Observation of an excess of di-charmonium events in the four-muon final state with the ATLAS detector

A search is made for potential $cc\bar{c}\bar{c}$ tetraquarks decaying into a pair of charmonium states in the four muon final state 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 LHC. Two decay channels, $J\mathrm{/}\psi +J\mathrm{/}\psi \to 4\mu$ and $J\mathrm{/}\psi +\psi (2S)\to 4\mu$, are studied. Backgrounds are estimated based on a hybrid approach involving Monte Carlo simulations and data-driven methods. Statistically significant excesses with respect to backgrounds dominated by the single parton scattering are seen in the di-$J\mathrm{/}\psi$ channel consistent with a narrow resonance at 6.9 GeV and a broader structure at lower mass. A statistically significant excess is also seen in the $J\mathrm{/}\psi$+$\psi$(2S) channel. The fitted masses and decay widths of the structures are reported.