Mu-MASS · Muonium Laser Spectroscopy

Striking anomalies in the muon sector have accumulated in recent years: notably the famous anomalous muon magnetic moment (g-2) and the muonic hydrogen Lamb shift measurement which prompted the so-called proton charge radius puzzle. These tantalizing results triggered vibrant activity on both experimental and theoretical sides. Different explanations have been put forward including exciting solutions invoking New Physics beyond the Standard Model. To contribute to clarifying the origin of these anomalies, I propose Mu-MASS, an experiment aiming for a 1000-fold improvement in the determination of the 1S-2S transition frequency of Muonium (M), the positive-muon/electron bound state. This substantial improvement beyond the current state-of-the-art relies on the novel cryogenic M converters and confinement techniques developed by the PI, and on the new laser and detection schemes which the PI implemented for positronium spectroscopy. This experiment will be performed at the Paul Scherrer Institute (PSI). With the Mu-MASS result our knowledge of the muon mass can be improved by almost two orders of magnitude. By using the expected results of the ongoing hyperfine splitting measurement of M in Japan, it will provide one of the most sensitive tests of bound-state Quantum Electrodynamics. It can also be used to extract the muon g-2 from the ongoing experiment at Fermilab. Since M is a unique system composed of two different leptons (point-like particles), the Mu-MASS results will provide the most stringent test of charge equality between the lepton generations. Moreover, it can be used to determine the Rydberg constant free from nuclear and finite-size effects and contribute to solving the proton charge radius puzzle. Mu-MASS is thus very timely and essential to the worldwide effort to understand the interesting observed discrepancies, which could be a hint of New Physics and therefore have profound implications on our understanding of the Universe.