Study of the rare $B^{0}_{s}$ and $B^{0}$ decays into the $\pi^{+}\pi^{-}\mu^{+}\mu^{-}$ final state at LHCb

A search for the rare decays B0s->pi pi mu mu and B0->pi pi mu mu 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 8TeV. 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->pi pi mu mu and the first evidence of the decay B0->pi pi mu mu are obtained and the branching fractions, restricted to the dipion-mass range considered, are measured to be Br(B0s->pi pi mu mu) = (8.6 +/- 1.5 (stat) +/- 0.7 (syst) +/- 0.7 (norm))*10^-8 and Br(B0->pi pi mu mu) = (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/psi K*, used as a normalisation.