Measurement of the η → e⁺e^-e⁺e^- double Dalitz decay and the search for new physics beyond the Standard Model in η → e⁺e^- with WASA-at-COSY

The Wide Angle Shower Apparatus (WASA) - operated at the Cooler Synchrotron COSY-Juelich - is a close-to-4π-detector used for the study of the production mechanisms and decay modes of light mesons. A huge amount of η meson data has been accumulated over the last few years and these allow one to measure even rare decay channels. One of these channels is the double Dalitz decay, where the η meson decays via two virtual photons into two electron-positron pairs. According to QED calculations, the expected branching ratio is BR(η → e⁺e^-e⁺e^-)=2.6 x 10^-5. In this work the analysis of 3 x 10⁷ η events from pd → ³Heη reactions is presented. Based upon 49.7±12.5_stat identified candidates, the branching ratio for η → e⁺e^-e⁺e^- is measured to be BR(η → e⁺e^-e⁺e^-)=(3.2±0.8_stat±0.8_syst) x 10^-5. The result is in agreement with the recently published value from the KLOE collaboration: BR(η → e⁺e^-e⁺e^-)=(2.4±0.2_stat±0.1_syst) x 10^-5. Sophisticated analysis methods have been developed and tuned to search for the very rare decay channel η → e⁺e^- which, according to the Standard Model, has a branching ratio on the order of 10^-9. Any significant deviation from this value would be a sign for new physics. Within the framework of this thesis, an upper limit for this branching ratio has been determined as BR(η → e⁺e^-)<6 x 10^-6 at the 90% confidence level. This is close to the recently published upper limit measured by the HADES collaboration: BR(η → e⁺e^-)<4.9^+0.7_-1.2 x 10^-6 at the 90% confidence level.