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

The Born cross section of the process $e^{+}{e}^{-}\to \eta J\mathrm{/}\psi$ at a center-of-mass energy $\sqrt{s}=3.773\mathrm{G}\mathrm{e}\mathrm{V}$ is measured to be ($8.89\pm 0.88\pm 0.42$)$\mathrm{p}\mathrm{b}$, using a data sample collected with the BESIII detector operating at the BEPCII storage ring. The decay $\psi (3770)\to \eta J\mathrm{/}\psi$ is observed for the first time with a statistical significance of $7.4\sigma$. From a fit to the dressed cross-section line-shape of $e^{+}{e}^{-}\to \eta J\mathrm{/}\psi$ from $\sqrt{s}=3.773$ to $4.600\mathrm{G}\mathrm{e}\mathrm{V}$ we obtain the branching fraction of the decay $\psi (3770)\to \eta J\mathrm{/}\psi$ to be $(11.6\pm 6.1\pm 1.0)\times {10}^{-4}$ when the $\psi (3770)$ decay amplitude is added coherently to the other contributions, and $(7.9\pm 1.0\pm 0.7)\times {10}^{-4}$ when it is added incoherently. Here the first uncertainties are statistical and the second are systematic.