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

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