The recently discovered fully charmed tetraquark candidate $X(6900)$ is analyzed within the frameworks of effective-range expansion, compositeness relation and width saturation, and a coupled multichannel dynamical study. By taking into account constraints from heavy-quark spin symmetry, the coupled-channel amplitude including the $J/\psi J/\psi$, ${\chi }_{c0}{\chi }_{c0}$, and ${\chi }_{c1}{\chi }_{c1}$ is constructed to fit the experimental di-$J/\psi$ event distributions around the energy region near 6.9 GeV. Another dynamical two-coupled-channel amplitude with the $J/\psi J/\psi$ and $\psi (3770)J/\psi$ is also considered to describe the same datasets. The three different theoretical approaches lead to similar conclusions that the two-meson components do not play dominant roles in the $X(6900)$. Our determinations of the resonance poles in the complex energy plane from the refined coupled-channel study are found to be consistent with the experimental analyses. The coupled-channel amplitudes also have another pole corresponding to a narrow resonance $X(6825)$ that we predict sitting below the ${\chi }_{c0}{\chi }_{c0}$ threshold and of molecular origin. We give predictions to the line shapes of the ${\chi }_{c0}{\chi }_{c0}$ and ${\chi }_{c1}{\chi }_{c1}$ channels, which could provide a useful guide for future experimental measurements.

• Received 5 November 2020
• Accepted 3 February 2021

DOI:https://doi.org/10.1103/PhysRevD.103.034024

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Published by the American Physical Society