Precision Measurements of ${\cal B}[\psi(3686) \to \pi^+\pi^- J/\psi]$ and ${\cal B}[J/\psi\to l^+l^-]$

TL;DR

This paper reports the most precise measurements to date of the branching fractions for $\psi(3686) o \pi^+\pi^- ext{ extbackslash} J/ ext{ extbackslash}psi$ and $J/ ext{ extbackslash}psi$ leptonic decays, confirming previous results and testing lepton universality.

Contribution

The study provides the most precise measurements of key charmonium decay branching fractions, improving accuracy and confirming previous findings with a large data sample.

Findings

Measured ${ m B}[\psi(3686) o \\pi^+\\\pi^- J/\\psi]$ as 34.98%

Determined ${ m B}[J/\\psi o e^+e^-]$ and ${ m B}[J/\\psi o \\mu^+\\\mu^-]$ around 6%

Confirmed lepton universality within 0.04%

Abstract

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.