Measurement of the mesonic decay branch of the K̅NN quasi-bound state

We conducted measurements of K^- + ^3 He→πY N + N' reactions using 1  GeV/c K^--beam, aiming to understand the broad decay width of K̅N N (≈ twice as broad as that of Λ(1405) considered to be the K̅N quasi-bound state). We successfully reproduced distributions of the π Y N invariant mass and momentum transfer of π Y N by employing model fitting functions for K̅ N N formation and quasi-free K̅ absorption (QF_K̅- abs) processes. The model can describe the experimental data quite well, and four K̅ N N →π Y N cross-sections were obtained. The result suggests that the mesonic decay is the dominant decay branch of K̅ N N. It was also suggested that Γ_πΛ N∼Γ_πΣ N, which indicates a significant contribution of the I_K̅ N=1 absorption channel to the K̅ N N decay as well as the I_K̅ N=0 makes the K̅ N N state ≈ twice as unstable as Λ(1405).