Observation of a $\bar{K}NN$ bound state in the ${}^3\mathrm{H}\mathrm{e}(K^{-},\mathrm{\Lambda }p)n$ reaction

We have performed an exclusive measurement of the $K^{-}+! ~^{3}{\rm He} \to \Lambda pn$reactionatanincidentkaonmomentumof$1\ {\rm GeV}/c$\mathrm{.}Inthe$\Lambda p$ invariant mass spectrum, a clear peak was observed below the mass threshold of $\bar{K}\text{\hspace{0.17em}⁣}+\text{\hspace{0.17em}⁣}N\text{\hspace{0.17em}⁣}+\text{\hspace{0.17em}⁣}N$, as a signal of the kaonic nuclear bound state, $\bar{K}NN$.The binding energy, decay width, and $S$-wave Gaussian reaction form-factor of this state were observed to be $B_{K} = 42\pm3({\rm stat.})^{+3}{-4}({\rm syst.})\ {\rm MeV}$,$\Gamma{K} = 100\pm7({\rm stat.})^{+19}{-9}({\rm syst.})\ {\rm MeV}$,and$Q{K} = 383\pm11({\rm stat.})^{+4}{-1}({\rm syst.})\ {\rm MeV}/c$, respectively. The total production cross-section of $\bar{K}NN$, determined by its $\mathrm{\Lambda }p$ decay mode, was $\sigma^{tot}{K} \cdot BR_{\Lambda p} = 9.3\pm0.8({\rm stat.})^{+1.4}_{-1.0}({\rm syst.})\ \mu{\rm b}$.We estimated the branching ratio of the $\bar{K}NN$ state to the $\mathrm{\Lambda }p$ and ${\mathrm{\Sigma }}^{0}p$ decay modes as $B{R}_{\mathrm{\Lambda }p}\mathrm{/}B{R}_{{\mathrm{\Sigma }}^{0}p}\sim 1.7$, by assuming that the physical processes leading to the $\mathrm{\Sigma }N\text{\hspace{0.17em}⁣}N$ final states are analogous to those of $\mathrm{\Lambda }pn$.