A search for the rare decay K L → π 0 νν was performed. With the data collected in 2015, corresponding to 2.2 × 10 19 protons on target, a single event sensitivity of ð1.30 AE 0.01 stat AE 0.14 syst Þ × 10 −9 was achieved and no candidate events were observed. We set an upper limit of 3.0 × 10 −9 for the branching fraction of K L → π 0 νν at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for K L → π 0 X 0 was also set as 2.4 × 10 −9 at the 90% C.L., where X 0 is an invisible boson with a mass of 135 MeV=c 2 .

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The KOTO (K 0 at Tokai) experiment aims to observe the CP-violating rare decay K L → π 0 ν ν by using a long-lived neutral-kaon beam produced by the 30 GeV proton beam at the Japan Proton Accelerator Research Complex. The K L flux is an essential parameter for the measurement of the branching fraction. Three K L neutral decay modes, K L → 3π 0 , K L → 2π 0 , and K L → 2γ were used to measure the K L flux in the beam line in the 2013 KOTO engineering run. A Monte Carlo simulation was used to estimate the detector acceptance for these decays. Agreement was found between the simulation model and the experimental data, and the remaining systematic uncertainty was estimated at the 1.4% level. The K L flux was measured as (4.183 ± 0.017 stat. ± 0.059 sys. ) × 10 7 K L per 2 × 10 14 protons on a 66-mm-long Au target.

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