Fully heavy tetraquark states and their evidences in LHC observations

Stimulated by the exciting progress on the observations of the fully charmed tetraquarks at LHC, we carry out a combined analysis of the mass spectra and fall-apart decays of the $1S$-, $2S$-, and $1P$-wave $cc\overline{c}\overline{c}$ states in a nonrelativistic quark model. It is found that the $X(6600)$ structure observed in the di-$J/\psi$ invariant mass spectrum can be explained by the $1S$-wave state $T_{(4c)0^{++}}(6550)$. This structure may also bear some feed-down effects from the higher $2S$ and/or $1P$ tetraquark states. The $X(6900)$ structure observed in both the di-$J/\psi$ and $J/\psi \psi (2S)$ channels can be naturally explained by the $2S$-wave state $T_{(4c)0^{++}}(6957)$. The small shoulder structure around 6.2–6.4 GeV observed at CMS and ATLAS may be due to the feed-down effects from some $1P$-wave states with $C=-1$ and/or some $2S$-wave states with $J^{PC}=0^{++}$. Other decay channels are implied in such a scenario and they can be investigated by future experimental analyses. Considering the large discovery potential at LHC, we also present predictions for the $bb\overline{b}\overline{b}$ states which can be searched for in the future.

Figure 1

The coordinates defined for a $T_{(4Q)}$ system and its fall-apart decays into a $BC$ meson pair via the quark rearrangement. The $BC$ final state can be formed via two quark rearrangement ways: $(Q_1{\overline{Q}}_3)(Q_2{\overline{Q}}_4)$ and $(Q_1{\overline{Q}}_4)(Q_2{\overline{Q}}_3)$ as shown in the figure.

Figure 2

Mass spectra for the $cc\overline{c}\overline{c}$ and $bb\overline{b}\overline{b}$ systems.

Figure 3

The fall-apart decays of a $T_{(4Q)}$ state induced by the interactions $V_{ij}$ ($ij\ne 13$, 24 or $ij\ne 14$, 23) between inner quarks of final hadrons $B$ and $C$.