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 [1]. QED corrections to Ps energy levels have been calculated up to order *mα*^{6} and the theoretical calculations [2] are currently more precise than experimental measurements. The first measurement of the *n* = 2 fine structure was of the 2^{3}S_{1} – 2^{3}P_{2} interval and was conducted in 1975 by Mills, Berko, and Canter [3]. This measurement was improved upon by Hatamian, Conti, and Rich in 1987 [4] and then by Hagena and co-workers in 1993 [5]. These experiments, however, relied on generation of Ps atoms in the 2^{3}S_{1} state via positron bombardment into metal targets [6], 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 [5].

I will present the results of a new measurement [7] of the ν* _{J}* = 2

^{3}S

_{1}- 2

^{3}P

_{J}(

*J*= 0,1,2) fine structure intervals. 2

^{3}S

_{1}atoms were produced via laser excitation [8] before flying through a waveguide where the transition to the 2

^{3}P

_{J}state was driven with microwave radiation. The ν

_{J}transitions were observed by monitoring the time-dependent ground state Ps annihilation radiation. We measured the ν

_{0}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 ν

_{1}and ν

_{2}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.

**References**

[1] S. G. Karshenboim. *Int. J. Mod. Phys. A*, **19** (2004)

[2] A. Czarnecki, K. Melnikov, A. Yelkhovsky, *Phys. Rev. Lett*. **82** (1999)

[3] A .P. Mills, Jr, S. Berko and K. F. Canter, *Phys. Rev. Lett*. **34** (1975)

[4] S. Hatamian, R. S. Conti and A. Rich, *Phys. Rev. Lett*. **58** (1987)

[5] D. Hagena, R. Ley, D. Weil, G. Werth, W. Arnold and H. Schneider, *Phys. Rev. Lett*. **71** (1993)

[6] K. F. Canter, A. P. Mills, Jr and S. Berko, *Phys. Rev. Lett*. **34** (1975)

[7] L. Gurung, T. J. Babij, S. D. Hogan and D. B. Cassidy, *Phys. Rev. Lett*. **125** (2020)

[8] A. M. Alonso, S. D. Hogan and D. B. Cassidy. *Phys. Rev. A* **95***,* (2017)