Measurement of the decay of the Υ(1S) and Υ(2S) resonances to muon pairs

Using the Crystal Ball detector at thee+e− storage ring DORIS II, we have measured the branching fraction to muon pairsBμμ of the Υ(1S) and Υ(2S) resonances and for the first time the product of the muonic partial widthΓμμ and the branching fraction to electronsBee for both resonances. We obtain $$\begin{gathered} B_{\mu \mu } (1S) = (2.31 \pm 0.12 \pm 0.10)\% , \hfill \\ \Gamma _{\mu \mu } (1S) \cdot B_{ee} (1S) = (31.2 \pm 1.6 \pm 1.7)eV, \hfill \\ \end{gathered} $$ and $$\begin{gathered} B_{\mu \mu } (2S) = (1.22 \pm 0.28 \pm 0.19)\% , \hfill \\ \Gamma _{\mu \mu } (2S) \cdot B_{ee} (2S) = (6.5 \pm 1.5 \pm 1.0)eV. \hfill \\ \end{gathered} $$ Inserting the present world average value ofBee(1S)=(2.52±0.17)%, we determine the muonic partial width of the Υ(1S) as $$\Gamma _{\mu \mu } (1S) = (1.24 \pm 0.06 \pm 0.11)keV.$$ In addition, we present the first indication of the expected interference between μ-pair production in the continuum and in Υ(1S) decays.