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

Abstract

To search for an ${\rm S} = -1$ di-baryonic state which decays to $\Lambda p$, the ${}^3{\rm He}(K^-,\Lambda p)n_{\rm missing}$ reaction was studied at 1.0 ${\rm GeV}/c$. Unobserved neutrons were kinematically identified from the missing mass $M_{X}$ of the ${}^3{\rm He}(K^-,\Lambda p)X$ 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 form in energy and with a Gaussian form factor. A minimum $\chi ^2$ method was applied to deduce its mass, $M_X = 2355^{+6}_{-8}$ (stat.) $\pm $12 (syst.) ${\rm MeV}/c^2$, and decay width, $\Gamma _X = 110^{+19}_{-17}$ (stat.) $\pm $27 (syst.) ${\rm MeV}/c^2$, respectively. The form factor parameter $Q_X \sim $ 400 ${\rm MeV}/c$ implies that the range of the interaction is about 0.5 fm.