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

A search for resonant structures in the J / ψ + ψ ( 2 S ) 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 ( 2 S ) → 4 μ and J / ψ + ψ ( 2 S ) → 4 μ + 2 π are analyzed. An excess near 6.9 GeV is observed in both channels with a combined significance of 8.9 σ . 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 ( 2 S ) and di- J / ψ channels of 1.08 ± 0.2 0 − 0.17 + 0.40 for the resonance near 6.9 GeV.