Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium 2S13→2PJ3

Abstract

We report new measurements of the positronium (Ps) 2 3 S1 → 2 3 PJ fine-structure intervals, ν J (J = 0, 1, 2). In the experiments, Ps atoms, optically excited to the radiatively metastable 2 3 S1 level, flew through microwave radiation fields tuned to drive transitions to the short-lived 2 3 PJ levels, which were detected via the time spectrum of subsequent ground-state Ps annihilation radiation. Both the ν 1 and ν 2 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 ν 0 line shape did not exhibit any significant asymmetry; the observed interval, however, was found to disagree with QED theory by 4.2 standard deviations.

Line-shape modeling in microwave spectroscopy of the positronium n=2 fine-structure intervals

“…However, it was found that the measured ν 0 transition interval disagreed with the QED calculation by more than four standard deviations [5]. Moreover, the ν 1 and ν 2 measurements exhibited unexplained asymmetric line shapes [6] which prevented a precise determination of the corresponding intervals.…”

“…Previously, simulations were performed that were designed to represent the experimental arrangements used in recent measurements of the Ps fine structure [5,6]. However, not all aspects of the experiments were simulated; for example, in the experiments Ps atoms in the 2 3 S 1 level were generated via a single-photon excitation process in a time varying electric field [16].…”

“…In the experiments [6] Ps decay rates were measured using the technique of single-shot positron annihilation lifetime spectroscopy (SSPALS) [25]. This method allows Ps lifetime spectra to be generated when many gamma-ray photons are detected nearly simultaneously.…”

“…Some recent measurements of the Ps n = 2 fine structure have been conducted with the aim of closing the gap between theory and experiment [5,6]. Previous measurements of the 2 3 S 1 → 2 3 P J intervals (where J = 0, 1, 2, and the transitions are henceforth denoted as ν J ) have been conducted decades ago, achieving uncertainties on the order of several MHz [7][8][9], while the calculations have estimated uncertainties of 80 kHz [4].…”

“…[13,14]). In the recent Ps fine-structure experiments a simplified form of this effect was * l.akopyan@rqc.ru considered [6] and found not to contribute at the level of the observed discrepancies. Here we present the results of a more complete density-matrix model using numerical integration of a master equation in the Lindblad form [15].…”