Observation of structure in the $J/\psi$-pair mass spectrum

Abstract

Using proton-proton collision data at centre-of-mass energies of $\sqrt{s}=7$, $8$ and $13\mathrm{TeV}$ recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$, the invariant mass spectrum of $J/\psi$ pairs is studied. A narrow structure around $6.9GeV/c^2$ matching the lineshape of a resonance and a broad structure just above twice the $J/\psi$ mass are observed. The deviation of the data from nonresonant $J/\psi$-pair production is above five standard deviations in the mass region between $6.2$ and $7.4GeV/c^2$, 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.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2020-011.html (LHCb public pages)