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Abstract

We report new measurements of the positronium (Ps) 2³S₁→2³P_J fine-structure intervals, νJ (J=0,1,2). In the experiments, Ps atoms, optically excited to the radiatively metastable 2³S₁ level, flew through microwave radiation fields tuned to drive transitions to the short-lived 2³P_J levels, which were detected via the time spectrum of subsequent ground-state Ps annihilation radiation. Both the ν₁ and ν₂ line shapes were found to be asymmetric, which, in the absence of a complete line-shape model, prevents accurate determination of these fine-structure intervals. Conversely, the ν₀ line shape did not exhibit any significant asymmetry; the observed interval, however, was found to disagree with QED theory by 4.2 standard deviations.