Precision spectroscopy with positronium *

Positronium (Ps) is a metastable hydrogenic atomic system composed of electron-positron pairs. Since they contain only leptons, Ps atoms can be considered to be described purely by QED, and thus are excellent systems with which to test (bound state) QED theory. Unfortunately, the practicalities of performing experiments with Ps atoms presents a significant barrier to performing measurements with a precision approaching what has been obtained in QED theory. In this talk I will describe how such experiments are done, highlighting some of the intrinsic limitations created by the nature of Ps atoms as well as recent technological developments that have enabled many new measurements. In particular, I will present some new measurements of the Ps n = 2 fine structure intervals that were performed using these new techniques. In this work we found that some measured lineshapes exhibited significant asymmetries, which have yet to be explained. Measurements in which the observed lineshapes were not asymmetric exhibited a disagreement with theory by 2.77 MHz, which amounts to more than 4 standard deviations. This observation also has not yet been explained.