Search for $C$ violation in the decay $��\rightarrow��^0+e^++e^-$ with WASA-at-COSY

We report on the investigation of the rare decay $��\rightarrow��^0+\text{e}^++\text{e}^-$ 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 J��lich, Germany. In total $3\times10^7$ events of the reaction $\text{p}+\text{d}\rightarrow{^3\text{He}}+��$ have been recorded at an excess energy of $Q = 59.8\,\textrm{MeV}$. Based on this data set the $C$ parity violating decay $��\rightarrow��^0+��^*\rightarrow��^0+\textrm{e}^++\textrm{e}^-$ via a single-photon intermediate state has been searched for, resulting in new upper limits of $��\left(��\rightarrow��^0+\textrm{e}^++\textrm{e}^-\right)/ ��\left(��\rightarrow��^++��^-+��^0\right) < 3.28\times10^{-5}$ and $��\left(��\rightarrow��^0+\textrm{e}^++\textrm{e}^-\right)/ ��\left(��\rightarrow\textrm{all}\right) < 7.5\times10^{-6}\ (\textrm{CL} = 90\,\%)$, respectively.
7 pages, 8 figures, submitted to PLB