The lineshape of the compact fully heavy tetraquark

Abstract

Hadrons and their distributions are the most direct observables in experiment, which would shed light on the non-perturbative mystery of quantum chromodynamics (QCD). As the result, any new hadron will challenge our current knowlege on the one hand, and provide additional inputs on the other hand. The fully heavy cccc system observed by LHCb recently opens 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/ψ lineshape, we find that the X( 6900) is only cusp effect from the J/ψψ(3770) channel. In addition, there is also a cusp slightly below 6.8 GeV stemming from the J/ψψ channel. The two 0 ++ tetraquarks behave as two resonances above the di-ηc and di-J/ψ threshold, respectively. The 2 ++ state is a bound state below the di-J/ψ threshold. Furthermore, we find that the X 0 ++ (6035) shows a significant structure in the di-ηc lineshape even after the coupled channel effect. This is an unique feature which can distinguish compact cccc tetraquark from the loosely hadronic molecules.

An updated review of the new hadron states

“…The lineshape of X(6900) was studied in Ref. [653] within the compact tetraquark picture. Properties of the fully-heavy tetraquark states at finite temperature were investigated in Ref.…”

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“…To date, the mass spectra of fully-heavy tetraquarks have been extensively studied using various models and techniques in the literature , most of which assume the di-J/ψ resonances as fully-charm compact tetraquark states. However, the identities of the observed resonances are still under debate; many works have been dedicated to the interpretation of them [61][62][63][64][65][66][67][68][69][70][71][72][73], most considering these resonances as effects of two charmonia produced simultaneously. We call such effects, regardless of whether the two charmonia are bound, a molecule-like state.…”