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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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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 $135~{rm MeV}/c^2$.
We performed a search for the decay $K_L^0 rightarrow 3gamma$ with the E391a detector at KEK. In the data accumulated in 2005, no event was observed in the signal region. Based on the assumption of $K_L^0 rightarrow 3gamma$ proceeding via parity-violation, we obtained the single event sensitivity to be $(3.23pm0.14)times10^{-8}$, and set an upper limit on the branching ratio to be $7.4times10^{-8}$ at the 90% confidence level. This is a factor of 3.2 improvement compared to the previous results. The results of $K_L^0 rightarrow 3gamma$ proceeding via parity-conservation were also presented in this paper.
The KTeV E799 experiment has conducted a search for the rare decay $K_{L}topi^{0}pi^{0}gamma$ via the topology $K_{L}topi^{0}pi^{0}_Dgamma$ (where $pi^0_Dtogamma e^+e^-$). Due to Bose statistics of the $pi^0$ pair and the real nature of the photon, the $K_{L}topi^{0}pi^{0}gamma$ decay is restricted to proceed at lowest order by the CP conserving direct emission (DE) of an E2 electric quadrupole photon. The rate of this decay is interesting theoretically since chiral perturbation theory predicts that this process vanishes at level $O(p^4)$. Therefore, this mode probes chiral perturbation theory at $O(p^6)$. In this paper we report a determination of an upper limit of $2.43times 10^{-7}$ (90% CL) for $K_{L}topi^{0}pi^{0}gamma$. This is approximately a factor of 20 lower than previous results.
We present measurements of D -> K0_S pi and D -> K0_L pi branching fractions using 281 pb-1 of psi(3770) data at the CLEO-c experiment. We find that B(D0 -> K0_S pi0) is larger than B(D0 -> K0_L pi0), with an asymmetry of R(D0) = 0.108 +- 0.025 +- 0.024. For B(D+ -> K0_S pi+) and B(D+ -> K0_L pi+), we observe no measurable difference; the asymmetry is R(D+) = 0.022 +- 0.016 +- 0.018. The D0 asymmetry is consistent with the value based on the U-spin prediction A(D0 -> K0 pi0)/A(D0 -> K0bar pi0) = -tan^2(theta_C), where theta_C is the Cabibbo angle.
Based on a sample of 1.31 billion $J/psi$ events collected with the BESIII detector, we perform a search for the rare decay $etarightarrow 4pi^{0}$ via $J/psirightarrowgammaeta$. No significant $eta$ signal is observed in the invariant mass spectrum of 4$pi^{0}$. With a Bayesian approach, the upper limit on the branching fraction of $etarightarrow 4pi^{0}$ is determined to be $mathcal{B}(etarightarrow 4pi^{0})$ $< 4.94times10^{-5}$ at the 90% confidence level, which is a factor of six smaller than the previous experimental limit.
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