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Register a new userStudy of the $K_L !to! pi^0 u overline{ u}$ Decay at the J-PARC KOTO Experiment
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The rare decay $K_L !to! pi^0 u overline{ u}$ was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of $( 7.20 pm 0.05_{rm stat} pm 0.66_{rm syst} ) times 10^{-10}$, three candidate events were observed in the signal region. After unveiling them, contaminations from $K^{pm}$ and scattered $K_L$ decays were studied, and the total number of background events was estimated to be $1.22 pm 0.26$. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of $4.9 times 10^{-9}$ on the branching fraction of $K_L !to! pi^0 u overline{ u}$ at the 90% confidence level.
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A search for the rare decay $K_L !to! pi^0 u overline{ u}$ was performed. With the data collected in 2015, corresponding to $2.2 times 10^{19}$ protons on target, a single event sensitivity of $( 1.30 pm 0.01_{rm stat} pm 0.14_{rm syst} ) times 10^{-9}$ was achieved and no candidate events were observed. We set an upper limit of $3.0 times 10^{-9}$ for the branching fraction of $K_L !to! pi^0 u overline{ u}$ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for $K_L !to! pi^0 X^0$ was also set as $2.4 times 10^{-9}$ at the 90% C.L., where $X^0$ is an invisible boson with a mass of $135~{rm MeV}/c^2$.
A charged-particle veto detector was constructed to study the rare decay $K_{L} rightarrow pi^{0} u bar{ u}$ in the J-PARC K$^{mathrm{O}}$TO experiment. This detector consists of 3mm-thick plastic scintillator strips and wavelength shifting fibers coupled with MPPCs at the both ends, which makes it possible to obtain large light output over the wide region. After the construction and installation to the K$^{mathrm{O}}$TO experimental area, its performance was studied using charged particles from $K_{mathrm{L}}$ decays. As a preliminary result, we found that the detector had the light yield larger than 10p.e./100keV and the timing resolution better than 3ns for most regions, which satisfied the requirements to achieve the standard model sensitivety($sim10^{-11}$) for the $K_{L} rightarrow pi^{0} u bar{ u}$ detection.
An upper limit on the branching ratio for the decay $K^+ ! rightarrow ! pi^+ u overline{ u}$ is set at $2.4 times 10^{-9}$ at the 90% C.L. using pions in the kinematic region $214~{rm MeV}/c < P_pi < 231~{rm MeV}/c$. An upper limit of $5.2 times 10^{-10}$ is found on the branching ratio for decays $K^+ ! rightarrow ! pi^+ X^0$, where $X^0$ is any massless, weakly interacting neutral particle. Limits are also set for cases where $M_{X^0}>0$.
The KOTO experiment recently reported four candidate events in the signal region of $K_Lto pi^0 ubar u$ search, where the standard model only expects $0.10pm 0.02$ events. If confirmed, this requires physics beyond the standard model to enhance the signal. We examine various new physics interpretations of the result including these: (1) heavy new physics boosting the standard model signal, (2) reinterpretation of $ ubar{ u}$ as a new light long-lived particle, or (3) reinterpretation of the whole signal as the production of a new light long-lived particle at the fixed target. We study the above explanations in the context of a generalized new physics Grossman-Nir bound coming from the $K^+ to pi^+ ubar{ u}$ decay, bounded by data from the E949 and the NA62 experiments.
We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} to pi^0 u overline{ u}$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34pm0.16$ background events were expected. We set an upper limit of $5.1times10^{-8}$ for the branching fraction at the 90% confidence level (C.L.). An upper limit of $3.7times10^{-8}$ at the 90% C.L. for the $K_{L} to pi^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an invisible particle with a mass of 135 MeV/$c^{2}$.
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