Ξ Resonances in Ξ⁻ Be interactions II. Properties of Ξ(1820) and Ξ(1960) in the$\Lambda \bar K^0$ and$\Sigma ^0 \bar K^0$ channels

Abstract

In an experiment at the CERN-SPS charged hyperon beam, we have investigated the inclusive$\Lambda \bar K^0$ and$\Sigma ^0 \bar K^0$ final states formed in Ξ⁻ Be interactions. In the$\Lambda \bar K^0$ channel, we observe a signal at 1826 MeV/c² which can be identified with the known Ξ(1820) resonance. We determine its mass and width to be:M=1826±4 MeV/c², Г=12±14 MeV/c². A moment analysis is consistent with a spin of 3/2 and indicates a negative parity for this spin assignment. Also in the$\Lambda \bar K^0$ channel, we observe a 3.6σ signal with the following parameters:M=1963±5 MeV/c², Г=25±15 MeV/c². This state, which we call Ξ(1960), is not observed in the$\Sigma ^0 \bar K^0$ channel, leading to an upper limit on the ratio of partial widths$\Sigma \bar K/\Lambda \bar K$ of 2.3 (90% confidence level). A moment analysis of the$\Lambda \bar K^0$ final state indicates a spin of 5/2 or greater in the natural spin-parity series 5/2⁺, 7/2⁻, etc.