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

A search for potential $cc\bar{c}\bar{c}$ tetraquarks decaying into a pair of charmonium states in the four muon final state is performed using proton-proton collision data at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS experiment at the Large Hadron Collider (LHC). Two different decay channels, $T_{cc\bar{c}\bar{c}}\to J/\psi~J/\psi\to 4\mu$ and $T_{cc\bar{c}\bar{c}}\to J/\psi~\psi(2S)\to 4\mu$, are studied. Various backgrounds to the resonance searches are estimated based on a hybrid approach involving both Monte Carlo simulations and data-driven methods. Statistically significant excesses 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 $J/\psi$+$\psi$(2S) channel. The fitted masses and decay widths of the structures are reported.