Observation of structures in the J/ψ + ψ(2S) mass spectrum with the ATLAS detector

A search for resonant structures in the J/ψ + ψ(2S) mass spectrum is performed using proton-proton collision data at √s = 13 TeV, corresponding to an integrated luminosity of 140 fb⁻¹ , recorded by the ATLAS experiment at the LHC. The decay channels of J/ψ + ψ(2S) → 4μ and J/ψ + ψ(2S)→ 4μ + 2π are analyzed. An excess near 6.9 GeV is observed in both channels with a combined significance of 8.9σ. No significant signal is observed near 7.2 GeV, and an upper limit on its yield relative to X(6900) is provided. A simultaneous fit with the di- J/ψ channel is carried out under assumptions regarding the resonance interferences, yielding a ratio of the partial decay widths between the J/ψ + ψ(2S) and di- J/ψ channels of 1.08 ± 0.20^+0.40_−0.17 for the resonance near 6.9 GeV.