Analysis of the resonant components in ¯¯¯B0→J/ψπ+π−

Abstract

Interpretation of C P violation measurements using charmonium decays, in both the ¯¯¯ B 0 and ¯¯¯ B 0 s 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 / ψ π + π − decays. The final state composition of this channel is investigated using a 1.0 fb − 1 sample of data produced in 7 TeV p p 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 / ψ π + 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 / ψ K − used as a normalization channel. Significant production of f 0 ( 500 ) and ρ ( 770 ) resonances is found in the substructure of the J / ψ π + π − final state, and this indicates that they are viable final states for C P 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 / ψ f 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.