We systematically study the 1D charmonium spin-triplet (with the Formula: see text) and spin-singlet (with the Formula: see text) via the QCD sum rules in comparison with the recent experimental results. The predicted mass Formula: see text supports identifying the Formula: see text as the Formula: see text, the value Formula: see text is consistent with the reported observation of the Formula: see text, the prediction Formula: see text supports identifying the Formula: see text as the Formula: see text. Additionally, we estimate the unobserved Formula: see text state lies at Formula: see text, and suggest detection prospects in the future. More experimental data will help us to unravel the mass spectrum of the charmonium states near the open-charm thresholds.
Xin et al. (Thu,) studied this question.