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 apair of charmonium states in the four muon final state using proton-protoncollision data at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosityof 140 fb$^{-1}$ recorded by the ATLAS experiment at LHC. Two decay channels,$J/\psi+J/\psi \rightarrow 4\mu$ and $J/\psi+\psi(2S) \rightarrow 4\mu$, arestudied. Backgrounds are estimated based on a hybrid approach involving MonteCarlo simulations and data-driven methods. Statistically significant excesseswith respect to backgrounds dominated by the single parton scattering are seenin the di-$J/\psi$ channel consistent with a narrow resonance at 6.9 GeV and abroader structure at lower mass. A statistically significant excess is alsoseen in the $J/\psi$+$\psi$(2S) channel. The fitted masses and decay widths ofthe structures are reported.