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

TL;DR

This paper measures the ratio of decay widths of eta mesons into different final states using the KLOE experiment, providing insights into the resonant and non-resonant decay contributions and their relation to the box anomaly.

Contribution

First precise measurement of the eta decay ratio with large data sample, confirming recent results and highlighting the non-resonant contribution's significance.

Findings

Measured ratio R_eta = 0.1838 ± 0.0005 (stat) ± 0.0030 (syst)

Result agrees with recent CLEO measurement

Discrepancy with previous averages exceeds 3 sigma

Abstract

We report the measurement of the ratio $\Gamma(\eta \to \pi^+\pi^-\gamma)/\Gamma(\eta \to \pi^+\pi^-\pi^0)$ analyzing a large sample of $\phi \to \eta \gamma$ decays recorded with the KLOE experiment at the DA$\Phi$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.