Search for the rare decay of $K^{0}_L\rightarrow\pi^{0}\nu\bar{\nu}$ at J-PARC

Abstract

The KOTO experiment is dedicated to study the rare decay $K^0 _L\rightarrow \pi^0 \nu \bar\nu$ at the J-PARC 30 GeV Main Ring. The $K^0 _L\rightarrow \pi^0 \nu \bar\nu$ decay breaks the CP symmetry directly and is highly suppressed in the Standard Model (SM). Thus, this decay is sensitive to new physics beyond the SM, in particular new sources of CP violation. After the first physics run in 2013, the KOTO collaboration improved the detector in order to suppress the backgrounds newly found in the analysis. We resumed physics run in 2015 and collected about 20 times larger amount of data in the year. From the 2015 data analysis, no events were observed with the expected background of $0.4\pm0.18$. We set an upper limit of $3.0\times10^{-9}$ at the 90% confidence level on the branching ratio. The result improves the previous limit by an order of magnitude.