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Precision spectroscopy of the positronium (Ps) $n = 2$ fine structure intervals has been performed several times to test bound state QED [1]. All previous measurements have used microwave waveguides with a fixed polarisation (e.g. [2,3,4]), and the effect of polarisation on these transitions has not been explored. The polarisation can change the subset of transitions driven by the radiation, changing Zeeman and Stark shifts, creating unwanted systematic effects.
We have used a horn antenna [5], which can be rotated to change the polarisation of the microwave radiation, to measure line shapes of the $2\,^3$S$_1 \rightarrow 2\,^{2S+1}$P$_1~(S = 0, 1)$ transitions. This was done in a large magnetic field to create large Zeeman splitting between the states, and to allow the $S = 0$ transition, which is forbidden by charge conjugation symmetry [6]. We report the effects of microwave polarisation on the resonance frequency and strength of the measured transitions. This work is a precursor to precision waveguide experiments which will measure these energy intervals as well as test for possible charge symmetry violation in the $S = 0$ transition.
[1] Precision spectroscopy of positronium: Testing bound-state QED theory and the search for physics beyond the Standard Model. G. S. Adkins, D. B. Cassidy, J. Pérez-Ríos; Phys. Rep. 975, 1 (2022).
[2] Measurements of the $2\,^3$S$_1 \rightarrow 2\,^3$P$_J~(J = 0, 1, 2)$ fine-structure splittings in positronium. S. Hatamian, R. S. Conti, and A. Rich; Phys. Rev. Lett. 58, 1833 (1987).
[3] Spectroscopy of excited state positronium. R. Ley, D. Hagena, D. Weil, G. Werth, W. Arnold, and H. Schneider; Hyperfine Interactions 89, 327–341 (1994).
[4] Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium $2\,^3$S$_1 \rightarrow 2\,^3$P$_J~(J = 0, 1, 2)$ fine-structure intervals. L. Gurung, T. J. Babij, J. Pérez-Ríos, S. D. Hogan and D. B. Cassidy; Phys. Rev. A. 103, 042805 (2021).
[5] Microwave spectroscopy of positronium atoms in free space. R. E. Sheldon, T. J. Babij, S. H. Reeder, S. D. Hogan, and D. B. Cassidy; Phys. Rev. A 107, 042810 (2023).
[6] Tests of the discrete symmetries C, P, and T in one-photon transitions of positronium. R. S. Conti, S. Hatamian, and A. Rich; Phys. Rev. A 33(5), 3495 (1986).
