η-η′ mixing from V → Pγ and J/ψ → V P decays

The η-η′ pseudoscalar mixing angle and the gluonium content of the η′ meson are deduced from an updated phenomenological analysis of V → Pγ and J/ψ → V P decays. In absence of gluonium, the value of the mixing angle in the quark-flavour basis is found to be φ_P = (41.5 ± 1.2)° from V → Pγ and φ_P = (40.5 ± 2.4)° from J/ψ → V P. In presence of gluonium, the values for the mixing angle and the gluonic content of the η′ wave function are φ_P = (41.4 ± 1.3)° and Z_η′² = 0.04 ± 0.09 from V → Pγ and φ_P = (44.5±4.3)° and Z _η′² = 0.28±0.21 from J/ψ → V P, respectively.