Nature of the First Excited State of 4 He

We study the first excited state of He in a microscopic {H + p,He + n} cluster model, including H and He distortions. The phenomenological S0 H+p scattering phase shift is well reproduced. We localize a complex pole of the S-matrix between the H+ p and He+n thresholds. The corresponding resonance parameters are Er = 93 keV position relative to H+p, and Γ = 390 keV width. A pole search is also performed in an extended R-matrix method, and a resonance is found with parameters Er = 114 keV and Γ = 392 keV. The R-matrix approach gives several additional poles, some of which may be connected with an enhanced threshold effect. PACS: 21.45.+v, 27.10.+h, 21.60.Gx, 25.10.+s Typeset using REVTEX