New limit on the $K_L^0\to {\pi }^0\nu \overline{\nu }$ decay rate

The first dedicated experiment for the rare kaon decay $K_L^0\to {\pi }^0\nu \overline{\nu }$ has been performed by the E391a Collaboration at the KEK 12-GeV proton synchrotron. A new upper limit of $2.1\times {10}^{-7}$ at the 90% confidence level was set for the branching ratio of the decay $K_L^0\to {\pi }^0\nu \overline{\nu }$ using about 10% of the data collected during the first period of data taking.

Figure 1

Cross section of the E391a detector. $K_L^0$’s enter from the left side.

Figure 2

Distribution of the invariant mass of four photons, $M_{4\gamma }$, with the cuts on the photon veto detectors and the shower shape of photons in the CsI calorimeter. The dots show data, the open solid histogram shows total MC, the closed solid histogram shows $K_L^0\to {\pi }^0{\pi }^0$ MC and the hatched histogram shows $K_L^0\to {\pi }^0{\pi }^0{\pi }^0$ MC.

Figure 3

$Z_{\mathrm{vtx}}$ versus $P_T$ with all the event selection cuts. The number of observed (total expected background) events are shown. The expected number of background events was consistent with the observed number of events for all the regions.

Figure 4

Distribution of the invariant mass (left) and the decay vertex (right) for the $K_L^0\to {\pi }^0{\pi }^0$ decays. In the top plot, the dots show the data and the histogram shows the MC. The bottom plot shows the ratio of the data to the MC.