Observation of CP Violation in B^{0}→J/ψρ(770)^{0} Decays

The time-dependent CP asymmetry in B^{0}→J/ψρ(770)^{0} decays is measured using proton-proton collision data corresponding to an integrated luminosity of 6 fb^{-1}, collected with the LHCb detector at a center-of-mass energy of 13 TeV during the years 2015-2018. The CP-violation parameters for this process are determined to be 2β_{cc[over ¯]d}^{eff}=0.710±0.084±0.051 rad and |λ|=1.019±0.034±0.024, where the first uncertainty is statistical and the second systematic. This constitutes the first observation of time-dependent CP violation in B^{0}→J/ψρ(770)^{0} decays. Assuming approximate SU(3) flavor symmetry, these results are combined with the previous consistent LHCb measurement to set the most stringent constraint on the penguin contribution, Δϕ_{s}, to the CP-violating phase ϕ_{s} in B_{s}^{0}→J/ψϕ(1020) decays, yielding Δϕ_{s}=5.0±4.6 mrad.