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

TL;DR

This paper reports a precise measurement of the long-lived neutral-kaon flux in the KOTO experiment, crucial for studying rare CP-violating decays, using multiple decay modes and Monte Carlo simulations to ensure accuracy.

Contribution

It presents the first detailed measurement of the $K_L$ flux for the KOTO experiment with systematic uncertainties estimated at 1.4%, using multiple decay modes and simulation validation.

Findings

Measured $K_L$ flux as (4.183 ± 0.017 (stat.) ± 0.059 (sys.)) × 10^7 per 2×10^14 protons.

Achieved agreement between Monte Carlo simulation and experimental data.

Estimated systematic uncertainty at 1.4% level.

Abstract

The KOTO ($K^0$ at Tokai) experiment aims to observe the CP-violating rare decay $K_L \rightarrow \pi^0 \nu \bar{\nu}$ 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, $K_L \rightarrow 3\pi^0$, $K_L \rightarrow 2\pi^0$, and $K_L \rightarrow 2\gamma$ 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 $(4.183 \pm 0.017_{\mathrm{stat.}} \pm 0.059_{\mathrm{sys.}}) \times 10^7$ $K_L$ per $2\times 10^{14}$ protons on a 66-mm-long Au target.