Kaon-Scatter Introduced Backgrounds in the KOTO Experiment

TL;DR

This paper investigates how kaon scattering backgrounds, especially from off-axis decays, affect the sensitivity of the KOTO experiment in searching for the rare $K_L ightarrow ext{pi}^0 uar{ u}$ decay.

Contribution

It introduces a detailed analysis of kaon scattering backgrounds using aluminum targets to improve background understanding in the KOTO experiment.

Findings

Kaon scattering can mimic the signal by producing large transverse momentum.

Beam profile measurements help identify background sources.

Understanding backgrounds improves the experiment's sensitivity.

Abstract

The KOTO experiment is a particle physics experiment located in J-PARC, Japan, aiming to explore physics beyond the Standard Model by measuring the branching ratio of the $K_L\rightarrow\pi^{0}\nu\bar{\nu}$ decay. This decay has not yet been observed. The branching ratio predicted by the Standard Model of $(3.0\pm0.3)\times10^{-11}$ and the current experimental upper limit established by KEK E391a is $2.6\times10^{-8}$. The signal of $K_L\rightarrow\pi^{0}\nu\bar{\nu}$ decay has the signature of two photons on the calorimeter with no signal on the veto detectors. It also has a large transverse momentum due to missing neutrinos. Kaons that decay outside the beam line with final product of two photons, such as $K_L\rightarrow\gamma\gamma$ and $K_L\rightarrow\pi^{+}\pi^{-}\pi^{0}$, can appear to have large transverse momentum due to kaon scattering and beam interaction with the detectors. These off-axis kaon decay events can impact the upper limit of $K_L\rightarrow\pi^{0}\nu\bar{\nu}$ branching ratio. Aluminum targets located at the upstream of the KOTO detector and inside the decay-volume were used to study kaon beam profile, which provided off-axis kaon decay vertex information. The beam profile provided insights on background contributions to the signal. Studies on the kaon beam profile and background identification from kaon scattering were presented in this talk.