Measurement of CP violation in B⁰ → D⁺D⁻ and B⁰_s → D⁺_s D⁻_s decays

A time-dependent, flavour-tagged measurement of CP violation is performed with B⁰→D⁺D⁻ and B⁰_s→D⁺_sD⁻_s 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 B⁰→D⁺D⁻ decays the CP-violation parameters are measured to be

S_D+D− = −0.552 ± 0.100(stat) ± 0.010(syst),
C_D+D− = 0.128 ± 0.103(stat) ± 0.010(syst).

In B⁰_s→D⁺_sD⁻_s decays the CP-violating parameter formulation in terms of ϕ_s and |λ| results in

ϕ_s = −0.086 ± 0.106(stat) ± 0.028(syst) rad,
|λ_D+sD−s| = 1.145 ± 0.126(stat) ± 0.031(syst).

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 B⁰→D⁺D⁻ decays with a significance exceeding six standard deviations.