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

• DOI: 10.1016/j.scib.2020.08.032
• Publication Date: 2020-08-29T07:42:02Z
• Authors: LHCb collaboration
• Keywords: Invariant mass; Charm quark

Using proton-proton collision data at centre-of-mass energies of $\sqrt{s} = 7$, $8$ and $13\mathrm{\,TeV}$ recorded by the LHCb experiment Large Hadron Collider, corresponding to an integrated luminosity $9\mathrm{\,fb}^{-1}$, invariant mass spectrum $J/ψ$ pairs is studied. A narrow structure around $6.9\mathrm{\,GeV/}c^2$ matching lineshape a resonance broad just above twice are observed. The deviation from nonresonant $J/ψ$-pair production five standard deviations in region between $6.2$ $7.4\mathrm{\,GeV/}c^2$, covering predicted masses states composed four charm quarks. natural width $X(6900)$ measured assuming Breit--Wigner lineshape.