Structure near the K^- + p + p threshold in the in-flight ³He(K^-, Λp)n reaction

To search for an S = -1 di-baryonic state which decays toΛp, the ³He(K^-,Λp)nmissing reaction was studied at 1.0 GeV/c. Unobserved neutrons were kinematically identified from the missing mass M_X of the ³He (K^-,Λp) X reaction in order to have a large acceptance for the Λpn final state. The observed Λpn events, distributed widely over the kinematically allowed region of the Dalitz plot, establish that the major component comes from a three-nucleon absorption process. A concentration of events at a specific neutron kinetic energy was observed in a region of low momentum transfer to the Λp. To account for the observed peak structure, the simplest S-wave polewas assumed to exist in the reaction channel, having a Breit-Wigner formin energy and with a Gaussian form factor. A minimum X² method was applied to deduce its mass, M_X = 2355⁺⁶ _-8 (stat.) ±12 (syst.)MeV/c², and decay width, γ_X = 110⁺¹⁹ _-17 (stat.) ±27 (syst.)MeV/c², respectively. The form factor parameter Q_X ∼ 400MeV/c implies that the range of the interaction is about 0.5 fm.