Measurements of Branching Ratios for $\eta$ Decays into Charged Particles

TL;DR

This study measures the branching ratios of various eta meson decays into charged particles using a large dataset from the WASA-at-COSY experiment, providing new quantitative insights into eta decay modes and CP violation limits.

Contribution

First measurement of eta decay branching ratios into specific charged particle channels normalized to a common decay mode.

Findings

Branching ratios for $ o ext{charged particles}$ decays determined.

Upper limit set on CP-violating asymmetry in $ o ext{charged particles}$ decay.

Data from $3\times10^{7}$ eta events analyzed.

Abstract

The WASA-at-COSY experiment has collected $3\times10^{7}$ events with $\eta$-mesons produced via the reaction $pd\rightarrow{^{3}\textrm{He}}\eta$ at $\textrm{T} = 1.0 \textrm{GeV}$. Using this data set, we evaluate the branching ratios of the decays $\eta\to\pi^{+}\pi^{-}\gamma$, $\eta\to e^{+}e^{-}\gamma$, $\eta\to\pi^{+}\pi^{-}e^{+}e^{-}$ and $\eta\to e^{+}e^{-}e^{+}e^{-}$. The branching ratios are normalized to the $\eta\rightarrow\pi^{+}\pi^{-}\pi^{0}$ decay. In addition an upper limit on a $CP$-violating asymmetry in $\eta\rightarrow \pi^{+}\pi^{-}e^{+}e^{-}$ is extracted.