Measurement of the CP-violating phase ϕs in B¯s0→J/ψπ+π− decays

Measurement of the mixing-induced CP-violating phase ϕs in B¯s0 decays is of prime importance in probing new physics. Here 7421±105 signal events from the dominantly CP-odd final state J/ψπ+π− are selected in 1fb−1 of pp collision data collected at s=7TeV with the LHCb detector. A time-dependent fit to the data yields a value of ϕs=−0.019−0.174−0.003+0.173+0.004 rad, consistent with the Standard Model expectation. No evidence of direct CP violation is found.