Physical Review C
The reaction gamma + p -> K+ + Sigma + pi was used to determine the invariant mass distributions or "line shapes" of the Sigma(+) pi-, Sigma - pi(+), and Sigma(0)pi(0) final states, from threshold at 1328 MeV/c(2) through the mass range of the Lambda(1405) and the Lambda(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 Lambda(1405), indicating the presence of substantial I = 1 strength in the reaction. Background contributions to the data from the Sigma(0)(1385) and from K*Sigma 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 N (K) over bar threshold. A best fit to all the data was obtained after including a phenomenological I = 1, J(P) = 1/2(-) amplitude with a centroid at 1394 +/- 20 MeV/c(2) and a second I = 1 amplitude at 1413 +/- 10 MeV/c(2). The centroid of the I = 0 Lambda(1405) strength was found at the Sigma pi threshold, with the observed shape determined largely by channel coupling, leading to an apparent overall peak near 1405 MeV/c(2).
Moriya, K., Schumacher, R. A., Adhikari, K. P., Adikaram, D., Aghasyan, M., Anderson, M. D., ... & Batourine, V. (2013). Measurement of the Σ π photoproduction line shapes near the Λ (1405). Physical Review C, 87(3), 035206.