Search for $C$ violation in the decay $\hat{I}\cdot \to \ddot{I}{€}^0+e^{+}+e^{-}$ with WASA-at-COSY

Abstract

We report on the investigation of the rare decay $\hat{I}\cdot \to \ddot{I}{€}^0+{\text{e}}^{+}+{\text{e}}^{-}$ which is of interest to study both $C$ violation in the electromagnetic interaction and to search for contributions from physics beyond the Standard Model, since the allowed decay via a two-photon intermediate state is strongly suppressed. The experiment has been performed using the WASA-at-COSY installation, located at the COSY accelerator of the Forschungszentrum Jülich, Germany. In total $3\times 10^7$ events of the reaction $\text{p}+\text{d}\to {}^3\text{He}+\hat{I}\cdot$ have been recorded at an excess energy of $Q=59.8\text{MeV}$. Based on this data set the $C$ parity violating decay $η\rightarrowπ^0+γ^*\rightarrowπ^0+\textrm{e}^++\textrm{e}^-$ via a single-photon intermediate state has been searched for, resulting in new upper limits of $\hat{I}“\left(\hat{I}\cdot \to \ddot{I}{€}^0+{\text{e}}^{+}+{\text{e}}^{-}\right)/\hat{I}“\left(\hat{I}\cdot \to \ddot{I}{€}^{+}+\ddot{I}{€}^{-}+\ddot{I}{€}^0\right)<3.28\times 10^{-5}$ and $\hat{I}“\left(\hat{I}\cdot \to \ddot{I}{€}^0+{\text{e}}^{+}+{\text{e}}^{-}\right)/\hat{I}“\left(\hat{I}\cdot \to \text{all}\right)<7.5\times 10^{-6}(\text{CL}=90\%)$, respectively.

7 pages, 8 figures, submitted to PLB