New Precision Measurement of the Hyperfine Splitting of Positronium

Akira Ishida (Tokyo Univ. and CERN)
DESY Auditorium, 16.45 h

Positronium (Ps) is an ideal system for precision tests of bound-state Quantum Electrodynamics (QED). The hyperfine Splitting (HFS) of the ground-state Ps, which is one of the most precisely tested quantities, has a discrepancy of 16 ppm (4.5 σ) between the averaged previous experimental value and the theoretical calculation with O(a3) corrections. A new experiment which reduces possible systematic uncertainties of Ps thermalization effect and non-uniformity of magnetic field was performed to check the discrepancy. It revealed that the Ps thermalization effect was as large as 10 ± 2 ppm. Taking into account this effect, a new result of 203.349 2 ± 0.0016 (stat., 8.0 ppm) ± 0.0013 (syst., 6.4 ppm) GHz was obtained. This result is consistent with the QED prediction within 1.1 σ, whereas it disfavours the previous experimental average by 2.6 σ. It shows that the Ps thermalization effect is crucial for precision measurement of HFS. In this seminar, I will explain the details of the new experiment. Future prospects to obtain further precision will be also briefly discussed.

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