Observation of CP Violation in B0 →j/ψρ (770)0 Decays

The time-dependent CP asymmetry in B0→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 2βcc¯deff=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 B0→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, Δφs, to the CP-violating phase φs in Bs0→J/ψφ(1020) decays, yielding Δφs=5.0±4.6 mrad.