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

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