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Recent measurements of the positronium (Ps) 23S1 → 23PJ fine-structure intervals, for νJ (J = 0, 1, 2), are presented. This experiment used slow Ps atoms, which were optically excited to the metastable 23S1 level. This metastable beam then passed through a microwave guide, which produced a radiation field tuned to drive the transition to the short-lived 23PJ levels. These short-lived Ps atoms were then detected via their subsequent annihilation radiation. For the ν0 transition, a discrepancy of 4.5 σ from QED theory was measured [1]. While the ν1, and ν2 transitions exhibited asymmetric lineshapes [2]. Simulations seem to suggest that this asymmetry was due to reflections of the RF field in the chamber [3]. Recent improvements have been made to the experiment, with asymmetry no longer observed.
[1] L. Gurung, T. J. Babij, S. D. Hogan, and D. B. Cassidy.Precision microwave spectroscopy of the positronium n = 2 fine structure. Phys. Rev. Lett, 125, 073002 (2020).
[2] L. Gurung, T. J. Babij, J. Pérez-Ríos, S. D. Hogan, and D. B. Cassidy. Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium 23S1 23PJ (J = 1, 2) fine-structure intervals. Phys. Rev. A, 103, 02805 (2021).
[3]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).
