Observation of the decay $B^0_s \to \phi\pi^+\pi^-$ and evidence for $B^0 \to \phi\pi^+\pi^-$

The first observation of the rare decay Bs0→ϕπ+π- and evidence for B0→ϕπ+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/c2 are [3.48±0.23±0.17±0.35]×10-6 and [1.82±0.25±0.41±0.14]×10-7 for Bs0→ϕπ+π- and B0→ϕπ+π- respectively, where the uncertainties are statistical, systematic and from the normalization mode Bs0→ϕϕ. A combined analysis of the π+π- mass spectrum and the decay angles of the final-state particles identifies the exclusive decays Bs0→ϕf0(980), Bs0→ϕf2(1270), and Bs0→ϕρ0 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.