Abstract
Abstract
We are searching for the decay
K
L
→
π
0
ν
ν
¯
in the KOTO experiment at J-PARC. The signal is identified by detecting two photons from the π
0 decay with a calorimeter composed of undoped CsI crystals. The main background “hadron cluster background” is caused by a neutron hitting the calorimeter to form two clusters: a neutron in the beam halo hits the calorimeter to generate the first cluster, and a secondary neutron generated in the interaction creates the second separated cluster. In order to reduce this background, we upgraded the calorimeter to have both-end readout by attaching MPPCs on the upstream face of each CsI crystal in addition to the original PMT attached on the downstream face. The background can be rejected exploiting the timing difference between the MPPC and PMT, since neutrons tend to interact deeper inside the crystals. We installed the MPPCs in 2018, and evaluated the performance with data taken in 2019. The principle in the calorimeter upgrade and the performance of the background reduction are reported.
Subject
General Physics and Astronomy