Measurement of branching fractions for $\psi (3686)\to \gamma {\eta }^{\prime }$, $\gamma \eta$, and $\gamma {\pi }^0$

Using a data sample of $448\times {10}^6\psi (3686)$ events collected with the BESIII detector operating at the BEPCII storage ring, the decays $\psi (3686)\to \gamma \eta$ and $\psi (3686)\to \gamma {\pi }^0$ are observed with a statistical significance of $7.3\sigma$ and $6.7\sigma$, respectively. The branching fractions are measured to be $\mathcal{B}(\psi (3686)\to \gamma \eta )=(0.85\pm 0.18\pm 0.05)\times {10}^{-6}$ and $\mathcal{B}(\psi (3686)\to \gamma {\pi }^0)=(0.95\pm 0.16\pm 0.05)\times {10}^{-6}$. In addition, we measure the branching fraction of $\psi (3686)\to \gamma {\eta }^{\prime }$ to be $\mathcal{B}(\psi (3686)\to \gamma {\eta }^{\prime })=(125.1\pm 2.2\pm 6.2)\times {10}^{-6}$, which represents an improvement of precision over previous results.

Figure 1

Projection of the simultaneous fit to the invariant mass of $M({\pi }^{+}{\pi }^{-}\eta )$ (left) and $M({\pi }^0{\pi }^0\eta )$ (right). Dots with error bars show data, the red solid curves show the total fit result, and the blue dashed lines represent the background contributions.

Figure 2

Projection of the simultaneous fit to the invariant mass of $M({\pi }^{+}{\pi }^{-}{\pi }^0)$ (left) and $M({\pi }^0{\pi }^0{\pi }^0)$ (right). Dots with error bars represent the data, the red solid curves show the total fit result, and the blue dashed lines correspond to the background contributions.

Figure 3

Unbinned maximum likelihood fit to the $M(\gamma \gamma )$ spectrum for the decay $\psi (3686)\to \gamma {\pi }^0$. Dots with error bars show data. The red solid curve shows the result of the fit, the blue dashed line represents the contribution of the QED background, and the green shaded histogram depicts the peaking background from ${\chi }_{cJ}\to {\pi }^0{\pi }^0$ decays.