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Abstract
The KOTO experiment at J-PARC aims to study CP-symmetry breaking in kaon decays. In particular, KOTO searches for the rare decay, KL → π0νν, which is sometimes called the "Golden Mode". The mode is direct CP-violating, and with a small theoretical uncertainty, this search is highly sensitive to new physics. During the analysis of 2021 data, no events were observed in the signal region, which allowed KOTO to set the best upper limit on BR(KL → π0νν) to date at < 2.01 × 10−9 (90% C.L.). This dissertation will include details on the analysis of KL → π0νν in 2021 data. Due to the high sensitivity required to study the KL → π0νν decay, KOTO is in a unique position to study rare KL interactions, such as sub-GeV quark coupling to dark matter. One avenue to study this is the mode KL → π0π0X, where X → γγ. This mode was studied in the E391a experiment at KEK in the X mass region 194.3-219.3 MeV, with the best upper limit on the branching ratio set with an X mass of 214.3 MeV at < 2.4 × 10−7 [48]. In the analysis of 2021 data, the scope is increased to include X mass in the range 160-220 MeV, and the first search for the rare decay KL → π0π0γγ which is described by ChPT [24]. Two events were observed in the signal region near an X mass of 177 MeV, and no events were observed for the analysis of KL → π0π0γγ, setting a world first upper limit of BR(KL → π0π0γγ) < 9.77 × 10−7 at 90% C.L.