Observation of the decay B-s(0) -> phi pi(+)pi(-) and evidence for B-0 -> phi pi(+)pi(-)

The first observation of the rare decayB0s→ϕπ+π− and evidence for B0→ϕπ+π− are reported, using pp collision data recorded by the LHCb detector at centre-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−)<1600MeV/c2 are [3.48±0.23±0.17±0.35]×10−6 and [1.82±0.25±0.41±0.14]×10−7 for B0s→ϕπ+π− and B0→ϕπ+π− respectively, where the uncertainties are statistical, systematic and from the normalisation mode B0s→ϕϕ. A combined analysis of the π+π− mass spectrum and the decay angles of the final-state particles identifies the exclusive decays B0s→ϕf0(980), B0s→ϕf2(1430), and B0s→ϕρ with branching fractions of [1.12±0.16+0.09−0.08±0.11]×10−6, [0.61±0.13+0.12−0.05±0.06]×10−6 and [2.7±0.7±0.2±0.2]×10−7, respectively.