Study of the rare And Decays Into the Final State

A search for the rare decays ${B}_{s}^{0}\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ and ${B}^{0}\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ is performed in a data set corresponding to an integrated luminosity of $3.0{\text{fb}}^{-1}$ collected by the LHCb detector in proton–proton collisions at centre-of-mass energies of 7 and $8\text{TeV}$. Decay candidates with pion pairs that have invariant mass in the range $0.5\text{–}1.3\text{GeV}/{c}^{2}$ and with muon pairs that do not originate from a resonance are considered. The first observation of the decay ${B}_{s}^{0}\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ and the first evidence of the decay ${B}^{0}\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ are obtained and the branching fractions, restricted to the dipion-mass range considered, are measured to be $B\left({B}_{s}^{0}\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\right)=\left(8.6±1.5\text{}\left(\text{stat}\right)±0.7\text{}\left(\text{syst}\right)±0.7\phantom{\rule{0.25em}{0ex}}\left(\text{norm}\right)\right)×{10}^{-8}$ and $B\left({B}^{0}\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\right)=\left(2.11±0.51\phantom{\rule{0.25em}{0ex}}\left(\text{stat}\right)±0.15\phantom{\rule{0.25em}{0ex}}\left(\text{syst}\right)±0.16\phantom{\rule{0.25em}{0ex}}\left(\text{norm}\right)\right)×{10}^{-8}$, where the third uncertainty is due to the branching fraction of the decay ${B}^{0}\to J/\psi \left(\to {\mu }^{+}{\mu }^{-}\right){K}^{⁎}{\left(892\right)}^{0}\left(\to {K}^{+}{\pi }^{-}\right)$, used as a normalisation.