Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium 2 S1 3 →2 PJ 3 (J=1,2) fine-structure intervals

We report new measurements of the positronium (Ps) 2S13→2PJ3 fine-structure intervals, νJ (J=0,1,2). In the experiments, Ps atoms, optically excited to the radiatively metastable 2S13 level, flew through microwave radiation fields tuned to drive transitions to the short-lived 2PJ3 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.