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

A search for the rare decay $B^{+}\to \bar{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-}$ 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 $m(\bar{\mathrm{\Lambda }}p)<2.8$ GeV/$c^2$. The branching fraction is measured to be ${\mathcal{B}}_{\mathrm{l}\mathrm{o}\mathrm{w}}(B^{+}\to \bar{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-})=(1.70_{-0.56}^{+0.65}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.17(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\pm 0.14(\mathrm{e}\mathrm{x}\mathrm{t}))\times {10}^{-8}$, where the last uncertainty is due to external inputs on $\mathcal{B}(B^{+}\to J\mathrm{/}\psi \bar{\mathrm{\Lambda }}p)\times \mathcal{B}(J\mathrm{/}\psi \to {\mu }^{+}{\mu }^{-})$. With no significant signal observed in the high $m(\bar{\mathrm{\Lambda }}p)$ region above 2.8 GeV/$c^2$, an upper limit is set to be ${\mathcal{B}}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}(B^{+}\to \bar{\mathrm{\Lambda }}p{\mu }^{+}{\mu }^{-})<2.8(3.7)\times {10}^{-9}$ at the $90\mathrm{\%}$ ($95\mathrm{\%}$) confidence level.