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 LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 9 fb - 1 , the invariant mass spectrum of J / ψ pairs is studied. A narrow structure around 6.9 GeV / c 2 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 / c 2 , 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.