Hadron Physics in a Polarized World : Exploring Electromagnetic Interactions with Spin Observables

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 thesis, I show how polarization observables can be used to gain an in-depth understanding of these processes and present two analyses of data from the BESIII experiment where such observables have been studied. In the first part of the thesis, I report on a study of the rare decay η' → π+π-e+e- based on a sample of 1.3×109 e+e- → J/ψ events using the J/ψ → γη' radiative decay as a source of η' mesons. The branching fraction is measured to be (2.42±0.05stat.±0.08syst.)×10-3 with statistical precision a factor of two better than the previous best measurement. Furthermore, I investigate the possibility of a CP-violating contribution to the decay beyond the standard model which would lead to an asymmetry in the angle between the decay planes of the e+e-- and π+π--pairs. The asymmetry is measured to be (2.9±3.7stat.±1.1syst.)%, consistent with zero in agreement with the standard model expectation of no CP-violation. In the second part of the thesis, the reaction e+e- → Λ anti-Λ is studied 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 Λ 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.