Measurement of the Sigma pi photoproduction line shapes near the Lambda(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 Σ0π0 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 NK⎯⎯⎯ 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 Σπ threshold, with the observed shape determined largely by channel coupling, leading to an apparent overall peak near 1405 MeV/c2.