Measurement of the Σπ photoproduction line shapes near the Λ(1405)

The reaction 𝛾+𝑝→𝐾++𝛴+𝜋 was used to determine the invariant mass distributions or “line shapes” of the 𝛴+⁢𝜋−, 𝛴−⁢𝜋+, and 𝛴0⁢𝜋0 final states, from threshold at 1328 MeV/𝑐2 through the mass range of the 𝛬⁡(1405) and the 𝛬⁡(1520). The measurements were made with the CLAS system at Jefferson Lab using tagged real photons, for center-of-mass energies 1.95<𝑊<2.85 GeV. The three mass distributions differ strongly in the vicinity of the 𝐼=0 𝛬⁡(1405), indicating the presence of substantial 𝐼=1 strength in the reaction. Background contributions to the data from the 𝛴0⁡(1385) and from 𝐾*⁢𝛴 production were studied and shown to have negligible influence. To separate the isospin amplitudes, Breit-Wigner model fits were made that included channel-coupling distortions due to the Math output error threshold. A best fit to all the data was obtained after including a phenomenological 𝐼=1, 𝐽𝑃=1/2− amplitude with a centroid at 1394±20 MeV/𝑐2 and a second 𝐼=1 amplitude at 1413±10 MeV/𝑐2. The centroid of the 𝐼=0 𝛬⁡(1405) strength was found at the 𝛴⁢𝜋 threshold, with the observed shape determined largely by channel coupling, leading to an apparent overall peak near 1405 MeV/𝑐2.