First observation of the ⁰b D^+ D^- decay

The ⁰b D^+ D^- decay is observed for the first time using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5. 3 fb^-1. Using the B⁰ D^+ D^- Kₒ⁰ decay as a reference channel, the product of the relative production cross-section and decay branching fractions is measured to be R=䃀₁₁䃀 B (⁰b D^+ D^-) { B (B⁰ D^+ D^- Kₒ⁰) }=0. 179 0. 022 0. 014 where the first uncertainty is statistical and the second is systematic. The known branching fraction of the reference channel, B (B⁰ D^+ D^- Kₒ⁰), and the cross-section ratio, 䃀₁ / ₁䃀, previously measured by LHCb are used to derive the branching fraction of the ⁰b D^+ D^- decay B (⁰b D^+ D^-) = (1. 24 0. 15 0. 10 0. 28 0. 11) 10^-4, where the third and fourth contributions are due to uncertainties of B (B⁰ D^+ D^- Kₒ⁰) and 䃀₁ / ₁䃀, respectively. Inspection of the D^+ and D^+ D^- invariant-mass distributions suggests a rich presence of intermediate resonances in the decay. The ⁰b D^*+ D^- decay is also observed for the first time as a partially reconstructed component in the D^+ D^- invariant mass spectrum.