Measurement of $\mathcal{B}(J/\psi \to \eta \prime e^{+}{e}^{-})$ and search for a dark photon

Using a data sample of $(1310.6\pm 7.0)\times {10}^{6}J/\psi$ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay $J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-}$ with two dominant ${\eta }^{\prime }$ decay modes, ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$ and ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$. The branching fraction is determined to be $\mathcal{B}(J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-})=(6.59\pm 0.07\pm 0.17)\times {10}^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon (${\gamma }^{\prime }$) is performed via $J/\psi \to {\eta }^{\prime }{\gamma }^{\prime }$, ${\gamma }^{\prime }\to e^{+}{e}^{-}$. Excluding the $\omega$ and $\varphi$ mass regions, no significant signal is observed in the mass range from 0.1 to $2.1\mathrm{GeV}/c^2$. We set upper limits at the 90% confidence level on $\mathcal{B}(J/\psi \to {\eta }^{\prime }{\gamma }^{\prime })\times \mathcal{B}({\gamma }^{\prime }\to e^{+}{e}^{-})$, $\mathcal{B}(J/\psi \to {\eta }^{\prime }{\gamma }^{\prime }$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.

Figure 1

Electron-positron pair vertex distributions. (a) The two-dimensional scatter plot of $R_y$ versus $R_x$ from $J/\psi \to \gamma {\eta }^{\prime }({\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-})$ MC simulated events. (b) Distribution of ${\delta }_{xy}$. The black crosses are data, the blue solid line is the total contribution from the MC and ${\eta }^{\prime }$ sideband, the green dotted-dashed line is signal MC, the dashed line is $J/\psi \to \gamma {\eta }^{\prime }({\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-})$ MC and the shaded area is the ${\eta }^{\prime }$ sideband. The quantities are described in the text.

Figure 2

The invariant mass spectra of ${\eta }^{\prime }$ candidate events (a) $M(\gamma {\pi }^{+}{\pi }^{-})$ and (b) $M(\gamma \gamma {\pi }^{+}{\pi }^{-})$. Black crosses are data, the blue solid line is the total fitting projection, the red dashed line is signal, the green dotted-dashed line is nonpeaking background, the pink cross-hatched area is $\gamma$ conversion $J/\psi \to \gamma {\eta }^{\prime }$ and the yellow solid area is $J/\psi \to {\eta }^{\prime }\varphi$, $\varphi \to e^{+}{e}^{-}$.

Figure 3

The total detection efficiency of the entire decay chain for different $m_{{\gamma }^{\prime }}$ of (a) mode I and (b) mode II. The blue curves show the fit with polynomial functions.

Figure 4

The upper plots in (a) and (b) show the electron-positron pair invariant mass distributions for mode I and mode II, respectively, where the crosses are data and the solid lines are the background-only fitting results. The lower plots in (a) and (b) show the ratio of data over fitting result for each bin. The shaded bands are the corresponding excluded $\omega$ and $\varphi$ regions. The plots in (c) and (d) are the number of signal events ($N_{\mathrm{sig}}$) and the significances ($S$) from the fit for mode I and mode II, respectively.

Figure 5

Upper limits at the 90% C.L. for the BFs (a) ${\mathcal{B}}_{{\gamma }^{\prime }}\times {\mathcal{B}}_{{\gamma }^{\prime }\mathrm{ee}}$ and (b) ${\mathcal{B}}_{{\gamma }^{\prime }}$, where ${\mathcal{B}}_{{\gamma }^{\prime }}$ and ${\mathcal{B}}_{{\gamma }^{\prime }\mathrm{ee}}$ are $\mathcal{B}(J/\psi \to {\eta }^{\prime }{\gamma }^{\prime })$ and $\mathcal{B}({\gamma }^{\prime }\to e^{+}{e}^{-})$, respectively. (c) The exclusion limit at the 90% C.L. on the kinematic mixing strength $ϵ$.