Search for $K_{L}^{0}→ π^{0}ν\overline{ν}$ at KOTO

In the Standard Model, the rare Kaon decay $K_{L}^{0}→ π^{0}ν\overline{ν}$ is identified as an ideal probe for new physics. This dissertation details the search for $K_{L}^{0} → π^{0}ν\overline{ν}$ decay, utilizing data collected from 2016 to 2018 and emphasizing the data acquisition (DAQ) system enhancements for the KOTO experiment. To improve data collection efficiency, we redesigned the clock distribution and trigger systems. We implemented new algorithms to manage pedestal data and cluster counts on the calorimeter, achieving a system dead time of 160 ns. This resulted in a signal loss of less than 1% and a systematic uncertainty also under 1%.

A blind analysis approach was used to ensure objectivity in data evaluation. All known background sources from previous studies were meticulously excluded. Upon examining the blind region, three events met all selection criteria for $K_{L}^{0}→ π^{0}ν\overline{ν}$ decay. These were reassessed with an estimated background level of $1.22 ± 0.26$, and the single event sensitivity (SES) was calculated to be $(7.19 ± 0.05_{stat} ± 0.62_{syst}) × 10^{−10}$.

Ultimately, statistical analysis confirmed that these three events are consistent with the background prediction, indicating that they are unlikely to be $K_{L}^{0}→ π^{0}ν\overline{ν}$ decay events. Consequently, an upper limit of $4.9 × 10^{−9}$ for the branching ratio of $K_{L}^{0}→ π^{0}ν\overline{ν}$ decay has been set at 90% confidence level (C.L.)