Reduction of charged kaon background in the KOTO experiment

Abstract

The KOTO experiment at J-PARC searches for the rare kaon decay . In the analysis of the data collected from 2016 to 2018, the largest background was caused by decays of K ±. To detect K ± in the beam line, we developed an in-beam charged particle detector in 2020. After installing it in the KOTO detector system, we collected physics data in 2021. Using a control sample of K ± events, we estimated the inefficiency to be , which reduces the K ± background by a factor of 13. The number of K ± background events was suppressed to be at the single event sensitivity of 7.9 × 10−10.