WASA-at-COSY

The study of $\eta$ mesons has been one of the main objectives of the WASA experiment, ever since its relocation to the accelerator complex COSY at the Research Center Jülich. As the interaction of $\eta$ mesons with nucleons is attractive in s-wave, with the $S_{11}(1535)$ resonance situated close to the $\eta N$ threshold, studying the properties of $\eta N$ and $\eta A$ interactions has long been an active research topic. With the $\eta$ meson being uncharged and short-lived, such studies are best performed in nucleon-nucleon and nucleon-nucleus collisions containing an $\eta$ meson in the final state. With both COSY and an internal pellet target being able to provide protons as well as deuterons, $\eta$ meson production can be studied in various reactions, most notably the proton-deuteron fusion. Here, experimental evidence for a strong final state interaction in the $\eta\;{}^3\textrm{He}$ system has lead to an ongoing discussion of a potential $\eta$-nucleus bound state.

Another major part of the $\eta$ physics program with WASA-at-COSY is the search for rare or forbidden decays. The $\eta$ meson, possessing no allowed strong decays, provides ideal surroundings to search for both rare standard model processes and symmetry violating decays potentially involving beyond standard model physics. Two dedicated datasets, containing $30\times10^6$ $\eta$ mesons in $pd\rightarrow{}^3\textrm{He}\eta$ and $500\times10^6$ $\eta$ mesons in $pp\rightarrowpp\eta$, allow precision studies of standard model decays and stringent limits on symmetry violating processes. Recent results on $\eta$ meson physics with the WASA-at-COSY experiment will be discussed.