Observation of ψ (3686 )→p p ¯η' and improved measurement of J /ψ →p p ¯η'

We observe the process ψ (3686 )→p p ¯η^' for the first time, with a statistical significance higher than 10 σ , and measure the branching fraction of J /ψ →p p ¯η^' with an improved accuracy compared to earlier studies. The measurements are based on 4.48 ×10⁸ ψ (3686 ) and 1.31 ×10⁹ J /ψ events collected by the BESIII detector operating at the BEPCII. The branching fractions are determined to be B (ψ (3686 )→p p ¯η^')=(1.10 ±0.10 ±0.08 )×10^-5 and B (J /ψ →p p ¯η^')=(1.26 ±0.02 ±0.07 )×10^-4, where the first uncertainties are statistical and the second ones systematic. Additionally, the η -η^' mixing angle is determined to be -2 4 ° ±1 1 ° based on ψ (3686 )→p p ¯η^', and -2 4 ° ±9 ° based on J /ψ →p p ¯η^', respectively.