Measurement of CP violation in B⁰D^+D^{-} and B^{0ₒ}D^+ₒD^{-ₒ} decays

A time-dependent, flavour-tagged measurement of CP violation is performed with B⁰D^+D^{-} and B^{0ₒ}D^+ₒD^{-ₒ} decays, using data collected by the LHCb detector in proton-proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 6 fb^-1. In B⁰D^+D^{-} decays the CP-violation parameters are measured to be align S₃^+D^{-} & = -0. 552 0. 100\, (stat) 0. 010\, (syst), C₃^+D^{-} & = -0. 128 0. 103\, (stat) 0. 010\, (syst). align In B^0ₒ D^+ₒD^-ₒ decays the CP-violating parameter formulation in terms of ₒ and || results in align ₒ & = -0. 086 0. 106 \, (stat) 0. 028\, (syst) \, rad, |₃^+ₒD^{-ₒ}| & = -1. 145 0. 126\, (stat) 0. 031\, (syst). align These results represent the most precise single measurement of the CP-violation parameters in their respective channels. For the first time in a single measurement, CP symmetry is observed to be violated in B⁰D^+D^{-} decays with a significance exceeding six standard deviations.