Speaker
Description
Muonium is a bound state of a positive muon and an electron. Precise measurements of the muonium hyperfine structure (HFS) provide a stringent test of quantum electrodynamics (QED), whose theoretical predictions are calculated with extremely high precision [1]. In the field of precision muon physics, there is ongoing discussion regarding the hadronic vacuum polarization contribution to the muon anomalous magnetic moment, $g-2$. A further improvement in the determination of the muonium HFS has attracted attention as a possible key to resolving this puzzle [2]. The Muonium Spectroscopy Experiment Using Microwave (MuSEUM) aims to improve the precision of the muonium HFS measurement by an order of magnitude compared to previous experiments.
The measurement principle of the muonium HFS is as follows. A 100% polarized muon beam is injected into krypton gas to produce muonium atoms. Microwave radiation is applied using a microwave cavity, and the resonance frequency is determined from a frequency scan, in which the number of decay positrons is measured at each microwave frequency. In high magnetic field measurements, two transition frequencies among the four Zeeman-split energy levels are measured. We have developed a magnetic field measurement system using an NMR probe with a precision of 15 ppb, as well as monitoring systems for magnetic field stability, krypton gas pressure, temperature, and microwave power. Estimation of systematic uncertainties associated with these parameters is also in progress.
Since 2025, MuSEUM has started test measurements in a high magnetic field at the J-PARC H-line. We have successfully observed resonance signals corresponding to the two transition frequencies. In addition, the dependence of the resonance peak on krypton gas pressure has been observed. Currently, we are performing a detailed analysis of these data and estimating the statistical uncertainty achievable in the final result. In this presentation, we report on the current status of the MuSEUM experiment.
[1] P. J. Mohr, D. B. Newell, B. N. Taylor, and E. Tiesinga, Rev. Mod. Phys. 97 025002 (2025).
[2] L. D. Luzio, A. Keshavarzi, A. Masiero, and P. paradisi, Phys. Rev. Lett. 134, 011902 (2025).