$p\bar\Lambda$ final-state interaction in the reactions $e^+e^- \to K^- p \bar \Lambda$ and $J/\psi \to K^- p \bar \Lambda$

Near-threshold $p\bar\Lambda$ mass spectra for the reactions $e^+e^- \to K^- p\bar\Lambda$ and $J/\psi \to K^- p\bar\Lambda$ are investigated with an emphasis on the role played by the interaction in the $p\bar\Lambda$ system. As guideline for the $p\bar\Lambda$ interaction a variety of $\Lambda\bar\Lambda$ potential models is considered that have been established in the analysis of data on $p\bar p\to \Lambda\bar\Lambda$ in the past. Arguments why the properties of the $p\bar\Lambda$ and $\Lambda\bar\Lambda$ interactions can be expected to be very similar are provided. It is shown that the near-threshold enhancement in the invariant mass observed for the $e^+e^-$ reaction can be reproduced quantitatively by the assumed $p\bar\Lambda$ final-state interaction in the partial wave suggested by an amplitude analysis of the experiment. The effect of the $p\bar\Lambda$ final-state interaction in other decays is explored, including the recently measured reactions $B^- \to J/\psi\, \bar p \Lambda$ and $B^+ \to J/\psi\, p \bar\Lambda$. It is found that the final-state interaction improves the description of the measured invariant mass near threshold in most cases.