Results of K-barNN search via the (K^–, n) reaction at J-PARC

There is a long-standing argument of the existence of kaonic nucleus, which is a bound state of anti-kaon and nucleus. Theoretically, its existence is strongly supported, however its binding energy and width have not been established yet. It is strongly desired to have an experiment to investigate the simplest kaonic nucleus, so-called K^–pp bound state. We performed an experiment to search for K^–pp via the in-flight (K^–, n) reaction at J-PARC. By studying Λp invariant mass spectrum in the Λpn final state and its dependence on momentum transfer of K^–N → K-barN, we observed a resonant state located below M_K^-pp. The mass and width of this resonant state are 2324±3(stat.)_-3⁺⁶(syst.) MeV/c², and 115 ± 7(stat.)_-20⁺¹⁰(syst.) MeV, respectively, which can be naturally interpreted as a theoretically predicted K^–pp bound state. (author)