Study of electromagnetic Dalitz decays χcJ→μ+μ−J/ψ

Using 4.48 × 108 ψð3686Þ events collected with the BESIII detector, we search for the decays χcJ → μþμ−J=ψ through the radiative decays ψð3686Þ → γχcJ, where J 1⁄4 0, 1, 2. The decays χc1;2 → μþμ−J=ψ are observed, and the corresponding branching fractions are measured to be Bðχc1 → μþμ−J=ψÞ 1⁄4 ð2.51 ` 0.18 ` 0.20Þ × 10−4 and Bðχc2 → μþμ−J=ψÞ 1⁄4 ð2.33 ` 0.18 ` 0.29Þ × 10−4, where the first uncertainty is statistical and the second one systematic. No significant χc0 → μþμ−J=ψ decay is observed, and the upper limit on the branching fraction is determined to be 2.0 × 10−5 at 90% confidence level. Also, we present a study of dimuon invariant mass dependent transition form factor for the decays χc1;2 → μþμ−J=ψ.