Microwave spectroscopy of the positronium fine structure

Using a Surko-type buffer gas positron trap [1] and single-shot annihilation lifetime techniques [2] we have performed new measurements of the positronium (Ps) fine structure using microwave spectroscopy [3]. Such experiments may be used to test QED but are presently much less precise than theory [4]. Measurements made in both free-space and in waveguides were found to be susceptible to line shape distortions arising from reflected microwave radiation [5], which in turn resulted in large apparent frequency shifts. We report here new waveguide measurements designed to mitigate these effects and which allow for increased precision and accuracy. We also describe a new technique in which microwave spectroscopy of dressed Rydberg He atoms is used to characterize the electric field in a waveguide [6].



[1] M. Surko, M. Leventhal, and A. Passner. “Positron plasma in the laboratory”. Physical Review Letters, 62(8), (1989)

[2] D. B. Cassidy, S. H.M. Deng, H. K.M. Tanaka, and A. P. Mills. “Single shot positron annihilation lifetime spectroscopy”. Applied Physics Letters, 88(19), (2006)

[3] R. E. Sheldon, T. J. Babij, S. H. Reeder, S. D. Hogan, and D. B. Cassidy. “Precision Microwave Spectroscopy of the Positronium 2 ³S₁ → 2 ³P₂ Interval”, In preparation

[4] A. Czarnecki, K. Melnikov, and A. Yelkhovsky, "Positronium S-state spectrum: Analytic results at o(mα⁶ )", Phys. Rev. A 59, 4316 (1999)

[5] L. A. Akopyan, T. J. Babij, K. Lakhmanskiy, D. B. Cassidy, and A. Matveev, “Line-shape modeling in microwave spectroscopy of the positronium n = 2 fine-structure intervals,” Phys. Rev. A 104, 062810 (2021)

[6] S. H. Reeder, S. D. Hogan, and D. B. Cassidy. “Microwave field characterisation in a waveguide using Rydberg helium”, In preparation