Observation of the Λb0→χc1 (3872) pK⁻ decay

Using proton-proton collision data, collected with the LHCb detector and corresponding to 1.0, 2.0 and 1.9 fb⁻¹ of integrated luminosity at the centre-of-mass energies of 7, 8, and 13 TeV, respectively, the decay Λb0→χc1(3872)pK⁻ with χ_c1(3872) → J/ψ π⁺π⁻ is observed for the first time. The significance of the observed signal is in excess of seven standard deviations. It is found that (58 ± 15)% of the decays proceed via the two-body intermediate state χ_c1(3872)Λ(1520). The branching fraction with respect to that of the Λb0 → ψ(2S)pK⁻ decay mode, where the ψ(2S) meson is reconstructed in the J/ψ π⁺π⁻ final state, is measured to be:β(Λb0→χc1(3872)pK−)β(Λb0→ψ(2S)pK−)×β(χc1(3872)→J/ψπ+π−)β(ψ(2S)→J/ψπ+π−)=(5.4±1.1±0.2)×10−2, where the first uncertainty is statistical and the second is systematic. [Figure not available: see fulltext.]