Long-lived neutral-kaon flux measurement for the KOTO experiment

Abstract

The KOTO (K at Tokai) experiment aims to observe the CP-violating rare decay KL → π→→ 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 → 3p 0, KL → 2π, and KL → 2→, 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.) × 107 KL per 2 × 1014 protons on a 66-mm-long Au target.