Observation of the decay $J/\psi \to e^{+}{e}^{-}\eta (1405)$ with $\eta (1405)\to {\pi }^0{f}_0(980)$

Abstract

Using a data sample of $(10087\pm 44)\times 10^6$ $J/\psi$ events collected by the BESIII detector in 2009, 2012, 2018 and 2019, the electromagnetic Dalitz process $J/\psi \to e^{+}{e}^{-}\eta (1405)$ is observed via the decay $\eta (1405)\to {\pi }^0{f}_0(980)$, $f_0(980)\to {\pi }^{+}{\pi }^{-}$, with a significance of about $9.6\sigma$. The branching fraction of this decay is measured to be $\mathcal{B}(J/\psi \to e^{+}{e}^{-}{\pi }^0\eta (1405)\to e^{+}{e}^{-}{\pi }^0{f}_0(980)\to e^{+}{e}^{-}{\pi }^0{\pi }^{+}{\pi }^{-})=(2.02\pm 0.24(stat.)\pm 0.09(syst.))\times 10^{-7}$. The branching-fraction ratio $\mathcal{B}(J/\psi \to e^{+}{e}^{-}\eta (1405))$/$\mathcal{B}(J/\psi \to \gamma \eta (1405))$ is determined to be $(1.35\pm 0.19(stat.)\pm 0.06(syst.))\times 10^{-2}$. Furthermore, an $e^{+}{e}^{-}$ invariant-mass dependent transition form factor of $J/\psi \to e^{+}{e}^{-}\eta (1405)$ is presented for the first time. The obtained result provides input for different theoretical models, and is valuable for the improved understanding the intrinsic structure of the $\eta (1405)$ meson.

Comments: 9 pages, 3 figures