Insights into the inner structures of the fully charmed tetraquark state X(6900)

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.