Analysis of the resonant components in B^̄0→J/ ψπ⁺π^-

Interpretation of CP violation measurements using charmonium decays, in both the B ^̄0 and B̄s0 systems, can be subject to changes due to "penguin" type diagrams. These effects can be investigated using measurements of the Cabibbo-suppressed B ^̄0→J/ ψπ ⁺π ^- decays. The final state composition of this channel is investigated using a 1.0 fb ^-1 sample of data produced in 7 TeV pp collisions at the LHC and collected by the LHCb experiment. A modified Dalitz-plot analysis is performed using both the invariant mass spectra and the decay angular distributions. An improved measurement of the B ^̄0→J/ψπ ⁺π ^- branching fraction of (3.97±0.09±0.11±0.16)×10 ^-5 is reported where the first uncertainty is statistical, the second is systematic, and the third is due to the uncertainty of the branching fraction of the decay B ^-→J/ψK ^- used as a normalization channel. Significant production of f ₀(500) and ρ(770) resonances is found in the substructure of the J/ψπ ⁺π ^- final state, and this indicates that they are viable final states for CP violation studies. In contrast evidence for the f ₀(980) resonance is not found. This allows us to establish the first upper limit on the branching fraction product B(B ^̄0→J/ψf ₀(980))×B(f ₀(980)→π ⁺π ^-)<1.1×10 ^-6, leading to an upper limit on the absolute value of the mixing angle of the f ₀(980) with the f ₀(500) of less than 31, both at 90% confidence level.