Measurement of the branching ratio for the decay η→μ+μ− (original) (raw)

A new measurement of the branching ratio for the decay,.,-+ JJ+ JJ-, made at tile SATURNE II proton synchrotron, resulted in f(,.,-+ JJ+ JJ-)lf(,.,-+ all) = 15.7 ± 0.7 (stat.)±0.5 (syst.)) x 10-8. The reaction pd-+ l He ,., close to threshold yielded 800 S-I tagged ,.,'s in a narrow momentum band around 257 MeV Ic. Muon pairs were detected in two range telescopes. The data obtained consist of 114 events ,.,-+ JJ+ JJ-on a background of 14 events. The new value for the branching ratio is 1.3 ± 0.2 times the unitarity lower limit, consistent with most quark and Vector Meson Dominance models which describe th~ decay as an electromagnetic transition with a two-photon intermediate state. The result resolves the discrepancy between the two previous measurements of this