Observation of the ψ(1³D₂) state in e⁺e^-→π⁺π^-γχ_c1at BESIII

We report the observation of the X(3823) state in the process e⁺e^-→π⁺π^-X(3823)→π⁺π^-γχ_c1with a statistical significance of 6.2σ, in data samples at center-of-mass energies s √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) state is (3821.7±1.3±0.7)MeV/c², 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^-→π⁺π^-X(3823) and the branching ratio B[X(3823)→γχ_c1,_c2] are also measured. These measurements are in good agreement with the assignment of the X(3823) state as the ψ(1³D₂) charmonium state.