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⁢cc¯¯tetraquarks decaying into a pair of charmonium states in the four muon final state using proton-proton collision data at √𝑠 = 13  TeV, corresponding to an integrated luminosity of 140  fb⁻¹ recorded by the ATLAS experiment at LHC. Two decay channels, 𝐽/𝜓+𝐽/𝜓→4⁢𝜇 and 𝐽/𝜓+𝜓⁡(2⁢𝑆)→4⁢𝜇, 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/𝜓 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/𝜓+𝜓⁡(2⁢S) channel. The fitted masses and decay widths of the structures are reported.