Study of the Rare B0s and B0 Decays into the π+ π− μ+ μ− Final State

A search for the rare decays B0s → π+ π− μ+ μ− and B0 → π+ π− μ+ μ− is performed in a data set corresponding to an integrated luminosity of 3.0 fb^−1 collected by the LHCb detector in proton–proton collisions at centre-of-mass energies of 7 and 8 TeV. Decay candidates with pion pairs that have invariant mass in the range 0.5–1.3 GeV/c2 and with muon pairs that do not originate from a resonance are considered. The first observation of the decay B0s → π+ π− μ+ μ− and the first evidence of the decay B0 → π+ π− μ+ μ− are obtained and the branching fractions, restricted to the dipion-mass range considered, are measured to be B(B0s → π+ π− μ+ μ−) = (8.6 ± 1.5(stat) ± 0.7(syst) ± 0.7(norm)) × 10^−8 and B(B0 → π+ π− μ+ μ−) = (2.11 ± 0.51(stat) ± 0.15(syst) ± 0.16(norm)) × 10^−8, where the third uncertainty is due to the branching fraction of the decay B0 → J/ψ (→ μ+ μ−) K⁎(892)0 (→ K+ π−), used as a normalisation.