Observation of $e^{+}{e}^{-}\to \gamma X(3872)$ at BESIII

With data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies from 4.009 to 4.420 GeV, the process $e^{+}{e}^{-}\to \gamma X(3872)$ is observed for the first time with a statistical significance of $6.3\sigma$. The measured mass of the $X(3872)$ is $(3871.9\pm 0.7_{\text{stat}}\pm 0.2_{\text{syst}})\mathrm{MeV}/c^2$, in agreement with previous measurements. Measurements of the product of the cross section $\sigma [e^{+}{e}^{-}\to \gamma X(3872)]$ and the branching fraction $\mathcal{B}[X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi ]$ at center-of-mass energies 4.009, 4.229, 4.260, and 4.360 GeV are reported. Our measurements are consistent with expectations for the radiative transition process $Y(4260)\to \gamma X(3872)$.

Figure 1

The ${\pi }^{+}{\pi }^{-}J/\psi$ invariant mass distributions at $\sqrt{s}=4.009$ (top left), 4.229 (top right), 4.260 (bottom left), and 4.360 GeV (bottom right). Dots with error bars are data, the green shaded histograms are normalized $J/\psi$ sideband events.

Figure 2

Fit of the $M({\pi }^{+}{\pi }^{-}J/\psi )$ distribution with a MC simulated histogram convolved with a Gaussian function for signal and a linear background function. Dots with error bars are data, the red curve shows the total fit result, while the blue dashed curve shows the background contribution.

Figure 3

The $\cos \theta$ distribution of the radiative photon in $e^{+}{e}^{-}\mathrm{c}\mathrm{.}\mathrm{m}\mathrm{.}$ frame (left) and the $M({\pi }^{+}{\pi }^{-})$ distribution (right). Dots with error bars are data in the $X(3872)$ signal region, the green shaded histograms are normalized $X(3872)$ sideband events, and the red open histogram in the left panel is the result from a MC simulation that assumes a pure $E1$ transition.

Figure 4

The fit to ${\sigma }^B[e^{+}{e}^{-}\to \gamma X(3872)]\times \mathcal{B}[X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi ]$ with a $Y(4260)$ resonance (red solid curve), a linear continuum (blue dashed curve), or a $E1$-transition phase space term (red dotted-dashed curve). Dots with error bars are data.