The electromagnetic interactions of hadrons serve as a valuable probe of the nature of the strong force at low energies and simultaneously allow for tests of fundamental discrete symmetries. In this talk, I will show how polarization observables can be used to gain an in-depth understanding of these processes. I will present two analyses of data from the BESIII experiment where such observables have been studied.
In the first part of the talk, I report on a study of the rare decay eta' -> pi+pi-e+e- based on a sample of 1.3 billion e+e- -> J/psi events. The possibility of a CP-violating beyond standard model contribution to the decay is investigated and the branching fraction is measured with statistical precision a factor of two better than the previous best measurement.
In the second part of the talk, I present a study of the reaction e+e- -> Lambda anti-Lambda at five center-of-mass energies from 2.3864 GeV to 3.0800 GeV. A combination of fully and partially reconstructed events is used to measure the Born cross section for the process and determine the ratio and the relative phase of the timelike electromagnetic form factors of the Lambda hyperon.
Finally, I present a benchmark test of the tracking software for the planned upgrade of the BESIII detector with a new cylindrical gas electron-multiplier tracker.
