We have performed an exclusive measurement of the $K^{ -}+\! ~^{ 3}{ \rm He} \to \Lambda pn$ reaction at an incident kaon momentum of $1\ { \rm GeV}/c$.In the $\Lambda p$ invariant mass spectrum, a clear peak was observed below the mass threshold of $\bar{ K}\!+\!N\!+\!N$, 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 $\Lambda p$ 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 $\Lambda p$ and $\Sigma^{ 0}p$ decay modes as $BR_{ \Lambda p}/BR_{ \Sigma^{ 0}p} \sim 1.7$, by assuming that the physical processes leading to the $\Sigma N\!N$ final states are analogous to those of $\Lambda pn$.
Categories: nucl-ex