Observation of structures in the $J\mathrm{/}\psi +\psi (2S)$ mass spectrum with the ATLAS detector

A search for resonant structures in the $J\mathrm{/}\psi +\psi$(2S) mass spectrum is performed using proton–proton collision data at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 140 fb${}^{-1}$, recorded by the ATLAS experiment at the LHC. The decay channels of $J\mathrm{/}\psi +\psi \text{(2S)}\to 4\mu$ and $J\mathrm{/}\psi +\psi \text{(2S)}\to 4\mu +2\pi$ are analyzed. An excess near 6.9 GeV is observed in both channels with a combined significance of 8.9$\sigma$. No significant signal is observed near 7.2 GeV, and an upper limit on its yield relative to $X$(6900) is provided. A simultaneous fit with the di-$J\mathrm{/}\psi$ channel is carried out under assumptions regarding the resonance interferences, yielding a ratio of the partial decay widths between the $J\mathrm{/}\psi +\psi \text{(2S)}$ and di-$J\mathrm{/}\psi$ channels of $1.08\pm 0.20_{-0.17}^{+0.40}$ for the resonance near 6.9 GeV.