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

Abstract

To search for an S= -1 di-baryonic state which decays to \(\Lambda p\), the \( {\rm{}^3He}(K^-,\Lambda p)n_{missing}\) reaction was studied at 1.0 GeV/\(c\). Unobserved neutrons were kinematically identified from the missing mass \(M_X\) of the \( {\rm{}^3He}(K^-,\Lambda p)X\) reaction in order to have a large acceptance for the \(\Lambda pn\) final state. The observed \(\Lambda p n\) 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 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.) MeV/c\(^2\), and decay-width \(\Gamma_X\ = \) 110 \( ^{+ 19}_{ - 17}\) (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