Measurement of the total and leptonic decay widths of the $J/\psi$ resonance with an energy scan method at BESIII

Abstract

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 ${\Gamma }_{ee}{\Gamma }_{ee}/{\Gamma }_{\mathrm{tot}}$ and ${\Gamma }_{ee}{\Gamma }_{\mu \mu }/{\Gamma }_{\mathrm{tot}}$ are determined to be ($0.346\pm 0.009$) and ($0.335\pm 0.006$) keV, respectively, where ${\Gamma }_{ee}$, ${\Gamma }_{\mu \mu }$, and ${\Gamma }_{\mathrm{tot}}$ are the electronic, muonic, and total decay widths of the $J/\psi$ resonance, respectively. Using the resultant ${\Gamma }_{ee}{\Gamma }_{\mu \mu }/{\Gamma }_{\mathrm{tot}}$ and ${\Gamma }_{ee}{\Gamma }_{ee}/{\Gamma }_{\mathrm{tot}}$, 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, ${\Gamma }_{\mathrm{tot}}$ 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.

13 pages, 3 figures, to be submitted to Physics Letters B