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

To search for an S = -1 di-baryonic state which decays to Lambda p, the He-3( K-, Lambda p) nmissing reaction was studied at 1.0 GeV/c. Unobserved neutrons were kinematically identified from the missing mass MX of the He-3(K-, Lambda p) X reaction in order to have a large acceptance for the Lambda pn final state. The observed Lambda pn 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 Lambda p. To account for the observed peak structure, the simplest S-wave pole was assumed to exist in the reaction channel, having a Breit-Wigner formin energy and with a Gaussian form factor. A minimum X-2 method was applied to deduce its mass, M-X = 2355(-8)(+6) (stat.) +/- 12 (syst.) MeV/ c2, and decay width, Gamma(X) = 110(-17)(+19) (stat.) +/- 27 (syst.) MeV/c(2), respectively. The form factor parameter Q(X) similar to 400MeV/ c implies that the range of the interaction is about 0.5 fm.