Abstract
The KOTO experiment at the J-PARC laboratory seeks to obtain the first observation of the decay. The branching ratio is calculated in the Standard Model (SM) to be 3 × 10−11. This is a good probe to explore new physics beyond the SM with small theoretical uncertainty of ∼2%. We installed a new barrel-shaped photon veto detector, named the Inner Barrel, inside the KOTO detector to improve detection efficiency of photons from the major background of KL → 2π0 decay. The Inner Barrel detector is a sampling calorimeter, consisting of 25 layers of 5-mm-thick scintillators and 24 layers of 1-mm-thick lead plates, corresponding to 5 radiation lengths. The volume is 3 m long along the beam direction, and inner and outer diameters are 1.5 m and 1.9 m, respectively. Scintillation light is read out by a photomultiplier at both ends via wavelength shifting fibers. The Inner Barrel was installed in April, 2016 and the performance was demonstrated at the beam time in June. In this paper, the detector design, construction and performance evaluated with the neutral beam are presented.
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