Precision measurement of the $\mathcal{B}({\rm Y}(3S)\to {\tau }^{+}{\tau }^{-})/\mathcal{B}({\rm Y}(3S)\to {\mu }^{+}{\mu }^{-})$ ratio

Abstract

We report on a precision measurement of the ratio ${\mathcal{R}}_{\tau \mu }^{{\rm Y}(3S)}=\mathcal{B}({\rm Y}(3S)\to {\tau }^{+}{\tau }^{-})/\mathcal{B}({\rm Y}(3S)\to {\mu }^{+}{\mu }^{-})$ using data collected with the BaBar detector at the SLAC PEP-II $e^{+}{e}^{-}$ collider. The measurement is based on a 28 fb${}^{-1}$ data sample collected at a center-of-mass energy of 10.355 GeV corresponding to a sample of 122 million ${\rm Y}(3S)$ mesons. The ratio is measured to be ${\mathcal{R}}_{\tau \mu }^{{\rm Y}(3S)}=0.966\pm 0.008_{\mathrm{stat}}\pm 0.014_{\mathrm{syst}}$ and is in agreement with the Standard Model prediction of 0.9948 within 2 standard deviations. The uncertainty in ${\mathcal{R}}_{\tau \mu }^{{\rm Y}(3S)}$ is almost an order of magnitude smaller than the only previous measurement.

8 pages, 4 figures, 3 tables

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