Measurement Of The Decay Of The Gamma-(1s) And Gamma(2s) Resonances To Muon Pairs

Using the Crystal Ball detector at the e+e-storage ring DORIS II, we have measured the branching fraction to muon pairs B(mu-mu) of the UPSILON(1S) and UPSILON(2S) resonances and for the first time the product of the muonic partial width GAMMA(mu-mu) and the branching fraction to electrons B(ee) for both resonances. We obtain B(mu-mu) (1S) = (2.31 +/- 0.12 +/- 0.10)%, GAMMA(mu-mu) (1S) . B(ee) (1S) = (31.2 +/- 1.6 +/- 1.7) eV, and B(mu-mu) (2S) = (1.22 +/- 0.28 +/- 0.19)%, GAMMA(mu-mu) (2S) . B(ee) (2S) = (6.5 +/- 1.5 +/- 1.0) eV. Inserting the present world average value of B(ee) (1S) = (2.52 +/- 0.17)%, we determine the muonic partial width of the UPSILON(1S) as GAMMA(mu-mu) (1S) = (1.24 +/- 0.06 +/- 0.11) keV. In addition, we present the first indication of the expected interference between mu-pair production in the continuum and in UPSILON(1S) decays.