Four-dimensional calorimeter to discriminate gammas from neutrons for the KOTO experiment

We study the rare decay K_L→π⁰νν-bar at the J-PARC KOTO experiment. This decay mode is highly suppressed in the Standard Model (SM). The signature of the decay is two gammas from a ${\pi }^0$ decay and no other detectable particles. In the KOTO experiment, the two gammas are measured with a calorimeter which is composed of undoped cesium iodide (CsI) crystals. Each crystal is read out with a photomultiplier tube (PMT) attached on its rear side. One of the main backgrounds to K_L→π⁰νν-bar is caused by neutrons in the beam halo. To reach the sensitivity predicted in the SM, we need to suppress the background further by a factor of ten. We developed a method to attach Multi Pixel Photon Counters (MPPCs) on the front end of the crystals. The depth of the interaction, measured with the timing difference between the MPPC signal from upstream and the PMT signal from downstream, is used to discriminate gammas from neutrons. We obtain four-dimensional information on the showers (timing and three-dimensional position) as well as their energies from the calorimeter. We installed MPPCs in the autumn of 2018 and took data during the beam time in 2019. Using the four-dimensional calorimeter, the background is suppressed by a factor of 45.