Experimental confirmation of the Λ(1405) ansatz from resonant formation of a K−p quasi-bound state in K− absorption by 3He and 4He

The Σπ invariant-mass spectra in the resonant capture of K− at rest in 4He, 3He and d are calculated by a coupled-channel procedure for a K−p quasi-bound state of an arbitrary chosen mass (M) and width (Γ). A χ2 analysis of old 4He bubble chamber data shows a dominance of the s-orbit absorption, and yielded M=1405.5−1+1.4 MeV/c2 and Γ=24−3+4 MeV, where a possible population of Σ0(1385) and also a small p-orbit capture contribution are taken into account. This result confirms the Λ(1405) ansatz, whereas recent chiral-SU(3) predictions (M∼1420 MeV/c2) are excluded. A more stringent test by using a 3He target is proposed.