Based on a sample of $(1310.6\pm 7.0)\times {10}^{6}J/\psi$ events collected with the BESIII detector, we present measurements of $J/\psi$ and ${\eta }^{\prime }$ absolute branching fractions using the process $J/\psi \to \gamma {\eta }^{\prime }$. By analyzing events where the radiative photon converts into an $e^{+}{e}^{-}$ pair, the branching fraction for $J/\psi \to \gamma {\eta }^{\prime }$ is measured to be $(5.27\pm 0.03\pm 0.05)\times {10}^{-3}$. The absolute branching fractions of the five dominant decay channels of the ${\eta }^{\prime }$ are then measured for the first time and are determined to be $\mathcal{B}({\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-})=(29.90\pm 0.03\pm 0.55)\%$, $\mathcal{B}({\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-})=(41.24\pm 0.08\pm 1.24)\%$, $\mathcal{B}({\eta }^{\prime }\to \eta {\pi }^0{\pi }^0)=(21.36\pm 0.10\pm 0.92)\%$, $\mathcal{B}({\eta }^{\prime }\to \gamma \omega )=(2.489\pm 0.018\pm 0.074)\%$, and $\mathcal{B}({\eta }^{\prime }\to \gamma \gamma )=(2.331\pm 0.012\pm 0.035)\%$, where the first uncertainties are statistical and the second systematic.

• Received 12 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.142002

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Published by the American Physical Society