Positronium Spectroscopy in a Magnetic Field

Hyperfine spectroscopy of positronium formed in the presence of a static magnetic field is considered. Generalising the situation hitherto developed in the literature, the magnetic field is not assumed to be parallel to the momentum of incoming polarised positrons, while the possibility of electron polarisation is also included in the analysis. The results are of application to high sensitivity positron polarimeters used in current $\beta$ decay experiments.