Measurement of $\mathrm{\Gamma }(\eta \to {\pi }^{+}{\pi }^{-}\gamma )\mathrm{/}\mathrm{\Gamma }(\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0)$ with KLOE experiment

We report the measurement of the ratio $\Gamma(\eta \to \pi^+\pi^-\gamma)/\Gamma(\eta \to \pi^+\pi^-\pi^0)$analyzingalargesampleof$\phi \to \eta \gamma$ decays recorded with the KLOE experiment at the DA$\mathrm{\Phi }$NE $e^{+}{e}^{-}$ collider, corresponding to an integrated luminosity of 558 pb${}^{-1}$. The $\eta \to {\pi }^{+}{\pi }^{-}\gamma$ process is supposed to proceed both via a resonant contribution, mediated by the $\rho$ meson, and a non resonant direct term, connected to the box anomaly. The presence of the direct term affects the partial width value. Our result $R_{\eta}=\Gamma(\eta \to \pi^+ \pi^- \gamma)/\Gamma(\eta \to \pi^+ \pi^- \pi^0)= 0.1838\pm 0.0005_{stat} \pm 0.0030_{syst}$ is in agreement with a recent CLEO measurement, which differs by more 3 $\sigma$ from the average of previous results.