The KOTO (K0 at Tokai) experiment aims to observe the CP-violating rare
decay KL→π0ννˉ by using a long-lived neutral-kaon
beam produced by the 30 GeV proton beam at the Japan Proton Accelerator
Research Complex. The KL flux is an essential parameter for the measurement
of the branching fraction. Three KL neutral decay modes, KL→3π0, KL→2π0, and KL→2γ were used to
measure the KL 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 KL flux was measured as (4.183±0.017stat.±0.059sys.)×107KL per 2×1014 protons on a
66-mm-long Au target.Comment: 27 pages, 16 figures. To be appeared in Progress of Theoretical and
Experimental Physic