Observation of $\psi (3770)\to \eta J/\psi$

BESIII Collaboration

The Born cross section of the process $e^{+}{e}^{-}\to \eta J/\psi$ at a center-of-mass energy $\sqrt{s}=3.773\mathrm{GeV}$ is measured to be ($8.89\pm 0.88\pm 0.42$)$\mathrm{pb}$, using a data…