Measurement of the pseudoscalar mixing angle and ${\eta }^{\prime }$ gluonium content with KLOE detector

Abstract

We have measured the ratio $R_{\varphi }=BR(\varphi \to {\eta }^{\prime }\gamma )/BR(\varphi \to \eta \gamma )$ by looking for the radiative decays $\varphi \to {\eta }^{\prime }\gamma$ and $\varphi \to \eta \gamma$ in the final states ${\pi }^{+}{\pi }^{-}$ 7 $\gamma$'s and 7 $\gamma$'s respectively, in a sample of $\sim 1.3\cdot 10^9$ $\varphi$ mesons produced at the Frascati $\varphi$-factory. We obtain $R_{\varphi }=(4.77\pm 0.09_{stat}\pm 0.19_{sys})\cdot 10^{-3}$ from which we derive $BR(\varphi \to {\eta }^{\prime }\gamma )=(6.20\pm 0.11_{stat}\pm 0.25_{sys})\cdot 10^{-5}$. In the hypothesis of no gluonium content we extract the pseudoscalar mixing angle in the quark-flavor basis ${\varphi }_P=(41.4\pm 0.3_{stat}\pm 0.7_{sys}\pm 0.6_{th}{)}^{\circ }$. Combining the value of $R_{\varphi }$ with other constraints, we estimate the gluonium fractional content of ${\eta }^{\prime }$ meson as $Z^2=0.14\pm 0.04$ and the mixing angle ${\varphi }_P=(39.7\pm 0.7{)}^{\circ }$.

13 pages, 8 figures