Search for the Decay ${K}_{L}\to {\pi }^{0}\nu \bar{\nu }$ at the J-PARC KOTO Experiment

Abstract

The KL →π0νν is a CP-violating decay process of the neutral K meson with a longer lifetime, KL. It is an excellent probe to search for new physics beyond the Standard Model (SM), because new physics can contribute to the process and this decay is highly suppressed in the SM. The KOTO experiment is designed to search for the KL→π0νν decay. In KOTO, we identify the KL→π0νν signal by detecting two photons from a π0 without any extra visible particles. The KOTO experiment is also sensitive to the KL→π0X0 decay, where X0 is an invisible light boson, because it has the same final state as the KL→π0νν decay. In this thesis, we report the results of a search for the KL→π0νν and KL→π0X0 decays based on the data collected in 2015. We learned from the data taken in 2013 that the background caused by a neutron hitting an electromagnetic calorimeter, which is called “hadron-cluster background”, is crucial. We collected a control sample for the hadron-cluster background in 2015. We developed new analysis methods to separate hadronic showers from electromagnetic showers. We found a new background source caused by an η meson production close to the calorimeter. A new analysis method was developed to eliminate this background. Finally, we achieved a single event sensitivity of (1.30±0.01stat±0.14syst)×10−9 for the KL→π0νν decay and no signal candidate events were observed. We set an upper limit of 3.0 × 10−9 for the branching fraction of the KL→π0νν decay at the 90% confidence level, which improves the previous limit by an order of magnitude. We also set an upper limit for the branching fraction of the KL→π0X0 decay to be 2.4 × 10−9 at the 90% confidence level with the X0 mass of 135 MeV/c2.