The third peak structure in the double $J/\psi$ spectrum

Recently, the CMS and ATLAS collaborations have reported their $J/\psi J/\psi $ and $J/\psi \psi^\prime$ invariant mass distributions, respectively, for searching for fully charmed tetraquarks. Both of them reported the existence of a peak structure around $7.2~\gev$. In this article, we exhibit the role of the $\eta_c(2S)\eta_c(2S)$ channel, which is close to this peak position, by studying the $J/\psi J/\psi $ and $J/\psi \psi^\prime$ invariant mass distributions for the potential quantum numbers $J^{PC}=0^{++}$ or $2^{++}$ by considering both the coherence and incoherence with the background contribution. All these frameworks can describe the experimental data very well, however with different pole structures. For instance, in the case of $2^{++}$ description of these structures, there always exists a pole slightly below the $J/\psi J/\psi$ threshold. For the $0^{++}$ case, a similar pole can also be found below the $J/\psi J/\psi$ threshold however with a much lower mass. More importantly, the number of the poles is found to be case-dependent. For the $0^{++}$ ($2^{++}$) case, the peak structure around $7.2~\gev$ can (cannot) be produced due to the presence (absence) of the $\eta_c(2S)\eta_c(2S)$ channel. Although the pole positions are case-dependent, the relation between the peak structure in the $J/\psi J/\psi$ invariant mass distribution and the dip structure in the $J/\psi \psi^\prime$ invariant mass distribution around $7.2~\gev$ is unambiguous. We suggest experimentalists to detailed scan both the $J/\psi J/\psi$ and the $J/\psi \psi^\prime$ invariant mass distributions, especially around $7.2~\gev$ to probe the nature of the third fully charmed state.