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Hadrons and their distributions are the most direct observables inexperiment, which would shed light on the non-perturbative mystery of quantumchromodynamics (QCD). As the result, any new hadron will challenge our currentknowlege on the one hand, and provide additional inputs on the other hand. Thefully heavy $cc\bar{c}\bar{c}$ system observed by LHCb recently opens a new erafor hadron physics. We first extract the internal structure of the fully heavytetraquarks directly from the experimental data, within the compact tetraquarkpicture. By fitting to the di-$J/\psi$ lineshape, we find that the $X(6900)$ isonly cusp effect from the $J/\psi\psi(3770)$ channel. In addition, there isalso a cusp slightly below $6.8~\mathrm{GeV}$ stemming from the$J/\psi\psi^\prime$ channel. The two $0^{++}$ tetraquarks behave as tworesonances above the di-$\eta_c$ and di-$J/\psi$ threshold, respectively. The$2^{++}$ state is a bound state below the di-$J/\psi$ threshold. Furthermore,we find that the $X_{0^{++}}(6035)$ shows a significant structure in thedi-$\eta_c$ lineshape even after the coupled channel effect. This is an uniquefeature which can distinguish compact $cc\bar{c}\bar{c}$ tetraquark from theloosely hadronic molecules.

The lineshape of the compact fully heavy tetraquark