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Positronium is an ideal system for research on Quantum Electrodynamics (QED), especially in a bound state. A discrepancy of 3.9 standard deviations has been found between the measured hyperfine structure (Ps-HFS) and the QED predictions. This may be due to the contribution of unknown new physics or common systematic effects in previous measurements, in all of which the Zeeman effect was used. We propose a new method to directly measure the Ps-HFS using a high power gyrotron. We compare two resonators which have been developed to supply sufficient power to drive the direct transition, a Fabry-Perot resonator and a ring resonator with a diffraction grating. We plan to perform first direct measurement of Ps-HFS within the next six months.

Positronium is an ideal system for research on QED, especially in a bound state. A discrepancy (3.9σ) is found recently between measured HFS values and the QED prediction (including up-to O(α3 log α − 1), where α is the fine-structure constant.). It might be due to a contribution of unknown new physics or common systematic problems in all the previous measurements. A new method to measure HFS directly is performed using a high power gyrotron. The transition from ortho-positronium to para-positronium has been observed with 5 σ CL, which is the first observation of M1 transition in (sub)Terahertz region. New technologies of high power gyrotrons are developed for precision spectroscopy.

Positronium is an ideal system for the research of the bound state QED. The hyperfine splitting of positronium (Ps-HFS, about 203 GHz) is an important observable but all previous measurements of Ps-HFS had been measured indirectly using Zeeman splitting. There might be the unknown systematic errors on the uniformity of magnetic field. We are trying to measure Ps-HFS directly using sub-THz radiation. We developed an optical system to accumulate high power (about 10 kW) radiation in a Fabry-Perot resonant cavity and observed the positronium hyperfine transition for the first time.

Positronium is an ideal system for the research of Quantum Electrodynamics (QED), especially for QED in bound state. The discrepancy of 3.9σ was found recently between the measured HFS values and the QED prediction of O(α3). It might be due to the contribution of unknown new physics or systematic problems in the all previous measurements. We propose a new method to measure HFS directly and precisely. A gyrotron, a novel sub-THz light source is adopted with a Fabry-Pérot cavity with high finesse and an efficient transportation system in order to obtain sufficient radiation power at 203 GHz. The present status of the optimization studies and the current design of the experiment are described.

The ground state hyperfine splitting of positronium, , is sensitive to high order corrections of QED. A new calculation up to O( ) has revealed a 3.9 discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure employing new methods designed to remedy the possible systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The prototype run has been performed and a result of = 203.3804 0.0022 (stat., 11 ppm) 0.0081 (sys., 40 ppm) GHz has been obtained. Compensation magnets to obtain O(ppm) magnetic field uniformity has been developed. The final run will start soon and a measurement with a precision of O(ppm) is expected within a few years.

We plan to directly measure the hyperfine structure of the ground-state positronium. The hyperfine structure between ortho-positronium and para-positronium is about 203 GHz. We develop a new optical system to accumulate about 20 kW power using a gyrotron and high finesse Fabry-Perot resonator. We report the current status of our experiment.

We plan to directly measure the hyperfine structure of the ground-state positronium. The hyperfine structure between ortho-positronium and para-positronium is about 203 GHz. We develop a new optical system to accumulate about 10 kW power using a gyrotron and a Fabry-Perot cavity, and have already observed the hyperfine transition at a frequency point of 202.9 GHz. The transition probability is measured to be A = 3.1 −1.2 +1.6×10−8 s−1, which is in good agreement with the theoretical value of 3.37×10−8 s−1. In order to measure the hyperfine structure, we have to repeat transition measurements at different frequencies around 203 GHz. We are now refining the optical system to quickly change output frequency and obtain more power.

We report the first direct measurement of the hyperfine transition of the ground state positronium. The hyperfine structure between ortho-positronium and para-positronium is about 203 GHz. We develop a new optical system to accumulate about 10 kW power using a gyrotron, a mode converter, and a Fabry-P\'erot cavity. The hyperfine transition has been observed with a significance of 5.4 standard deviations. The transition probability is measured to be $A={3.1}_{\ensuremath{-}1.2}^{+1.6}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ for the first time, which is in good agreement with the theoretical value of $3.37\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.

The direct transition from ortho-positronium to para-positronium with high-power sub-THz radiation has been observed with 5.1 σ significance. The observed transition rate is consistent with the theoretical calculation. A sub-THz gyrotron is used as the light source, and a quasi-optical Fabry-Perot cavity is utilized to accumulate the radiation. This is the first observation of the positronium hyperfine splitting (Ps-HFS), which leads to the first direct measurement of Ps-HFS with a target accuracy of 100 ppm in several years.

The direct transition from o-Ps to p-Ps was firstly observed, using a gyrotron, a mode-converter and a Fabry-Perot cavity. The Einstein's A coefficient is measured as 3.9 (+1.8 −1.7) • 10−8 s−1. It is the first step to directly examine a reported discrepancy of positronium hyperfine splitting.

