We formulated the Λ (1405)(abbreviated as Λ*)→(Σ π) 0 invariant-mass spectra produced in
the K−+ p→ Σ+(1660)+ π−, followed by Σ+(1660)→ Λ (1405)+ π+→ Σ π+ π+, processes at p
(K−)= 4.2 GeV/c, in which both the incident channel for a quasibound K− p state and its
decay process to (Σ π) 0 were taken into account realistically. We calculated M (Σ π) spectral
shapes using mixed transition matrices, T 21= T Σ π← K− p and T 22= T Σ π← Σ π, for
various theoretical models involving Λ*. The asymmetric spectra were compared to old …

Historically, in 1959 Dalitz and Tuan [1] predicted the existence of a strange quasi-bound
state of K−+ p→ Σ+ π with I= 0 in their analysis of experimental KN scattering data. In 1961
its experimental evidence was found from the mass spectrum, M ((Σπ) 0), in the reaction K−+
p→(Σπ) 0+ π+ π− at pK−= 1.15 GeV/c [2]. The resonant state of Λ (1405) with J= 1/2, I= 0,
S=− 1, called Λ (1405), is located below the KN threshold, and decays to Σπ. After half a
century, this state has been certified as a four-star state in Particle Data Group data [3] …