First measurement of the CP -violating phase in Bs0→J/ψ(→ e⁺e^-) ϕ decays

A flavour-tagged time-dependent angular analysis of Bs0→J/ψϕ decays is presented where the J/ ψ meson is reconstructed through its decay to an e⁺e^- pair. The analysis uses a sample of pp collision data recorded with the LHCb experiment at centre-of-mass energies of 7 and 8 \,Te V , corresponding to an integrated luminosity of 3 \,fb ^{- 1}. The CP-violating phase and lifetime parameters of the Bs0 system are measured to be ϕ_s= 0.00 ± 0.28 ± 0.07 \,rad , Δ Γ _s= 0.115 ± 0.045 ± 0.011 \,ps ^{- 1} and Γ _s= 0.608 ± 0.018 ± 0.012 \,ps ^{- 1} where the first uncertainty is statistical and the second systematic. This is the first time that CP-violating parameters are measured in the Bs0→J/ψϕ decay with an e⁺e^- pair in the final state. The results are consistent with previous measurements in other channels and with the Standard Model predictions.