Observation of a $\bar{K}NN$ bound state in the ${}^3\mathrm{He}(K^{-},\hat{I}›p)n$ reaction

Abstract

We have performed an exclusive measurement of the $K^{-}+{}^3\mathrm{He}\to \hat{I}›pn$ reaction at an incident kaon momentum of $1\mathrm{GeV}/c$.In the $\hat{I}›p$ 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 $B_K=42\pm 3(stat.{)}_{-4}^{+3}(syst.)\mathrm{MeV}$, $\hat{I}{“}_K=100\pm 7(stat.{)}_{-9}^{+19}(syst.)\mathrm{MeV}$, and $Q_K=383\pm 11(stat.{)}_{-1}^{+4}(syst.)\mathrm{MeV}/c$, respectively. The total production cross-section of $\bar{K}NN$, determined by its $\hat{I}›p$ decay mode, was $\ddot{I}{ƒ}_K^{tot}\cdot B{R}_{\hat{I}›p}=9.3\pm 0.8(stat.{)}_{-1.0}^{+1.4}(syst.)\hat{I}¼\mathrm{b}$.We estimated the branching ratio of the $\bar{K}NN$ state to the $\hat{I}›p$ and $\hat{I}{\pounds }^{0}p$ decay modes as $B{R}_{\hat{I}›p}/B{R}_{\hat{I}{\pounds }^{0}p}\sim 1.7$, by assuming that the physical processes leading to the $\hat{I}\pounds NN$ final states are analogous to those of $\hat{I}›pn$.

13 pages, 12 figures