Positronium (Ps) is a hydrogenic atom made of an electron and its antiparticle, a positron. As a purely leptonic system, positronium is an ideal system for testing QED theory . QED corrections to Ps energy levels have been calculated up to order mα6 and the theoretical calculations  are currently more precise than experimental measurements. The first measurement of the n = 2 fine structure was of the 23S1 – 23P2 interval and was conducted in 1975 by Mills, Berko, and Canter . This measurement was improved upon by Hatamian, Conti, and Rich in 1987  and then by Hagena and co-workers in 1993 . These experiments, however, relied on generation of Ps atoms in the 23S1 state via positron bombardment into metal targets , which is intrinsically inefficient and necessarily results in Ps with energies of several eV. The last measurement is now over 25 years old and has a precision of approximately 200 ppm .
I will present the results of a new measurement  of the νJ = 23S1 - 23PJ (J = 0,1,2) fine structure intervals. 23S1 atoms were produced via laser excitation  before flying through a waveguide where the transition to the 23PJ state was driven with microwave radiation. The νJ transitions were observed by monitoring the time-dependent ground state Ps annihilation radiation. We measured the ν0 resonance frequency to be 18501.02 ± 0.61 MHz, which is in 4.5σ disagreement with the theoretical value of 18498.25 ± 0.08 MHz. Systematics effects and known interference effects are not large enough to account for this disagreement. The ν1 and ν2 transitions exhibit asymmetric lineshapes which prevent the true resonance frequency from being determined. Future work to investigate the disagreement and asymmetry will also be discussed.
 S. G. Karshenboim. Int. J. Mod. Phys. A, 19 (2004)
 A. Czarnecki, K. Melnikov, A. Yelkhovsky, Phys. Rev. Lett. 82 (1999)
 A .P. Mills, Jr, S. Berko and K. F. Canter, Phys. Rev. Lett. 34 (1975)
 S. Hatamian, R. S. Conti and A. Rich, Phys. Rev. Lett. 58 (1987)
 D. Hagena, R. Ley, D. Weil, G. Werth, W. Arnold and H. Schneider, Phys. Rev. Lett. 71 (1993)
 K. F. Canter, A. P. Mills, Jr and S. Berko, Phys. Rev. Lett. 34 (1975)
 L. Gurung, T. J. Babij, S. D. Hogan and D. B. Cassidy, Phys. Rev. Lett. 125 (2020)
 A. M. Alonso, S. D. Hogan and D. B. Cassidy. Phys. Rev. A 95, (2017)