. To reduce the hits by neutral particles, the detector has been upgraded to a thin-gap wire chamber. Keeping a high detection efficiency over 99.6% for penetrating charged particles, a 65% reduction of the single counting rate was achieved, corresponding to 40% recovery of acceptance for .