First evidence of $CP$ violation in beauty baryon to charmonium decays

A study of the difference in the $CP$ asymmetries between ${\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi p{\pi }^{-}$ and ${\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi p{K}^{-}$ decays, $\mathrm{\Delta }{\mathcal{A}}_{CP}$, is performed using proton-proton collision data collected by the LHCb experiment in the years 2015–2018, corresponding to an integrated luminosity of $6{\mathrm{f}\mathrm{b}}^{-1}$. This quantity is measured to be $\mathrm{\Delta }{\mathcal{A}}_{CP}=(4.03\pm 1.18\pm 0.23)\mathrm{\%}$, where the first uncertainty is statistical and the second is systematic. When combined with the previous LHCb result, a value of $\mathrm{\Delta }{\mathcal{A}}_{CP}=(4.31\pm 1.06\pm 0.28)\mathrm{\%}$ is obtained, corresponding to a significance of $3.9\sigma$ against the $CP$ symmetry hypothesis. Studies of triple-product asymmetries, which provide an additional probe of $CP$ violation, show no significant deviation from $CP$ symmetry.