High resolution pion-proton elastic scattering differential cross-section near the eta meson threshold

A high resolution π-rho elastic scattering differential cross-section is measured in the range of CosTHETA(deg) = -0.60 to CosTHETA(deg) = 0.80, in the centre of mass system using a system of drift chambers surrounding a liquid hydrogen target. The measurements are made in the range of incident pion momentum near the eta meson threshold (0.6855 +- 0.0005 GeV/c). A cusp i.e. discontinuity has been observed in the elastic differential cross-section across the threshold for the eta meson production. Relative phases of the S11 wave and f(THETA(double apostrophe),α) the non-spin flip amplitude are extracted from the behaviour of the cusp as a function of CosTHETA(double apostrophe). The production cross-section of π-rho → eta n is also extracted from the behaviour of the cusp. The phase of the S11 wave at the eta threshold is delta0 = 25.130 +- 1.590. (auth.)