Hadron shower background in KOTO 2019 - 2021 data

Abstract

One of the keys for the search of KOTO experiment is to suppress the backgrounds caused by hadron showers in CsI calorimeter. We developed three methods to discriminate the hadron showers from gamma showers. The methods use 1) pulse shape of signals, 2) shower shape on the calorimeter, and 3) depth of the showers in the calorimeter. We report on the discrimination power of these methods. The performance of the third method remained the same even if the dark current of silicon photomultipliers increased by a factor of 85 due to irradiation.