Searching for eta to 3 gamma decay

TL;DR

This study analyzed data from the KLOE experiment to search for a rare, C-violating decay of the eta meson into three photons, setting an upper limit on its branching ratio due to no observed signal.

Contribution

First search for the eta to three gamma decay using KLOE data, establishing a new upper limit on its branching ratio at 95% confidence level.

Findings

No signal observed for eta to three gamma decay.

Upper limit on branching ratio set at 1.6×10⁻⁵ at 95% CL.

Systematic uncertainties estimated at 35%.

Abstract

Data collected by the KLOE experiment in years 2001/2002 for a total integrated luminosity of 410 pb$^{-1}$, corresponding to 17 million of produced $\eta$'s from the $\Phi \to \eta\gamma$ radiative decay, have been analyzed to look for the C-violating decay $\eta\to\gamma\gamma\gamma$. The signal is searched in events with 4 photons in the final state by using the spectrum of the most energetic photon. The background is evaluated from the data themself, doing a fifth degree polynomial fit outside the signal region, and extrapolating inside it. No signal has been observed and taking into account a selection efficiency $\epsilon = 20.3 %$ we get from a likelihood fit Br$(\eta\to\gamma\gamma\gamma) \le 1.6\times10^{-5}$ at 95% CL. The procedure has a systematic ambiguity at the 35 % level.