Lineshape of the compact fully heavy tetraquark

Hadrons and their distributions are the most direct observables in experiments, which would shed light on the nonperturbative mystery of quantum chromodynamics. As the result, any new hadron will challenge our current knowledge on the one hand, and provide additional inputs on the other hand. The fully heavy cc (c) over bar(c) over bar system observed by LHCb recently opened a new era for hadron physics. We first extract the internal structure of the fully heavy tetraquarks directly from the experimental data, within the compact tetraquark picture. By fitting to the di-J/psi lineshape, we find that the X(6900) is only the cusp effect from the J/psi psi(3770) channel. In addition, there is also a cusp slightly below 6.8 GeV stemming from the J/psi psi' channel. The two 0(++ )tetraquarks behave as two resonances 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 the di-eta(c) lineshape even after the coupled channel effect. This is an unique feature which can distinguish compact cc (c) over bar(c) over bar tetraquark from the loosely hadronic molecules.

Automated summary

Observing hadrons and their distributions directly in experiments sheds light on the nonperturbative mystery of quantum chromodynamics, with new hadrons challenging current knowledge and providing additional inputs; the observation of fully heavy cc over bar(c) over bar system by LHCb opens a new era for hadron physics, where internal structure is extracted from experimental data.