Evidence for the rare decay $B^{+} \to \bar \Lambda p \mu^{+} \mu^{-}$

Abstract

A search for the rare decay $B^{+} \to \bar \Lambda p \mu^{+} \mu^{-}$ 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 $m(\bar \Lambda p)<2.8$ GeV/$c^2$. The branching fraction is measured to be ${\cal B}_{\rm low}({B^{+}}\to {\bar \Lambda p \mu^{+} \mu^{-}})=\left(1.70 ^{+0.65}_{-0.56}(\rm stat) \pm 0.17(\rm syst) \pm 0.14(\rm ext)\right) \times 10^{-8}$, where the last uncertainty is due to external inputs on ${\cal B}(B^+\to{J/\psi \bar \Lambda p})\times {\cal B}({J/\psi}\to{\mu^+\mu^-})$. With no significant signal observed in the high $m(\bar \Lambda p)$ region above 2.8 GeV/$c^2$, an upper limit is set to be ${\cal B}_{\rm high}(B^{+}\to {\bar \Lambda p \mu^{+} \mu^{-}})<2.8\,(3.7) \times 10^{-9}$ at the $90\%$ ($95\%$) confidence level.