First evidence of CP violation in beauty baryon to charmonium decays

A study of the difference in the CP asymmetries between Λ_b ⁰→J/ψpπ^- and Λ_b ⁰→J/ψpK^- decays, ΔA_CP, is performed using proton-proton collision data collected by the LHCb experiment in the years 2015–2018, corresponding to an integrated luminosity of 6fb^-1. This quantity is measured to be ΔA_CP=(4.03±1.18±0.23)%, where the first uncertainty is statistical and the second is systematic. When combined with the previous LHCb result, a value of ΔA_CP=(4.31±1.06±0.28)% is obtained, corresponding to a significance of 3.9σ 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.