Observation of a Resonant Structure near the Ds+ Ds− Threshold in the B+ → Ds+ Ds− K+ Decay

An amplitude analysis of the B+ → Ds+ Ds− K+ decay is carried out to study for the first time its intermediate resonant contributions, using proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. A near-threshold peaking structure, referred to as X(3960), is observed in the Ds+ Ds− invariant-mass spectrum with significance greater than 12 standard deviations. The mass, width, and the quantum numbers of the structure are measured to be 3956 ± 5 ± 10 MeV, 43 ± 13 ± 8 MeV, and J^PC = 0^++, respectively, where the first uncertainties are statistical and the second systematic. The properties of the new structure are consistent with recent theoretical predictions for a state composed of c anti-c s anti-s quarks. Evidence for an additional structure is found around 4140 MeV in the Ds+ Ds− invariant mass, which might be caused either by a new resonance with the 0^++ assignment or by a J/ψ ϕ ↔ Ds+ Ds− coupled-channel effect.