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

We have measured the ratio Rφ = BR(φ→η'γ )/BR(φ→ηγ ) by looking for the radiative decays φ→η'γ and φ→ηγ into the final states π+π−7γ and 7γ , respectively, in a sample of ∼1.4 × 10^9 φ mesons produced at the Frascati φ factory. We obtain Rφ = (4.77 ± 0.09stat ± 0.19syst) × 10^−3, from which we derive BR(φ →η'γ ) = (6.20 ± 0.11stat ± 0.25syst) × 10^−5. Assuming the η' has zero gluonium content, we extract the pseudoscalar mixing angle in the quark-flavor basis, ϕP= (41.4 ± 0.3stat ± 0.7syst ± 0.6th)◦. Combining the value of Rφ with other constraints, we estimate the fractional gluonium content of the η' to be Z2 = 0.14 ±0.04 and the mixing angle to be ϕP = (39.7± 0.7)◦.