Observation of the rare baryonic decay $B^{+}\to p\bar{\Lambda }$ and measurement of its weak decay parameter

The first observation of the decay $B^{+}\to p\bar{\Lambda }$ is presented, using proton-proton collision data collected by the LHCb experiment between 2016 and 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{f}\mathrm{b}}^{-1}$. The signal significance exceeds seven standard deviations. Using the $B^{+}\to K_{\mathrm{S}}^0{\pi }^{+}$ decay as a normalization channel, the branching fraction is measured and combined with previous LHCb results based on data collected at 7 and 8 TeV in 2011 and 2012, yielding $\mathcal{B}(B^{+}\to p\bar{\Lambda })=(1.24\pm 0.17\pm 0.05\pm 0.03)\times {10}^{-7}$, where the first uncertainty is statistical, the second is systematic, and the third comes from the uncertainty on the branching fraction of the normalization channel. The $B^{+}\to p\bar{\Lambda }$ weak decay parameter is measured to be ${\alpha }_B=0.87_{-0.29}^{+0.26}\pm 0.09$, indicating the presence of comparable S-wave and P-wave decay amplitudes.