Search for $K_L\to {\pi }^0\nu \overline{\nu }$ and $K_L\to {\pi }^0{X}^0$ Decays at the J-PARC KOTO Experiment

A search for the rare decay $K_L\to {\pi }^0\nu \overline{\nu }$ was performed. With the data collected in 2015, corresponding to $2.2\times {10}^{19}$ protons on target, a single event sensitivity of $(1.30\pm 0.01_{\mathrm{stat}}\pm 0.14_{\mathrm{syst}})\times {10}^{-9}$ was achieved and no candidate events were observed. We set an upper limit of $3.0\times {10}^{-9}$ for the branching fraction of $K_L\to {\pi }^0\nu \overline{\nu }$ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for $K_L\to {\pi }^0{X}^0$ was also set as $2.4\times {10}^{-9}$ at the 90% C.L., where $X^0$ is an invisible boson with a mass of $135\mathrm{MeV}/c^2$.

Figure 1

Cross-sectional view of the KOTO detector. The beam enters from the left. Detector components with their abbreviated names written in blue (in green and underlined) are photon (charged particle) veto counters. BPCV, newBHCV, and BHGC are new counters installed after 2013. BHCV and BHTS were not used in the analysis.

Figure 2

Four-photon invariant mass distribution of the $K_L\to 2{\pi }^0$ events after imposing all the cuts except for the $K_L$ mass cut. The bottom panel shows the ratio of data and MC (sum of $K_L\to 2{\pi }^0$ and $K_L\to 3{\pi }^0$) for each histogram bin.

Figure 3

Reconstructed ${\pi }^0$ transverse momentum ($P_t$) vs ${\pi }^0$ decay vertex position ($Z_{\mathrm{vtx}}$) plot of the events with all the cuts imposed. The region surrounded by red lines is the signal region. The black dots represent observed events, and the contour indicates the $K_L\to {\pi }^0\nu \overline{\nu }$ signal distribution derived from the MC simulation. The black italic (red regular) numbers indicate the numbers of observed (expected background) events for the regions inside the lines.

Figure 4

Upper limit at the 90% C.L. for the $K_L\to {\pi }^0{X}^0$ branching fraction as a function of the $X^0$ mass. For comparison, the limit for the $K_L\to {\pi }^0\nu \overline{\nu }$ decay is shown with the red line.