The KOTO experiment conducted at J-PARC was designed to observe the K→π0νν decay and measure its branching ratio. The current SM prediction for the branching ratio is 3.0 x 10-11 with a small theoretical uncertainty of about 2-3%. Studying this rare “golden” decay in quark flavor physics is an ideal candidate to search for physics beyond the standard model. The results of this measurement would determine the height of the CKM unitary triangle, which is related to the imaginary part (η) of the CKM matrix. The previous search performed by the KEK E391a collaboration set a limit on the branching ratio of 2.6 x 10-8.
As a result of the favorable outcome of pilot search (KEK E391), the K0TO experiment is intended to pursue the search with increased beam power, an upgraded data acquisition system, and improved detector hardware. The signature of the decay is a pair of photons from the π0 decay and no other detected particles. For the measurement of the energies and positions of the photons, K0TO uses a Cesium Iodide (CSI) electromagnetic calorimeter as the main detector, and hermetic veto counters to guarantee that there are no other detected particles.
The initial data was collected in 2013, followed by additional runs in 2015-2016 at beam powers of roughly 24 and 40 kW, respectively. In this talk, I will discuss the results of the first run with K0TO, upgrades and improvements to the detector, and present status.