High Energy Accelerator Research Organization (KEK)

J-PARC Center

Nagoya University

School of Science,The University of Tokyo

Summary of Presentations

A muonic helium atom, which is composed of an ordinary helium atom with one of its two electrons replaced by a negative muon, is a special atom that is not found in nature. It has a small energy level structure, called hyperfine structure, that results from the interaction between the remaining electron and the negative muon (due to the intrinsic properties of their respective spin).

Precision spectroscopy measurements of the muonic helium atom hyperfine structure are the only available experimental results for three-body muonic atoms (i.e., helium nucleus, electron, and negative muon) that can be used to verify and improve the current theory of particle physics for three-body atomic system and quantum electrodynamics (QED), which accurately describes how matter and light interact at the quantum level. It can also be used to determine the mass of the negative muon to test CPT invariance by comparing the masses of positive and negative muons (second-generation leptons).

The hyperfine structure of muonic helium atoms has only been measured twice in the 1980s at the Paul Scherrer Institute in Switzerland (directly at zero magnetic field) and Los Alamos National Laboratory in the United States (indirectly at high magnetic field).

In this study, we succeeded in measuring the hyperfine structure of muonic helium atoms directly at zero magnetic field using the Muon Science Facility (MUSE) D-line at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC) with a precision 3 times better than the previous direct measurement. The result obtained is also 1.5 times more precise than the previous indirect measurement at high field improving the current world record and establishing a highly precise spectroscopic method. It was also performed for the first time with methane admixture used as an electron donor to form neutral muonic helium atoms efficiently, the prerequisite to measuring the hyperfine structure.

This technique will soon be used at the H-line, which offers about ten times higher muon beam intensity than at the D-line and the possibility of longer measurement time. This will allow us to measure the hyperfine structure of muonic helium atoms at high magnetic field a hundred times more precisely and determine the negative muon mass with greater precision.

Figure：Experimental apparatus for muonic helium atom spectroscopy measurements in the D2 area of J-PARC MLF MUSE.

Associate Professor Hiroyuki Torii of the Department of Chemistry participated in this research.

For more information, please visit the website of High Energy Accelerator Research Organization (KEK).

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Journal name	Physical Review Letters
Title of paper	Improved Measurements of Muonic Helium Ground-State Hyperfine Structure at a Near-Zero Magnetic Field