First Observation of the Charmless Baryonic Decay $B^+\to \bar{\Lambda}p\bar{p}p$

A search for the charmless baryonic decay $B^+\to \bar{\Lambda} p\bar{p}p$ 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 $B^+\to J/\psi K^+$, with $J/\psi \to \bar{\Lambda} p K^-$. The branching fraction is measured to be $\mathcal{B}(B^+\to \bar{\Lambda} p\bar{p}p) = (2.15 \pm 0.35 \pm 0.12 \pm 0.28) \times 10^{-7}$, 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 $\mathcal{A}_{CP}=(5.4\pm 15.6\pm 2.4)%$, where the uncertainties are statistical and systematic. The background-subtracted invariant-mass distributions of $\bar{\Lambda}p$ and $\bar{p}p$ pairs exhibit pronounced enhancements at both kinematic thresholds, in contrast to a uniform phase-space distribution.