Searching for eta to 3 gamma decay

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.