Search for the Rare Decay of the Neutral Kaon, KL-->?0??

Abstract

The KOTO experiment located at the J-PARC research facility in Tokai, Japan aims to to measure the branching ratio (BR) of the rare decay of the neutral kaon, KL-->pi0nunubar. This decay has a Standard Model (SM) predicted BR of (3.00 +/- 0.30) x 10^(-11). While it is highly suppressed, it is an excellent probe to search for new physics beyond the SM as it directly violates charge-parity (CP) and has small theoretical uncertainties.

This thesis performs an analysis with data collected in 2016-2018 and reports on the KL-->pi0nunubar search. KL-->pi0nunubar events were identified by detecting two photons from the pi0 decay with a large transverse momentum and no other particles present. The data was analyzed using a blind analysis technique and Monte Carlo simulations to ensure the reproducibility of the data. The analysis included event reconstruction and selection, normalization, background estimations, and the estimation of systematic uncertainties.

The Single Event Sensitivity (SES) achieved with the 2016-2018 data was (7.20 +/- 0.05_stat +/- 0.66_syst) x 10^(-10) which is the highest sensitivity level ever reached for the KL-->pi0nunubar search. The expected background in the signal region was estimated to be 0.05 +/- 0.02. After unblinding the data, three events were observed in the signal region and two new backgrounds from K+ decays and scattered KLs were considered. Consequently, the background estimation in the signal region was updated to be 1.22 +/- 0.26. Based on these results, an upper limit on the branching ratio of KL-->pi0nunubar was set to be 4.9 x 10^(-9) at the 90% Confidence Level.