Evidence for the Rare Decay B+ → Λ ¯ pμ+μ

A search for the rare decay B+→Λ¯pμ+μ- is performed using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of 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¯p)<2.8 GeV/c2. The branching fraction is measured to be Blow(B+→Λ¯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¯p)×B(J/ψ→μ+μ-). With no significant signal observed in the high m(Λ¯p) region above 2.8 GeV/c2, an upper limit is set to be Bhigh(B+→Λ¯pμ+μ-)<2.8(3.7)×10-9 at the 90% (95%) confidence level.