Measurement of the CP-violating phase β in B0→J/ψπ+π−B0→J/ψπ+π− decays and limits on penguin effects

Time-dependent CP   violation is measured in the B0→J/ψπ+π− channel for each π+π−π+π− resonant final state using data collected with an integrated luminosity of 3.0 fb−1 in pp   collisions using the LHCb detector. The final state with the largest rate, J/ψρ0(770)J/ψρ0(770), is used to measure the CP  -violating angle 2βeff2βeff to be (41.7±9.6−6.3+2.8)°. This result can be used to limit the size of penguin amplitude contributions to CP   violation measurements in, for example, Bs0→J/ψϕ decays. Assuming approximate SU(3) flavour symmetry and neglecting higher order diagrams, the shift in the CP  -violating phase ϕsϕs is limited to be within the interval [−1.05°,+1.18°−1.05°,+1.18°] at 95% confidence level. Changes to the limit due to SU(3) symmetry breaking effects are also discussed.