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

TL;DR

This study precisely measures the decay widths of the $J/ar{ ext{psi}}$ resonance using an energy scan at BESIII, confirming lepton universality and providing updated decay width values.

Contribution

First simultaneous measurement of $J/\psi$ leptonic decay widths at multiple energies with an energy scan method at BESIII.

Findings

Measured $ ext{Br}(J/\psi \to e^+e^-)$ and $ ext{Br}(J/\psi \to \mu^+\mu^-)$ with high precision.

Confirmed lepton universality within two standard deviations.

Provided updated total and leptonic decay widths of $J/\psi$.

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 $\Gamma_{ee} \Gamma_{ee} / \Gamma_{\rm tot}$ 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.