Search for the rare decay of K$^{0}_{L} \rightarrow \pi^{0}\nu\bar{\nu}$ with the KOTO detector

Abstract

The goal of the J-PARC K0TO experiment is to observe the K0L → π0νν decay and measure its branching ratio. The prediction for the branching ratio from (SM) processes is 3.0 x 10−11 with a theoretical uncertainty of 2.5%. The previous experimental limit is 2.6 x 10−8, set by the KEK E391a collaboration [J. Ahn et al., Phys. Rev. D81, 072004 (2010)]. A comparison of experimentally obtained results with SM calculations permits a test of the quark flavor region and is an ideal candidate to search for physics beyond the SM (BSM). Despite of the success of the KEK E391a collaboration, it highlighted the need for further upgrades with the anticipated increase of beam power, and motivated the development of the K0TO detector. A characteristic of the process of interest is a pair of photons from the π0 decay and no detected particles. K0TO uses a Cesium Iodide (CsI) electromagnetic calorimeter as the main detector to measure the energies and positions of the two photons, and hermetic veto counters to guarantee that there is no other detectable particle. The first data was collected in spring 2013, and since then we have had four additional data runs in 2015−2016 at beam powers of roughly 24 and 39 kW, respectively. In this report, we present a description of the upgrades to the detector, results of the first search with K0TO, and the current status of the analysis with the aim to reach the sensitivity of the Grossman-Nir [Y. Grossman;..et al.: Adv. Ser. Direct. High Energy Phys.15:755-794, (1998)] bound for the larger 2015−2016 data.