T. K. Komatsubara

Experiment E949 at Brookhaven National Laboratory has observed three new events consistent with the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ in the pion momentum region $140<{P}_{\ensuremath{\pi}}<199\text{ }\text{ }\mathrm{MeV}/c$ in an exposure of $1.71\ifmmode\times\else\texttimes\fi{}{10}^{12}$ stopped kaons with an estimated total background of $0.93\ifmmode\pm\else\textpm\fi{}0.17(\mathrm{stat}{)}_{\ensuremath{-}0.24}^{+0.32}(\mathrm{syst})$ events. This brings the total number of observed ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ events to seven. Combining this observation with previous results, assuming the pion spectrum predicted by the standard model, results in a branching ratio of $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})=({1.73}_{\ensuremath{-}1.05}^{+1.15})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$. An interpretation of the results for alternative models of the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}+nothing$ is also presented.

Three events for the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ have been observed in the pion momentum region below the ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{0}$ peak, $140<{P}_{\ensuremath{\pi}}<199\text{ }\text{ }\mathrm{MeV}/c$, with an estimated background of $0.93\ifmmode\pm\else\textpm\fi{}0.17(\mathrm{stat}.{)}_{\ensuremath{-}0.24}^{+0.32}(\mathrm{syst}.)$ events. Combining this observation with previously reported results yields a branching ratio of $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})=({1.73}_{\ensuremath{-}1.05}^{+1.15})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ consistent with the standard model prediction.

A search for the rare decay ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ was performed. With the data collected in 2015, corresponding to $2.2\ifmmode\times\else\texttimes\fi{}{10}^{19}$ protons on target, a single event sensitivity of $(1.30\ifmmode\pm\else\textpm\fi{}0.0{1}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.1{4}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ was achieved and no candidate events were observed. We set an upper limit of $3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ for the branching fraction of ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{X}^{0}$ was also set as $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ at the 90% C.L., where ${X}^{0}$ is an invisible boson with a mass of $135\text{ }\text{ }\mathrm{MeV}/{c}^{2}$.

An additional event near the upper kinematic limit for ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ has been observed by experiment E949 at Brookhaven National Laboratory. Combining previously reported and new data, the branching ratio is $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})=({1.47}_{\ensuremath{-}0.89}^{+1.30})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ based on three events observed in the pion momentum region $211<P<229\text{ }\mathrm{M}\mathrm{e}\mathrm{V}/c$. At the measured central value of the branching ratio, the additional event had a signal-to-background ratio of 0.9.