Observation of structure in the J/ψ-pair mass spectrum

Using proton-proton collision data at centre-of-mass energies of √s = 7; 8 and 13 TeV recorded by the ffiffi LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 9 fb^-1, the invariant mass spectrum of J/w pairs is studied. A narrow structure around 6:9 GeV=c² matching the lineshape of a resonance and a broad structure just above twice the J/ψ mass are observed. The deviation of the data from nonresonant J/ψ-pair production is above five standard deviations in the mass region between 6:2 and 7:4 GeV=c², covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow X(6900) structure are measured assuming a Breit-Wigner lineshape.