Evidence for the Rare Decay B^{+}→Λ[over ¯]pμ^{+}μ^{-}

A search for the rare decay B^{+}→Λ[over ¯]pμ^{+}μ^{-} is performed using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of sqrt[s]=13  TeV, corresponding to an integrated luminosity of 5.4  fb^{-1}. An excess of events is found with respect to the background-only expectation, with a signal significance of 3.5 standard deviations, in the low invariant-mass region of m(Λ[over ¯]p)<2.8  GeV/c^{2}. The branching fraction is measured to be B_{low}(B^{+}→Λ[over ¯]pμ^{+}μ^{-})=(1.70_{-0.56}^{+0.65}(stat)±0.17(syst)±0.14(ext))×10^{-8}, where the last uncertainty is due to external inputs on B(B^{+}→J/ψΛ[over ¯]p)×B(J/ψ→μ^{+}μ^{-}). With no significant signal observed in the high m(Λ[over ¯]p) region above 2.8  GeV/c^{2}, an upper limit is set to be B_{high}(B^{+}→Λ[over ¯]pμ^{+}μ^{-})<2.8(3.7)×10^{-9} at the 90% (95%) confidence level.