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

Abstract

Using the Crystal Ball detector at thee ⁺ e ⁻ storage ring DORIS II, we have measured the branching fraction to muon pairsB _μμ of the Υ(1S) and Υ(2S) resonances and for the first time the product of the muonic partial widthΓ _μμ and the branching fraction to electronsB _ee 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 ofB _ee (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.