A determination of the muon pair branching ratio of the γ' meson

Using the ARGUS detector at thee+e− storage ring DORIS-II, we have observed the decays of γ(9,460) andΥ'(10,023) into muon pairs. The ratio of the two observed rates determines Bμμ(γ') largely independent of acceptance uncertainties and absolute luminosity calibration, ifBμμ(γ) is known. From data with an integrated luminosity of 6.11/pb on the γ and 9.93/pb on the γ, using the present world averageBμμ(γ)=(2.9±0.2)%, we obtainBμμ(γ)=(1.57±0.59±0.68)%.