Description
The Mu-MASS experiment at PSI focuses on studying the spectroscopic properties of Muonium (M), the positive-muon/electron bound state.
The experiment was proposed in 2019 and since then is running approximately 10 days/year at the PSI low energy muon (LEM) facility. After successfully demonstrating the production and detection of a 2S metastable M beam (J. Janka et al. EPJC 80, 804 (2020)), the Mu-MASS collaboration could perform a measurement of the Muonium Lamb shift at the LEM beamtime (B. Ohayon et al. PRL 128, 011802 (2022)). Since the measured value matches the theoretical calculations within one standard deviation, one could set stringent limits on Lorentz/CPT violation in the muonic sector, as well as on new physics coupled to muons and electrons which could provide an explanation to the muon $g-2$ anomaly. Additionally, Mu(3S) was detected and the $2S_{1/2}(F=0) \rightarrow 2P_{1/2}(F=1)$ transition was measured, with its resonance extracted at \SI{580.4+-7.2} MHz (G. Janka et al. arxiv:2205.06202, accepted in Nat. Comm.). Being isolated, this transition holds the promise to provide an improved determination of the Muonium Lamb shift at a level where bound state QED recoil corrections not accessible in hydrogen could be tested and new physics scenario could be probed. To reach this goal the setup is being upgraded and a measurement of the fine structure at the same level is also foreseen.
The longer term goal of the Mu-MASS experiment at PSI is a 1000-fold improvement in the determination of the 1S-2S transition frequency of Muonium. This substantial improvement beyond the current state-of-the-art relies on novel cryogenic M converters, new excitation and detection schemes which we implemented for positronium spectroscopy, and the advances in generation of UV radiation (Z. Burkley, L. de Sousa Borges, B. Ohayon, A. Golovizin, J. Zhang, and P. Crivelli, Optics Express 29, 27450 (2021)). This will provide the best determination of the muon mass at a level of 1 ppb. Moreover, combined with the results of the ongoing hyperfine splitting measurement (MUSEUM) at the Japan Proton Accelerator Research Complex (JPARC) this will yield one of the most sensitive tests of bound state Quantum ElectroDynamics (QED) with a relative precision of $1\times10^{-9}$.