Positronium is an ideal system for the research of the quantum electrodynamics (QED) in bound state. The hyperfine splitting (HFS) of positronium, $\Delta_{\mathrm{HFS}}$, gives a good test of the bound state calculations and probes new physics beyond the Standard Model. A new method of QED calculations has revealed the discrepancy by 15\,ppm (3.9$\sigma$) of $\Delta_{\mathrm{HFS}}$ between the QED prediction and the experimental average. There would be possibility of new physics or common systematic uncertainties in the previous all experiments. We describe a new experiment to reduce possible systematic uncertainties and will provide an independent check of the discrepancy. We are now taking data and the current result of $\Delta_{\mathrm{HFS}} = 203.395\,1 \pm 0.002\,4 (\mathrm{stat.}, 12\,\mathrm{ppm}) \pm 0.001\,9 (\mathrm{sys.}, 9.5\,\mathrm{ppm})\,\mathrm{GHz} $ has been obtained so far. A measurement with a precision of $O$(ppm) is expected within a year., Comment: 8 pages, 8 figures, 2 tables, proceeding of LEAP2011, accepted by Hyperfine Interactions

We report on the first measurement of the Breit-Wigner resonance of the transition from {\it ortho-}positronium to {\it para-}positronium. We have developed an optical system to accumulate a power of over 20 kW using a frequency-tunable gyrotron and a Fabry-P\'{e}rot cavity. This system opens a new era of millimeter-wave spectroscopy, and enables us to directly determine both the hyperfine interval and the decay width of {\it p-}Ps.

The positronium hyperfine splitting is a good target to study Quantum Electrodynamics in the bound state. There is a discrepancy between precision measurements and a theoretical calculation. We are planning to directly measure the positroinum hyperfine structure for the first time. A gyrotron oscillator is used as a novel radiation source in terahertz region. A Fabry-Perot resonator is also developed to increase photon density. We have already observed the direct transition at 202.9 GHz. The direct measurement of the order of 100 ppm will be performed within about a year.

Positronium is an ideal system for the research of the bound state QED. The hyperfine splitting of positronium (Ps-HFS: about 203 GHz) is a good tool to test QED and also sensitive to new physics beyond the Standard Model via a quantum oscillation between an ortho-Ps and a virtual photon. Previous experimental results show 3.9 σ (15 ppm) discrepancy from the QED calculation. All previous experiments used an indirect method with static magnetic field to cause Zeeman splitting (a few GHz) between triplet states of ortho-Ps, from which the HFS value was derived. One possible systematic error source of the indirect method is non-uniformity of the static magnetic field. We are developing a new direct Ps-HFS measurement system without static magnetic field. In this measurement we use a gyrotron, a novel sub-THz light source, with a high-finesse Fabry-Perot cavity to obtain enough radiation power at 203 GHz. The present status of the optimization studies and current design of the experiment are described.

Positronium is a QCD-free system and the measurement of its hyperfine splitting provides a strict test of quantum electrodynamics (QED). Recent research revealed a discrepancy of 3.9? between the QED prediction and previous experimental results. We report on the prototype run of an improved experimental set-up and the performance of its RF system.

Interference between different energy eigenstates in a quantum system results in an oscillation with a frequency which is proportional to the difference in energy between the states. Such an oscillation is observable in polarized positronium when it is placed in a magnetic field. In order to measure the hyperfine splitting of positronium, we perform the precise measurement of this oscillation using a high quality superconducting magnet and fast photon-detectors. A result of $203.324 \pm 0.039\rm{~(stat.)} \pm 0.015\rm{(~sys.)}$~GHz is obtained which is consistent with both theoretical calculations and previous precise measurements., Comment: 4 figures accepted by Phys. Lett. B

The ground state hyperfine splitting of positronium, ΔHFS, is sensitive to high order corrections of QED. A new calculation up to O(α3) has revealed a 3.9 σ discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure ΔHFS employing new methods designed to remedy the systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The measurement is in progress and a result of ΔHFS = 203.385 ± 0.011 GHz (58ppm) has been obtained from the prototype run. A measurement with a precision of O(ppm) is expected within a few years.

The ground state hyperfine splitting in positronium, $\Delta _{\mathrm{HFS}}$, is sensitive to high order corrections of QED. A new calculation up to $O(\alpha ^3)$ has revealed a $3.9 \sigma$ discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure $\Delta_{\mathrm{HFS}}$ employing new methods designed to remedy the systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The measurement is in progress and a preliminary result of $\Delta_{\mathrm{HFS}} = 203.399 \pm 0.029 \mathrm{GHz} (143 \mathrm{ppm})$ has been obtained. A measurement with a precision of O(1) ppm is expected within a few years., Comment: 5 pages, 6 figures, contributed to POSMOL 2009, will be published in J. Phys.: Conf. Series

Positronium is an ideal system for the research of the bound state QED. The hyperfine splitting of positronium (Ps-HFS: about 203 GHz) is sensitive to new physics beyond the Standard Model via a vacuum oscillation between an ortho-Ps and a virtual photon. Previous experimental results of the Ps-HFS show 3.9 sigma(15 ppm) discrepancy from the QED calculation. All previous experiments used an indirect method with static magnetic field to cause Zeeman splitting (a few GHz) between triplet states of ortho-Ps, from which the HFS value was derived. One possible systematic error source of the indirect method is the static magnetic field. We are developing a new direct measurement system of the Ps-HFS without static magnetic field. In this measurement we use a gyrotron, a novel sub-THz light source, with a high-Finesse Fabry-Perot cavity to obtain enough radiation power at 203 GHz. The present status of the optimization studies and current design of the experiment are described., Comment: 6 pages, 6 figures, Proceedings of POSMOL 2009