Observation of the Rare Baryonic Decay B + → p Λ ¯ and Measurement of its Weak Decay Parameter

The first observation of the decay B^{+}→pΛ[over ¯] 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 fb^{-1}. The signal significance exceeds seven standard deviations. Using the B^{+}→K_{S}^{0}π^{+} 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 B(B^{+}→pΛ[over ¯])=(1.24±0.17±0.05±0.03)×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^{+}→pΛ[over ¯] weak decay parameter is measured to be α_{B}=0.87_{-0.29}^{+0.26}±0.09, indicating the presence of comparable S-wave and P-wave decay amplitudes.