Akira Ishida

New Precision Measurement of Hyperfine Splitting of Positronium

Abstract The ground state hyperfine splitting of positronium Δ HFS is sensitive to high order corrections of quantum electrodynamics (QED) in bound state. The theoretical prediction and the averaged experimental value for Δ HFS have a discrepancy of 15 ppm, which is equivalent to 3.9 standard deviations (s.d.). A new precision measurement which reduces the systematic uncertainty from the positronium thermalization effect was performed, in which the non-thermalization effect was measured to be as large as 10 ± 2 ppm in a timing window we used. When this effect is taken into account, our new result becomes Δ HFS = 203.394 2 ± 0.001 6 ( stat. , 8.0 ppm ) ± 0.001 3 ( sys. , 6.4 ppm ) GHz , which favors the QED prediction within 1.2 s.d. and disfavors the previous experimental average by 2.6 s.d.

Probing the energy structure of positronium with a 203 GHz Fabry-Perot Cavity

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 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 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.

The first direct measurement of the hyperfine splitting in positronium

Positronium is an ideal system for the research of the QED. The hyperfine splitting of positronium (Ps-HFS) is sensitive to the new physics beyond the Standard Model via a vacuum oscillation. Previous experimental results of the Ps-HFS show 3.5 σ discrepancy from the QED calculation, and it might be caused by uncertainties of the indirect method with static magnetic field and a few GHz RF. We developed a new direct measurement system of the Ps-HFS without static magnetic field, using a sub-THz gyrotron and a quasi-optical Fabry-Perot cavity. Status (hopefully the first result) of the direct positronium hyperfine transition observation will be presented.

Experiment for the first direct measurement of the hyperfine splitting of positronium

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.

Precision measurements of positronium decay rate and energy level

Positronium is an ideal system for the research of the bound state QED. New precise measurement of orthopositronium decay rate has been performed with an accuracy of 150 ppm, and the result combined with the last three is 7.0401±0.0007u2009μs−1. It is the first result to validate the 2nd order correction. The Hyper Fine Splitting of positronium is sensitive to the higher order corrections of the QED prediction and also to the new physics beyond Standard Model via the quantum oscillation into virtual photon. The discrepancy of 3.5σ is found recently between the measured values and the QED prediction (O(α3)). It might be due to the contribution of the new physics or the systematic problems in the previous measurements: (non‐thermalized Ps and non‐uniformity of the magnetic field). We propose new methods to measure HFS precisely without the these uncertainties.

Precise measurement of HFS of positronium using Zeeman effect

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.

Precise measurement of the HFS of positronium using the zeeman effect I: Experimental set-up and RF system

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.

First test of O(α) correction on the energy spectrum in the orthopositronium decay

Positronium(Ps) is an ideal system for the precision test of quantum electrodynamics (QED) in bound state. Orthopositronium(o-Ps) decays into three gamma rays dominantly, and their energy spectrum has detail information on bound-state QED. The purpose of this study is to check the spectrum at the O(α) level accuracy. We constructed a setup for the precise measurement. Ps is created at silica aerogel in a vacuum chamber, and its decay gamma rays are measured with a LaBr3 (Ce) scintillator. In this setup, the materials around the Ps assembly and the detector are well-controlled, and all their responses are coded in a GEANT4 simulator in detail. After the measurement for 100 days, the obtained spectrum is compared with the expected spectrum from the QED calculations and the GEANT4 simulator. The obtained spectrum is consistent with O(a) calculation and excludes the lowest- order calculation at 92% C.L.

The first direct observation of positronium hyperfine splitting

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.

Precise Measurement of Hyperfine Splitting of Positronium Using the Zeeman Effect

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.