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

Abstract

The first observation of the rare decay $B^0_s \to \phi\pi^+\pi^-$ and evidence for $B^0 \to \phi\pi^+\pi^-$ 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 $\pi^+\pi^-$ invariant mass range $400 < m(\pi^+\pi^-) < 1600  MeV/c^2$ are $[3.48 \pm 0.23 \pm 0.17 \pm 0.35] \times 10^{−6}$ and $[1.82 \pm 0.25 \pm 0.41 \pm 0.14] \times 10^{−7}$ for $B^0_s \to \phi\pi^+\pi^-$ and $B^0 \to \phi\pi^+\pi^-$ respectively, where the uncertainties are statistical, systematic and from the normalization mode $B^0_s \to \phi\phi$. A combined analysis of the $\pi^+\pi^-$ mass spectrum and the decay angles of the final-state particles identifies the exclusive decays $B^0_s \to \phi f_0(980), B^0_s \to \phi f_2(1270)$, and $B^0_s \to \phi\rho^0$ with branching fractions of $[1.12 \pm 0.16^{+0.09}_{−0.08} \pm 0.11] \times 10^{−6}, [0.61 \pm 0.13^{+0.12}_{−0.05} \pm 0.06] \times 10^{−6}$ and $[2.7 \pm 0.7 \pm 0.2 \pm 0.2] \times 10^{−7}$, respectively.