Scattered Kaon Study in the KOTO Experiment at J-PARC

Abstract

The KOTO experiment at J-PARC aims to explore physics beyond the Standard Model (SM) by measuring the branching ratio of the $K_L\to\pi^0\nu\bar{\nu}$ decay, predicted by the SM to be $(3.0\pm0.3)\times10^{-11}$ and is characterized by a pair of photons from the $\pi^0$ decay and no other detected particles. The decay is identified by observing two photons with a large transverse momentum on an undoped Cesium Iodide (CsI) electromagnetic calorimeter and no other signals on the hermetic veto counters around the decay region.\\ Kaons which decay outside the beam line with two photons in the final state, such as $K_L\to\gamma\gamma$ and $K_L\to\pi^+\pi^-\pi^0$, can appear to have a large transverse momentum due to kaon scattering and beam interaction with the detector components. These off-axis kaon decay events can impact the measurement of the $K_L\to\pi^0\nu\bar{\nu}$ branching ratio. An Aluminum target inserted at the upstream of the KOTO detector was used to study the kaon beam profile, which provided the vertex information of off-axis kaon decays. The beam profile provided insights on background contributions to the signal. Studies on the kaon beam profile and background estimation from kaon scattering is presented in this paper.