Using $4.48\times {10}^8\psi (3686)$ events collected with the BESIII detector, we search for the decays ${\chi }_{cJ}\to {\mu }^{+}{\mu }^{-}J/\psi$ through the radiative decays $\psi (3686)\to \gamma {\chi }_{cJ}$, where $J=0$, 1, 2. The decays ${\chi }_{c1,2}\to {\mu }^{+}{\mu }^{-}J/\psi$ are observed, and the corresponding branching fractions are measured to be $\mathcal{B}({\chi }_{c1}\to {\mu }^{+}{\mu }^{-}J/\psi )=(2.51\pm 0.18\pm 0.20)\times {10}^{-4}$ and $\mathcal{B}({\chi }_{c2}\to {\mu }^{+}{\mu }^{-}J/\psi )=(2.33\pm 0.18\pm 0.29)\times {10}^{-4}$, where the first uncertainty is statistical and the second one systematic. No significant ${\chi }_{c0}\to {\mu }^{+}{\mu }^{-}J/\psi$ decay is observed, and the upper limit on the branching fraction is determined to be $2.0\times {10}^{-5}$ at 90% confidence level. Also, we present a study of dimuon invariant mass dependent transition form factor for the decays ${\chi }_{c1,2}\to {\mu }^{+}{\mu }^{-}J/\psi$.

• Received 20 January 2019

DOI:https://doi.org/10.1103/PhysRevD.99.051101

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Published by the American Physical Society