Upper limit on the $\eta\to\pi^{+}\pi^{-}$ branching fraction with the KLOE experiment

TL;DR

This study sets a new upper limit on the rare decay of eta mesons into charged pion pairs using KLOE data, improving previous constraints and searching for CP violation effects.

Contribution

It provides the most stringent upper limit to date on the eta to pi+ pi- decay branching fraction using KLOE experiment data.

Findings

No signal observed for eta to pi+ pi- decay.

Upper limit on branching fraction is 4.9 x 10^-6 at 90% CL.

Combined analysis yields an upper limit of 4.4 x 10^-6.

Abstract

Based on an integrated luminosity of 1.61 fb$^{-1}$ $e^+e^-$ collision data collected with the KLOE detector at DA$\Phi$NE, the Frascati $\phi$-factory, a search for the $P$- and $CP$-violating decay $\eta\to\pi^{+}\pi^{-}$ has been performed. Radiative $\phi\to\eta\gamma$ decay is exploited to access the $\eta$ mesons. No signal is observed in the $\pi^{+}\pi^{-}$ invariant mass spectrum, and the upper limit on the branching fraction at 90\% confidence level is determined to be ${\mathcal B}(\eta\to\pi^{+}\pi^{-})<4.9\times10^{-6}$, which is approximately three times smaller than the previous KLOE result. From the combination of these two measurements we get ${\mathcal B}(\eta\to\pi^{+}\pi^{-}) < 4.4\times10^{-6}$ at 90\% confidence level.