Based on $(106.41 \pm 0.86)\times 10^{ 6}$ $\psi(3686)$ events collected with the BESIII detector at the BEPCII collider, the branching fractions of $\psi(3686) \to \pi^+\pi^- J/\psi$, $J/\psi \to e^+e^- $, and $J/\psi \to \mu^+\mu^-$ are measured. We obtain ${ \cal B}[\psi(3686) \to \pi^+\pi^-J/\psi]=(34.98\pm 0.02\pm 0.45)%$, ${ \cal B}[J/\psi \to e^+e^-] = (5.983 \pm 0.007 \pm 0.037)%$ and ${ \cal B}[J/\psi \to \mu^+\mu^-] = (5.973 \pm 0.007 \pm 0.038)%$. The measurement of ${ \cal B}[\psi(3686) \to \pi^{ +}\pi^{ -}J/\psi]$ confirms the CLEO-c measurement, and is apparently larger than the others. The measured $J/\psi$ leptonic decay branching fractions agree with previous experiments within one standard deviation. These results lead to ${ \cal B}[J/\psi \to l^+l^-] = (5.978 \pm 0.005 \pm 0.040)%$ by averaging over the $e^{ +}e^{ -}$ and $\mu^{ +}\mu^{ -}$ channels and a ratio of ${ \cal B}[J/\psi \to e^+e^-] / { \cal B}[J/\psi \to \mu^+\mu^-] = 1.0017 \pm 0.0017 \pm 0.0033$, which tests $e$-$\mu$ universality at the four tenths of a percent level. All the measurements presented in this paper are the most precise in the world to date.
Categories: hep-ex