Observation of the electromagnetic Dalitz transition $J/\psi \to e^{+}{e}^{-}{\eta }_c$

Abstract

Using $(10.087\pm 0.044)\times 10^9$ $J/\psi$ events collected with the BESIII detector at the $e^{+}{e}^{-}$ BEPCII collider, we present the first observation of the electromagnetic Dalitz decay $J/\psi \to e^{+}{e}^{-}{\eta }_c$. The relative branching fraction $R\equiv \frac{\Gamma (J/\psi \to e^{+}{e}^{-}{\eta }_c)}{\Gamma (J/\psi \to \gamma {\eta }_c)}$ is determined to be $(0.65\pm 0.02_{stat.}\pm 0.07_{sys.})\%$, where the first uncertainty is statistical and the second systematic. The $q^2$-dependent form factors are also extracted, and no significant deviation from the theoretical prediction is seen.

13 pages, 7 figures, submitted to PRD