Observation of the $J\mathrm{/}\psi$ and $\psi (3686)$ decays into $\eta {\mathrm{\Sigma }}^{+}{\bar{\mathrm{\Sigma }}}^{-}$

The decays $J\mathrm{/}\psi \to \eta {\mathrm{\Sigma }}^{+}\bar{\mathrm{\Sigma }}{}^{-}$ and $\psi (3686)\to \eta {\mathrm{\Sigma }}^{+}\bar{\mathrm{\Sigma }}{}^{-}$ are observed for the first time, using $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ and $(448.1\pm 2.9)\times {10}^6$ $\psi (3686)$ events collected with the BESIII detector at the BEPCII collider. We determine the branching fractions of these two decays to be $\mathcal{B}(J\mathrm{/}\psi \to \eta {\mathrm{\Sigma }}^{+}\bar{\mathrm{\Sigma }}{}^{-})=(6.34\pm 0.21\pm 0.37)\times {10}^{-5}$ and $\mathcal{B}(\psi (3686)\to \eta {\mathrm{\Sigma }}^{+}\bar{\mathrm{\Sigma }}{}^{-})=(9.59\pm 2.37\pm 0.61)\times {10}^{-6}$, where the first uncertainties are statistical and the second are systematic. The ratio of these two branching fractions is determined to be $\frac{\mathcal{B}(\psi (3686)\to \eta {\mathrm{\Sigma }}^{+}\bar{\mathrm{\Sigma }}{}^{-})}{\mathcal{B}(J\mathrm{/}\psi \to \eta {\mathrm{\Sigma }}^{+}\bar{\mathrm{\Sigma }}{}^{-})}=(15.1\pm 3.8)\mathrm{\%}$, which is in agreement with the “12% rule.”