Observation of a K̅NN bound state in the ^3 He (K^-, Λ p)n reaction

We have performed an exclusive measurement of the K^-+ ^3 He→Λ pn reaction at an incident kaon momentum of 1 GeV/c.In the Λ p invariant mass spectrum, a clear peak was observed below the mass threshold of K̅+N+N, as a signal of the kaonic nuclear bound state, K̅NN.The binding energy, decay width, and S-wave Gaussian reaction form-factor of this state were observed to be B_K = 42±3( stat.)^+3_-4( syst.) MeV, Γ_K = 100±7( stat.)^+19_-9( syst.) MeV, and Q_K = 383±11( stat.)^+4_-1( syst.) MeV/c, respectively. The total production cross-section of K̅NN, determined by its Λ p decay mode, was σ^tot_K· BR_Λ p = 9.3±0.8( stat.)^+1.4_-1.0( syst.) μ b.We estimated the branching ratio of the K̅NN state to the Λ p and Σ^0p decay modes as BR_Λ p/BR_Σ^0p∼ 1.7, by assuming that the physical processes leading to the Σ NN final states are analogous to those of Λ pn.