MEASUREMENT OF THE BRANCHING RATIO FOR THE DECAY ETA-]MU-(MU)

A new measurement of the branching ratio for the decay eta --> mu+mu-, made at the SATURNE II proton synchrotron, resulted in GAMMA(eta --> mu+mu-)/GAMMA(eta -->all)=[5.7 +/-; 0.7(stat.)+/-O.5 (syst.)] x 10(-6) . The reaction pd -->He-3eta close to threshold yielded 800 s-1 tagged eta's in a narrow momentum band around 257 MeV/c. Muon pairs were det ected in two range telescopes. The data obtained consist of 114 events eta --> mu+mu- on a background of 14 events. The new value for the br anching ratio is 1.3 +/- 0.2 times the unitarity lower limit, consiste nt with most quark and vector meson dominance models which describe th e decay as an electromagnetic transition with a two-photon intermediat e state. The result resolves the discrepancy between the two previous measurements of this branching ratio.