Observation of the Bs0 → χc1(3872)π⁺π⁻ decay

The first observation of the B _s ⁰→ (χ _c1(3872) → J/ψπ ⁺π ⁻)π ⁺π ⁻ decay is reported using proton-proton collision data, corresponding to integrated luminosities of 1, 2 and 6 fb ⁻¹, collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, respectively. The ratio of branching fractions relative to the B _s ⁰→ (ψ(2S) → J/ψπ ⁺π ⁻)π ⁺π ⁻ decay is measured to be (Formula presented.) where the first uncertainty is statistical and the second systematic. The mass spectrum of the π ⁺π ⁻ system recoiling against the χ _c1(3872) meson exhibits a large contribution from B _s ⁰→ χ _c1(3872) (f ₀(980) → π ⁺π ⁻) decays.