Di Donato, C (reprint author), Univ Naples Federico II, Dipartimento Sci Fisiche, Naples, Italy.
We have measured the ratio R-phi = BR(phi ->eta'gamma)/BR phi ->eta gamma) by looking for the radiative decays phi ->eta'gamma and phi ->eta gamma into the final states pi(+)pi(-)7 gamma and 7 gamma, respectively, in a sample of similar to 1.4 x 10(9) phi mesons produced at the Frascati phi factory. We obtain R phi = (4.77 +/- 0.09(stat) +/- 0.19(syst)) x 10(-3) from which we derive BR(phi ->eta'gamma) = (6.20 +/- 0.11(stat) +/- 0.25(syst)) x 10(-5). Assuming the eta' has zero gluonium content, we extract the pseudoscalar mixing angle in the quark-flavor basis, phi(P) = (41.4 +/- 0.3(stat) +/- 0.7(syst) +/- 0.6(th))degrees. Combining the value of R-phi with other constraints, we estimate the fractional gluonium content of the eta' to be Z(2) = 0.14 +/- 0.04 and the mixing angle to be phi(P) = (39.7 +/- 0.7)degrees. (C) 2007 Elsevier B.V. All rights reserved.