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Register a new userNew physics implications of recent search for $K_L to pi^0 ubar{ u}$ at KOTO
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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.
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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 $135~{rm MeV}/c^2$.
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.
The rare pion decays ${pi}^+{rightarrow}{mu}^+{ u}_{mu}{ u}bar{ u}$ and ${pi}^+{rightarrow}e^+{ u}_{e}{ u}bar{ u}$ are allowed in the Standard Model but highly suppressed. These decays were searched for using data from the PIENU experiment. A first result for ${Gamma}({pi}^+{rightarrow}{mu}^+{ u}_{mu}{ u}bar{ u})/{Gamma}({pi}^+{rightarrow}{mu}^+{ u}_{mu})<8.6{times}10^{-6}$, and an improved measurement ${Gamma}({pi}^+{rightarrow}{e}^+{ u}_{e}{ u}bar{ u})/{Gamma}({pi}^+{rightarrow}{mu}^+{ u}_{mu})<1.6{times}10^{-7}$ were obtained.
We provide a comprehensive, up-to-date analysis of possible New Physics contributions to the mass difference $Delta M_D$ in $D^0$-${bar D}^0$ mixing. We consider the most general low energy effective Hamiltonian and include leading order QCD running of effective operators. We then explore an extensive list of possible New Physics models that can generate these operators, which we organize as including Extra Fermions, Extra Gauge Bosons, Extra Scalars, Extra Space Dimensions and Extra Symmetries. For each model we place restrictions on the allowed parameter space using the recent evidence for observation of $D$ meson mixing. In many scenarios, we find strong constraints that surpass those from other search techniques and provide an important test of flavor changing neutral currents in the up-quark sector. We also review the recent BaBar and Belle findings, and describe the current status of the Standard Model predictions of $D^0$-${bar D}^0$ mixing.
We report the first search for the $K_L to pi^0 gamma$ decay, which is forbidden by Lorentz invariance, using the data from 2016 to 2018 at the J-PARC KOTO experiment. With a single event sensitivity of $(7.1pm 0.3_{rm stat.} pm 1.6_{rm syst.})times 10^{-8}$, no candidate event was observed in the signal region. The upper limit on the branching fraction was set to be $1.7times 10^{-7}$ at the 90% confidence level.
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