Measurement of the time-dependent CP asymmetries in B s 0 → J / ψK S 0

The first measurement of decay-time-dependent CP asymmetries in the decay B s 0 → J / ψK S 0 and an updated measurement of the ratio of branching fractions ℬ(B_s^0→ J/ψK_S^0)/ℬ(B^0→ J/ψK_S^0) are presented. The results are obtained using data corresponding to an integrated luminosity of 3.0 fb −1 of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of 7 and 8 TeV. The results on the CP asymmetries are [ 𝒜_ΔΓ(B_s^0→ J/ψK_S^0)=0.49±_0.65^0.77(stat)± 0.06(syst),; C_dir(B_s^0→ J/ψK_S^0)=-0.28± 0.41(stat)± 0.08(syst),; S_mix(B_s^0→ J/ψK_S^0)=-0.08± 0.40(stat)± 0.08(syst). ] The ratio ℬ(B_s^0→ J/ψK_S^0)/ℬ(B^0→ J/ψK_S^0) is measured to be 0.0431± 0.0017(stat)± 0.0012(syst)± 0.0025(f_s/f_d), where the last uncertainty is due to the knowledge of the B s 0 and B 0 production fractions.