Σ Excitation in Nuclei and its Decay

We examine phenomenologically the ⁴He(K^-, π^-) spectra in flight and at rest within the distorted-wave impulse approximation by using a coupled (3N-Λ) + (3N-Σ) model. The calculated spectra can explain consistently the available data overall from Λ and Σ regions. The result shows that there exists a pole of the ⁴_Σ He unstable bound state on the second Riemann sheet of the complex energy plane, and an asymmetric peak for ⁴_Σ He appears because of the Σ branch cut associated with opening the Σ channel. The decay properties of ⁴_Σ He are also discussed.