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

We observe the process ψ(3686)→p$\overline{p}$η' for the first time, with a statistical significance higher than 10σ, and measure the branching fraction of J/ψ→p$\overline{p}$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 $\mathcal{B(ψ(3686)→p\overline{p}η')}$ = (1.10 ± 0.10 ± 0.08)×10-5 and $\mathcal{B(J/ψ →p\overline{p}η')}$ = (1.26 ± 0.02 ± 0.07) ×10-4, where the first uncertainties are statistical and the second ones systematic. Also, the η- η' mixing angle is determined to be -24° ± 11° based on ψ(3686)→p$\overline{p}$η', and -24° ± 9° based on J/ψ→p$\overline{p}$η', respectively.