First Measurement of the Absolute Branching Fraction of $\eta_c \to \gamma \gamma$

Abstract

We apply a tag-and-probe method to precisely measure the absolute branching fraction of the decay $\eta_c \to \gamma \gamma$ with the BESIII experiment at BEPCII. Starting with a large initial sample of $2712.4\pm 14.3$ million $\psi(3686)$ events, a sample of 0.16 million $\eta_c$ events are tagged via the golden channel $\psi(3686)\to \pi^0 h_c$, $h_c\to \gamma \eta_c$, effectively avoiding interference effects. The absolute branching fraction of $\eta_c \to \gamma \gamma$ is measured for the first time to be $\mathcal{B}(\eta_c \to \gamma \gamma) = (2.45 \pm 0.48_{\rm stat.} \pm 0.09_{\rm syst.}) \times 10^{-4}$. Using the world average value of the total width of the $\eta_c$, the partial decay width of $\eta_c \to \gamma \gamma$ is calculated to be $\Gamma(\eta_c \to \gamma \gamma) = (7.48 \pm 1.48_{\rm stat.} \pm 0.30_{\rm syst.})~{\rm keV}$.