First observation of the charmless baryonic decay $B^{+}\to \bar{\mathrm{\Lambda }}p\bar{p}p$

A search for the charmless baryonic decay $B^{+}\to \bar{\mathrm{\Lambda }}p\bar{p}p$ is performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of 5.4~${\text{fb}}^{-1}$. The branching fraction for this decay is measured for the first time relative to that of the topologically similar decay $B^{+}\to J\mathrm{/}\psi K^{+}$, with $J\mathrm{/}\psi \to \bar{\mathrm{\Lambda }}p{K}^{-}$. The branching fraction is measured to be \mbox{$\mathcal{B}(B^{+}\to \bar{\mathrm{\Lambda }}p\bar{p}p)=(2.15\pm 0.35\pm 0.12\pm 0.28)\times {10}^{-7}$}, where the first uncertainty is statistical, the second is systematic, and the third arises from the uncertainty in the normalization channel branching fraction. The $CP$ asymmetry is measured to be ${\mathcal{A}}_{CP}=(5.4\pm 15.6\pm 2.4)\mathrm{\%}$, where the uncertainties are statistical and systematic. The background-subtracted invariant-mass distributions of $\bar{\mathrm{\Lambda }}p$ and $\bar{p}p$ pairs exhibit pronounced enhancements at both kinematic thresholds, in contrast to a uniform phase-space distribution.