2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/178709We report the first measurements of the branching fractions: $B(ψ(2S) -> γη') = (1.54 \pm 0.31 \pm 0.23) x 10^{-4}$ and $B(ψ(2S) -> γη) = (0.53 \pm 0.31 \pm 0.08) x 10^{-4}$. The $ψ(2S)$ to $γη'$ result is consistent with expectations of a model that considers the possibility of $η' - η$ mixing. The ratio of the $ψ(2S)$ to $γη'$ and $ψ(2S)$ to $γη$ rates is used to determine the pseudoscalar octet-singlet mixing angle.10 pages with 3 figuresHigh Energy Physics - Experimenttext