Using $e^+e^-$ annihilation data sets collected with the BESIII detector, we measure the cross sections of the processes $e^+e^- \to e^+e^-$ and $e^+e^- \to \mu^+\mu^-$ at fifteen center-of-mass energy points in the vicinity of the $J/\psi$ resonance. By a simultaneous fit to the measured, center-of-mass energy dependent cross sections of the two processes, the combined quantities $\Gamma_{ ee} \Gamma_{ ee} / \Gamma_{ \rm tot}$ and $\Gamma_{ ee} \Gamma_{ \mu\mu} / \Gamma_{ \rm tot}$ are determined to be ($0.346 \pm 0.009$) and ($0.335 \pm 0.006$) keV, respectively, where $\Gamma_{ ee}$, $\Gamma_{ \mu\mu}$, and $\Gamma_{ \rm tot}$ are the electronic, muonic, and total decay widths of the $J/\psi$ resonance, respectively. Using the resultant $\Gamma_{ ee} \Gamma_{ \mu\mu} / \Gamma_{ \rm tot}$ and $\Gamma_{ ee} \Gamma_{ ee} / \Gamma_{ \rm tot}$, the ratio $\Gamma_{ ee} / \Gamma_{ \mu\mu}$ is calculated to be $1.031 \pm 0.015$, which is consistent with the expectation of lepton universality within about two standard deviations. Assuming lepton universality and using the branching fraction of the $J/\psi$ leptonic decay measured by BESIII in 2013, $\Gamma_{ \rm tot}$ and $\Gamma_{ ll}$ are determined to be ($93.0 \pm 2.1$) and ($5.56 \pm 0.11$) keV, respectively, where $\Gamma_{ ll}$ is the average leptonic decay width of the $J/\psi$ resonance.
Categories: hep-ex