Observation of an Excess of Dicharmonium Events in the Four-Muon Final State with the ATLAS Detector

A search is made for potential $cc\overline{c}\overline{c}$ tetraquarks decaying into a pair of charmonium states in the four muon final state using proton-proton collision data at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$, corresponding to an integrated luminosity of $140\text{}\text{}{\mathrm{fb}}^{-1}$ recorded by the ATLAS experiment at LHC. Two decay channels, $J/\psi +J/\psi \to 4\mu $ and $J/\psi +\psi \left(2S\right)\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/\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 $J/\psi +\psi \left(2S\right)$ channel. The fitted masses and decay widths of the structures are reported.