Observation of the Bc⁺→ J/ψ π⁺π⁰ decay

The first observation of the Bc⁺ → J/ψ π⁺ π⁰ decay is reported with high significance using proton-proton collision data, corresponding to an integrated luminosity of 9fb⁻¹, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV. The ratio of its branching fraction relative to the Bc⁺ → J/ψ π⁺ channel is measured to be B( Bc⁺ → J/ψ π⁺π⁰ ) B( Bc⁺ → J/ψ π⁺ ) = 2.80 ± 0.15 ± 0.11 ± 0.16 \,, where the first uncertainty is statistical, the second systematic and the third related to imprecise knowledge of the branching fractions for B⁺ → J/ψ K*+ and B⁺ → J/ψ K⁺ decays, which are used to determine the π⁰ detection efficiency. The π⁺π⁰ mass spectrum is found to be consistent with the dominance of an intermediate ρ⁺ contribution in accordance with a model based on QCD factorisation.