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Observation of the $ψ(1^3D_2)$ state in $e^+e^-\toπ^+π^-γχ_{c1}$ at BESIII

We report the observation of the $X(3823)$ in the process $e^+e^-\to π^+π^-X(3823) \to π^+π^-γχ_{c1}$ with a statistical significance of $6.2σ$, in data samples at center-of-mass energies $\sqrt{s}=$4.230, 4.260, 4.360, 4.420 and 4.600~GeV collected with the BESIII detector at the BEPCII electron positron collider. The measured mass of the $X(3823)$ is $(3821.7\pm 1.3\pm 0.7)$~MeV/$c^2$, where the first error is statistical and the second systematic, and the width is less than $16$~MeV at the 90\% confidence level. The products of the Born cross sections for $e^+e^-\to π^+π^-X(3823)$ and the branching ratio $\mathcal{B}[X(3823)\to γχ_{c1,c2}]$ are also measured. These measurements are in good agreement with the assignment of the $X(3823)$ as the $ψ(1^3D_2)$ charmonium state.