http://inspirehep.net/record/691220/files/arXiv:hep-ph_0509010.pdf

Precision physics of simple atoms: QED tests, nuclear structure and fundamental constants

As an unstable light pure leptonic system, positronium is a very specific probe atom to test bound state QED. In contrast to ordinary QED for free leptons, the bound state QED theory is not so well understood and bound state approaches deserve highly accurate tests. We present a brief overview of precision studies of positronium paying special attention to uncertainties of theory as well as comparison of theory and experiment. We also consider in detail advantages and disadvantages of positronium tests compared to other QED experiments.

Precision study of positronium: Testing bound state QED theory

Positronium and muonium are purely leptonic atoms and hence free of an internal substructure. This qualifies them as potentially well suited systems to probe the existence of physics beyond the Standard Model. We hence carry out a comprehensive study of the sensitivity of current positronium and muonium precision spectroscopy to several new physics scenarios. By taking properly into account existing experimental and astrophysical probes, we define clear experimental targets to probe new physics via precise spectroscopy. For positronium we find that, in order for the spectroscopy bounds to reach a sensitivity comparable to the electron gyromagnetic factor, an improvement of roughly five orders of magnitude from state-of-the-art precision is required, which would be a challenge based on current technology. More promising is instead the potential reach of muonium spectroscopy: in the next few years experiments like Mu-MASS at PSI will probe new regions of the parameter space testing the existence of medium/short range (MeV and above) spin-dependent and spin-independent dark forces between electrons and muons.

Current and future perspectives of positronium and muonium spectroscopy as dark sectors probe

Precision spectroscopy of positronium: Testing bound-state QED theory and the search for physics beyond the Standard Model

/pdf/xuv-frequency-comb-metrology-and-the-ground-state-of-helium-1m77uuuybq.pdf

XUV Frequency Comb Metrology and the Ground State of Helium

Electric dipole transitions in the microwave range have been induced between the fine-structure levels of positronium in the excited staten=2. As an indication of the transitions, we used the increase in Lyman-α radiation when the metastable 23S1-level is depopulated. The results for the transitions 23S1→23P0,1,2 areΝ
0=18499.65±1.20±4.00 MHz,Ν
1=13012.42 ±0.65±1.54 MHz andΝ
2=8624.38±0.54±1.40 MHz. The first error is statistical and the second systematic. The precision of the present measurement has improved by a factor of 3, compared to previous data. Recent bound state QED-calculations have been extended to the orderR
t8
α
4lnα
−1. The not yet completely calculated orderR
t8
α
4 is estimated to contribute less than 1 MHz. Our experimental results are in good agreement with theory. By applying a weak magnetic field, we were able to observe the transition 23S1→21P1 which is strictly forbidden byC-invariance in zero field. Our result, corrected for Zeeman- and motional Starkeffect, isΝ
3=11180.0±5.0±4.0 MHz. An upper limit for theC-violating matrix element of
 
$$|\langle 2^1 P_1 |H_ \not\subset |2^3 P_1 \rangle |< 76$$

 MHz could be deduced. Our experiment used moderated slow positrons from the bremsstrahlung and pair production of a pulsed electron linear accelerator (TEPOS facility at the university of Giessen).

Spectroscopy of excited state positronium

The slow positron facility TEPOS at the Giessen electron LINAC (36 MeV, 120 µA) has been used to produce an intense beam of moderated positrons which is magnetically guided over a distance of 9 m. At a transportation energy of 100 eV about 106 slow e+/s could be extracted out of the magnetic field (0.01 T) and have been electrostatically focussed inside a microwave guide. A small fraction of the positrons form positronium in the excited staten=2. The spontaneous emission of Lyman-α photons (λ=243 nm) from the 2P-states is observed by a photomultiplier. Microwave induced fine-structure transitions 23S1©23P2,1,0 have been observed at 8617(2), 13010(3) and 18494(2) MHz by an increase of the Lyman-α counting rate. The present errors take into consideration only statistical contributions; systematic errors in the same order of magnitude may originate from frequency dependent variations of the microwave power. The observed linewidth exceeds the natural linewidth of 50 MHz by Doppler-effect and power broadening. Values around 100 MHz could be reached at the lowest applied power levels.

Positronium spectroscopy at a LINAC-based slow positron source

The [ital n]=2 fine-structure transitions in positronium have been measured by observation of increased Lyman-[alpha] radiation at microwave induced resonances. We used an accelerator based slow positron beam to produce positronium in the 2[sup 3][ital S][sub 1] excited state inside a waveguide. Our results are [nu][sub 0]=18 499.65[plus minus]1.20[plus minus]4.00 MHz, [nu][sub 1]=13 012.42[plus minus]0.67[plus minus]1.54 MHz, and [nu][sub 2]=8 624.38[plus minus]0.54[plus minus]1.40 MHz for the 2[sup 3][ital S][sub 1][r arrow][ital k]2[sup 3][ital P][sub 0,1,2] transition frequencies, respectively. The first error is statistical and the second is systematic. This is in agreement with recent bound state QED calculations.

Precise measurement of n=2 positronium fine-structure intervals.

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Spectroscopy of excited state positronium

Positronium spectroscopy at a LINAC-based slow positron source

Precise measurement of n=2 positronium fine-structure intervals.