Halo Neutron Measurement for KOTO Experiment

The KL → π0νν decay is a rare CP-violating process and is a powerful probe to search for new physics beyond the Standard Model. The KOTO experiment studies for this decay using a high intensity KL beam line constructed at J-PARC. In this KL beam line, there are remaining neutrons, called ”halo neutrons”, and the π0’s produced by the interaction of halo neutrons with KOTO detectors become backgrounds. We developed a photon and neutron counter, called Neutron Collar Counter (NCC) to suppress and estimate the neutron background. NCC consists of 48 modules, which are made of undoped CsI crystals, divided into 3 parts along the beam direction and read out with wavelength shifting fibers. We established the clustering method to measure the flux and energy of the halo neutrons, distinguishing neutron events from other kaon-decay events using the difference of three-dimensional shower shapes. We analyzed the data taken in March 2013 to demonstrate the effectiveness of the neutron selection with the clustering method.