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-Pérot 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 +1.6 −1.2 × 10 −8 s −1 for the first time, which is in good agreement with the theoretical value of 3.37 × 10 −8 s −1 .Positronium (Ps) [1], a bound state of an electron and a positron, is a purely leptonic system and is a good target to study quantum electrodynamics (QED) in bound state. The triplet (1 3 S 1 ) state of Ps, ortho-positronium (o-Ps), decays into three gamma rays with a lifetime of τ o = 142 ns [2,3]. On the other hand, the singlet (1 1 S 0 ) state of Ps, para-positronium (p-Ps), decays into two gamma rays in τ p = 125 ps [4]. The energy level of the ground state o-Ps is higher than that of the ground state p-Ps due to the spin-spin interaction between the electron and the positron. This difference is called the hyperfine structure of the ground state positronium (Ps-HFS), which is about 203 GHz. Although precise measurements of Ps-HFS have been performed in 1970s and 1980s [5,6], all of them are indirect measurements using Zeeman splitting of about 3 GHz caused by a static magnetic field of about 1 T. There is a discrepancy of 3.9 standard deviations (15 ppm) between the measured and the theoretical value [7]. The largest systematic uncertainty common to all previous measurements is the non-uniformity of the static magnetic field. It is important to directly measure Ps-HFS, in order to avoid the systematic uncertainty of the static magnetic field. Here we present a direct observation of the hyperfine transition between Ps-HFS, which is the first great step toward a direct measurement of Ps-HFS. The hyperfine transition of the ground state Ps, which is M 1 transition, has not yet been observed directly, since the transition probability (Einstein's A coefficient is A = 3.37 × 10 −8 s −1 [8]) is 10 14 times smaller than the decay rate of o-Ps (7.0401(6)×10 6 s −1 [2,3]). In order to cause sufficient amount of stimulated emission from o-Ps to p-Ps, we develop a new optical system which consists of a gyrotron as a sub-THz radiation source, a mode converter to convert the gyrotron output to a Gaussian beam, and a Fabry-Pérot cavity to accumulate high power sub-THz radiation. The gyrotron is a novel FIG. 1: Schematic diagrams of our experimental setup. Top view of the gas chamber is shown in the box. M1 and M2 are parabolic mirrors made of aluminum. We use a gold mesh plane mirror with a transmittance of about 3 % as a beam splitter (BS). Three pyroelectric detectors (PY) are used to monitor the incident, the reflected and the transmitted power.high power radiation source for sub-THz to THz region, which enables us to perform a direct measurement of the...
Influence of reflections on operation of gyrotrons with axial output is studied both theoretically and experimentally. By way of example the Fukui large orbit gyrotron with a permanent magnet operating in third harmonic at frequency 89 GHz is considered. In the case of strong reflection (= 0 6), extreme sensitivity of output power on the reflection phase is found. A qualitative agreement between theory and experiment is observed.
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 +- 0.0007 mu s^-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 3.5 sigma is found recently between the measured values and the QED prediction (O(alpha^3)). 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.Comment: This note contributes to pbar0
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(α 3 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.
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