Observation of the Decay B0s → ϕ π+ π− and Evidence for B0 → ϕ π+ π−

The first observation of the rare decay B0s → ϕ π+ π− and evidence for B0 → ϕ π+ π− are reported, using pp collision data recorded by the LHCb detector at center-of-mass energies √s = 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb^−1. The branching fractions in the π+ π− invariant mass range 400 < m(π+ π−) < 1600 MeV/c^2 are [3.48 ± 0.23 ± 0.17 ± 0.35] × 10^−6 and [1.82 ± 0.25 ± 0.41 ± 0.14] × 10^−7 for B0s → ϕ π+ π− and B0 → ϕ π+ π− respectively, where the uncertainties are statistical, systematic and from the normalization mode B0s → ϕ ϕ . A combined analysis of the π+ π− mass spectrum and the decay angles of the final-state particles identifies the exclusive decays B0s → ϕ f0(980), B0s → ϕ f2(1270), and B0s → ϕ ρ0 with branching fractions of [1.12 ± 0.16{−0.08}{+0.09} ± 0.11] × 10^−6, [0.61 ± 0.13{−0.05}{+0.12} ± 0.06] × 10^−6 and [2.7 ± 0.7 ± 0.2 ± 0.2] × 10^−7, respectively.