Speaker
Description
Muonic helium is an exotic, hydrogen-like atom formed when a negative muon replaces one of the two electrons in an ordinary helium atom. Its ground-state hyperfine structure (HFS), arising from the interaction between the magnetic moments of the negative muon and the remaining electron, is very similar to muonium HFS but inverted. The same microwave magnetic resonance technique used to measure muonium HFS can be applied to precisely determine muonic helium HFS and the negative muon magnetic moment and mass [1]. The world's most intense pulsed negative muon beam at the J-PARC Muon Science Facility (MUSE) enables a more precise determination of the muonic helium HFS, which will be beneficial to test and improve the QED theory of the three-body atomic system and test CPT invariance by comparing the magnetic moments and masses of positive and negative muons (second-generation leptons).
Improved measurements of muonic helium HFS at zero magnetic field have already been obtained at MUSE D-line [2]. Since 2025, the MuSEUM collaboration started test measurements of muonium HFS in a high magnetic field at MUSE H-line. In high magnetic field measurements, two transition frequencies among the four Zeeman-split energy levels are measured, and the corresponding resonance curves have already been observed [3]. Muonic helium HFS measurements at high field are now in preparation and will follow shortly. Using ten times more muon beam intensity than at the D-line, and with decay electrons being more focused on the detector due to the high magnetic field, we aim at improving the accuracy of previous measurements done 40 years ago [4], nearly a hundred times.
In addition, to drastically improve measurement precision potentially by tenfold, a new method using Spin-Exchange Optical Pumping (SEOP) [5] is also being investigated. This technique aims to restore the negative muon polarization lost in the muon cascade process in helium. The first laser repolarization experiments for this approach were recently completed.
We will present an overview of these new muonic helium HFS measurements and the latest results.
[1] V. W. Hughes and G. zu Putlitz, in Quantum Electro-dynamics, ed. T. Kinoshita (World Scientific, Singapore, 1990), pp. 822–904.
[2] P. Strasser et al., Phys. Lett. 131, 253003 (2023).
[3] S. Nishimura, this conference.
[4] C. J. Gardner et al., Phys. Rev. Lett. 48, 1168 (1982).
[5] A. S. Barton et al., Phys. Rev. Lett. 70, 758 (1993).