Measurement of $\eta \to {\pi }^0\gamma \gamma$ branching fraction with the KLOE detector

Abstract

We present a measurement of the radiative decay $\eta \to {\pi }^0\gamma \gamma$ using 82 million $\eta$ mesons produced in $e^{+}{e}^{-}\to \varphi \to \eta \gamma$ process at the Frascati $\varphi$-factory DA$\Phi$NE. From the data analysis $1246\pm 133$ signal events are observed. By normalising the signal to the well-known $\eta \to 3{\pi }^0$ decay the branching fraction $\mathcal{B}(\eta \to {\pi }^0\gamma \gamma )$ is measured to be $(0.98\pm 0.11_{\text{stat}}\pm 0.14_{\text{syst}})\times 10^{-4}$. This result agrees with a preliminary KLOE measurement, but is twice smaller than the present world average. Results for $d\Gamma (\eta \to {\pi }^0\gamma \gamma )/d{M}^2(\gamma \gamma )$ are also presented and compared with latest theory predictions.

16 pages, 9 figures, prepared for submission to JHEP. Small changes in affiliation and acknowledgments