Precision microwave spectroscopy of the positronium n = 2 fine structure
Prof. David Cassidy (University College London, UK)

Positronium (Ps) is a hydrogenic atom composed of an electron bound to a positron. Since it contains only leptons Ps is, for all practical purposes, a pure QED system, unaffected by nuclear structure effects. Also, being composed of a particle-antiparticle pair, Ps atoms are metastable, and may decay via self-annihilation, as well as through the usual radiative decay channels seen in regular atoms. The energy levels of Ps can be calculated to arbitrary precision (in principle), and precision spectroscopy of Ps can therefore be used to perform rigorous tests of bound-state QED theory. Moreover, since the theoretical description is limited only by the order of the calculations performed, rather than unknown physical constants or incalculable terms, any observed (and confirmed) disagreement with theory could indicate the existence of “new physics” such as particles or fields not currently included in the Standard Model.

In this talk I will describe some new measurements of the Ps n = 2 fine structure, specifically 2 ^3S_1--> 2 ^3P_J (J = 0,1,2) transitions. The experiments were performed using a radioactive isotope-based positron beam coupled to a buffer gas/Penning trap. This allows positron pulses to be generated, which are converted into a dilute Ps gas with in vacuum an initial number density on the order of 10^6 cm^-3. A pulsed dye laser was used to optically excite atoms to the 2 ^3S_1 level, and microwave radiation was used to drive transitions to the 2 ^3P_J levels, which decay radiatively to the ground state before annihilation. The different annihilation decay rates of the ground and excited (S) states allows the fine structure transitions to be monitored via the time spectrum of the Ps annihilation radiation.

We found that the measured J = 1 and J = 2 lineshapes exhibited significant asymmetries, whereas a symmetric lineshape was observed for the J = 0 transition. The observed asymmetries are not consistent with the most obvious quantum interference or line-pulling phenomena arising from nearby (off-resonant) transitions, and in the absence of a complete lineshape model we are therefore unable to determine the fine structure intervals for these transtions. Since the J = 0 lineshape did not exhibit any significant asymmetry it was possible to extract a value for the centre frequency: however, the obtained interval was found to disagree with theory by 2.77 MHz, which amounts to 4.5 standard deviations. No mechanism for a line shift of this magnitude has so far been identified.