Measurement of the pseudoscalar mixing angle and eta ' gluonium content with the KLOE detector

We have measured the ratio R ϕ = BR ( ϕ → η ′ γ ) / BR ( ϕ → η γ ) by looking for the radiative decays ϕ → η ′ γ and ϕ → η γ into the final states π + π − 7 γ and 7γ, respectively, in a sample of ∼ 1.4 × 10 9 ϕ mesons produced at the Frascati ϕ factory. We obtain R ϕ = ( 4.77 ± 0.09 stat ± 0.19 syst ) × 10 −3 , from which we derive BR ( ϕ → η ′ γ ) = ( 6.20 ± 0.11 stat ± 0.25 syst ) × 10 −5 . Assuming the η ′ has zero gluonium content, we extract the pseudoscalar mixing angle in the quark-flavor basis, φ P = ( 41.4 ± 0.3 stat ± 0.7 syst ± 0.6 th ) ° . Combining the value of R ϕ with other constraints, we estimate the fractional gluonium content of the η ′ to be Z 2 = 0.14 ± 0.04 and the mixing angle to be φ P = ( 39.7 ± 0.7 ) ° .