Search for the η → e+e−e+e− double Dalitz decay

Abstract

The Wide Angle Shower Apparatus (WASA) operated at the Cooler Synchrotron COSY-Jülich is a 4p-spectrometer to study the decay channels of light mesons ranging up into the strange quark sector. A large number of η mesons is being produced in proton-deuteron and proton-proton reactions, permitting the study of very rare η-decay channels. One of the channels is the double Dalitz decay, where the η meson decays via two virtual photons into two electron-positron pairs. The coupling between the η meson and the two virtual photons can be described by the transition form factor. It depends on the squared invariant mass of the lepton pairs and allows to study the structure of the decay mechanism. Currently, there is only an experimental upper limit for the branching ratio. One objective of the WASA-at-COSY experiment is to reduce the upper limit for this decay channel or to determine a finite value of the branching ratio. The goal is to study the transition form factor. The status of the continuing analysis and preliminary results are presented.