The KOTO experiment at J-PARC is dedicated to searching for the rare decay $K_L\rightarrow \pi^0\nu \bar{\nu}$. This
decay violates CP symmetry and is sensitive to new physics beyond the Standard Model (SM)
because its branching ratio is predicted to be 3 × 10−11 with a small theoretical uncertainty in
SM. One of main backgrounds is caused by a small contamination of charged kaons in the neutral
beam. We installed a new charged particle detector in the beam to reject the background events
by detecting charged kaons directly. This detector consists of a 0.2-mm-thick plastic scintillator
film and 12-𝜇m-thick aluminized mylar. The scintillation photons escaping from the scintillator
surface are reflected by the mylar and are detected with multiple photomultiplier tubes on the
sides. In the data taken in 2024, we conclude light yield is 18.5 photoelectrons and inefficiency is
$5×10^{−4}$ at the 0.4 MIP threshold. As a result, we can eliminate charged kaon background.