## pi. N S/sub 11/ partial-wave amplitude near the etan production threshold. [Cusp size and orientation, partial wave unitarity]

From an analysis of the ..pi../sup -/p backward differential cross-section data, which clearly shows a cusplike structure near the etan threshold, we deduce, using partial-wave unitarity relations, the size and the orientation of the cusp. The cusp appears because of the strong coupling of the etan channel to an N' resonance just above its threshold. Using the above information on the cusp, the ..pi..N S/sub 11/ partial-wave amplitude is fitted through the etan threshold and the resonance. The effect of the size of the cusp, acting as a constraint, on the resonance parameters determined in the fit is emphasized. Themore »