Study of the rare $B_s^0$ and $B^0$ decays into the $π^+π^-µ^+µ^-$ final state

A search for the rare decays Bs0→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 Bs0→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(Bs0→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.