η-PHYSICS AT GEM

Abstract

Some experimental studies of η–production and η–interactions performed or presently under way by the GEM collaboration at COSY Jülich are reviewed. The Germanium Wall, a stack of annular detectors made of high purity Germanium, was used to measure a series of differential and total cross sections for η–production with proton and deuteron beams on light nuclei. The unique combination of a high resolution magnetic spectrograph (BIG KARL) and 7an electron cooled beam delivered by COSY is ideally suited to perform high precision experiments like the η–mass determination. In a self calibrating experiment three particles from two reactions were detected simultaneously. Kinematical coincidence of 3H and π+ from p + d → 3H + π+ reaction allowed to calibrate the spectrograph and determine the beam momentum with great accuracy. The η–meson was seen clearly as a sharp missing-mass peak on a slowly varying background in the p + d → 3He + X reaction. A new value for the η-mass has been derived with extremely small error bars.