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تسجيل مستخدم جديدStudy 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 c
oupled 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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