Observation of electromagnetic Dalitz decays $J/\psi \to P{e}^{+}{e}^{-}$

Based on a sample of $(225.3\pm 2.8)\times 10^6$ $J/\psi$ events collected with the BESIII detector, the electromagnetic Dalitz decays of $J/\psi \to P{e}^{+}{e}^{-}(P={\eta }^{\prime }/\eta /{\pi }^0)$ are studied. By reconstructing the pseudoscalar mesons in various decay modes, the decays $J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-}$, $J/\psi \to \eta e^{+}{e}^{-}$, and $J/\psi \to {\pi }^0{e}^{+}{e}^{-}$ are observed for the first time. The branching fractions are determined to be $\mathcal{B}(J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-})=(5.81\pm 0.16\pm 0.31)\times 10^{-5}$, $\mathcal{B}(J/\psi \to \eta e^{+}{e}^{-})=(1.16\pm 0.07\pm 0.06)\times 10^{-5}$, and $\mathcal{B}(J/\psi \to {\pi }^0{e}^{+}{e}^{-})=(7.56\pm 1.32\pm 0.50)\times 10^{-7}$, where the first errors are statistical and the second ones systematic.

Figure 1

Veto of $\gamma$-conversion events. (a) a scatter plot of $R_y$ vs $R_x$ for the MC-simulated $J/\psi \to {\eta }^{\prime }\gamma$ (${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$) events. (b) ${\delta }_{xy}$ distributions. The (green) shaded histogram shows the MC-simulated $J/\psi \to e^{+}{e}^{-}{\eta }^{\prime }$ (${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$) signal events. The (red) dots with error bars are data. The (blue) dotted histogram shows the background from the $\gamma$-conversion events. In (b), the solid arrow indicates the requirement on ${\delta }_{xy}$.

Figure 2

The $|\cos {\theta }_{\text{decay}}|$ distributions (a) for $\eta$ and (c) for ${\pi }^0$ and two-photon invariant-mass distributions (b) for the $J/\psi \to \eta e^{+}{e}^{-}(\eta \to \gamma \gamma )$ and (d) for the $J/\psi \to {\pi }^0{e}^{+}{e}^{-}({\pi }^0\to \gamma \gamma )$ modes. In (a) and(c), the (green) solid histograms are the MC-simulated signals, the (red) dots with error bars are data, and the (blue) dotted histograms are from the $\psi (3770)$ data. The arrows indicate the requirement $|\cos {\theta }_{\text{decay}}|<0.9$. In (b) and (d), the (red) histograms and the (blue) dots with error bars are $\psi (3770)$ data (used as a continuum sample) without and with the requirement, respectively.

Figure 3

Mass distributions of the pseudoscalar meson candidates in $J/\psi \to P{e}^{+}{e}^{-}$: (a) ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$, (b) ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$ ($\eta \to \gamma \gamma$), (c) $\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0$, (d) $\eta \to \gamma \gamma$, and (e) ${\pi }^0\to \gamma \gamma$. The (black) dots with error bars are data, the (red) dashed lines represent the signal, the (green) dotted-dashed curves show the nonpeaking background shapes, and the (yellow) shaded components are the shapes of the peaking backgrounds from the $J/\psi \to P\gamma$ decays. Total fits are shown as the (blue) solid lines.

Figure 4

The $M_{e^{+}{e}^{-}}$ mass distributions in $J/\psi \to P{e}^{+}{e}^{-}$: (a) ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$, (b) ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$ ($\eta \to \gamma \gamma$), (c) $\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0$, (d) $\eta \to \gamma \gamma$, and (e) ${\pi }^0\to \gamma \gamma$. The (red) dots with error bars are data, the (yellow) shaded components are from the $\gamma$-conversion backgrounds in the $J/\psi \to P\gamma$ decays, and the (green) light-shaded histograms are from nonpeaking backgrounds estimated from the sidebands on the pseudoscalar mass spectra. The (blue) histograms represent the sum of backgrounds and MC-simulated signals.

Figure 5

Data of $J/\psi \to {\pi }^{+}{\pi }^{-}{\pi }^0$, ${\pi }^0\to \gamma e^{+}{e}^{-}$. The distributions of ${\pi }^0$ masses are in (a) and (c); the distributions of the $M_{e^{+}{e}^{-}}$ are in (b) and (d). The upper two plots [(a) and (b)] are for events without the requirement of ${\delta }_{xy}<2$ cm; the lower two plots [(c) and (d)] are for events with the requirement. The dots with error bars are data. In (b) and (d), the (red) dashed curves are the MC-simulated signals, and the (green) dot-dashed curves are the MC-simulated shapes from $J/\psi \to {\pi }^{+}{\pi }^{-}{\pi }^0(\gamma \gamma )$ in which one of the photons converts to an electron-positron pair. Total fits are shown as the (blue) solid lines.

Figure 6

Form factor for $J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-}({\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-})$. The crosses are data, the (red) dotted-dashed curve is the prediction of the simple pole model with the pole mass $\mathrm{\Lambda }=m_{{\psi }^{\prime }}=3.686\mathrm{GeV}/c^2$, and the fit is shown as the (blue) solid curve.