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

Abstract

The BaBar collaboration has measured the ratio $R_{\tau \mu }^{{\rm Y}(3S)}=\mathcal{B}({\rm Y}(3S)\to {\tau }^{+}{\tau }^{-})/\mathcal{B}({\rm Y}(3S)\to {\mu }^{+}{\mu }^{-})$ with a high level of precision. This measurement utilized a 28 fb${}^{-1}$ dataset collected at a center-of-mass energy of 10.355 GeV. The measured ratio, $R_{\tau \mu }^{{\rm Y}(3S)}$, is measured to be $R_{\tau \mu }^{{\rm Y}(3S)}=0.966\pm 0.008_{\text{stat}}\pm 0.014_{\text{syst}}$. This value is within 2 standard deviations of the standard model prediction $R_{\tau \mu }^{{\rm Y}(3S)}$=0.9948. The new measurement is approximately a factor of 6$\times$ more precise than the only prior measurement. This increased precision is in part due to a more complete analysis of the radiative tail in the ${\rm Y}(3S)$ decay, in addition to a significant increase in statistics.

contribution to the 2021 EW session of the 55th Rencontres de Moriond