Study of the rare Bs0 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 B 0 → 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.