Despite its incredible success, the Standard Model of particle physics does not provide a complete description of nature. For example, it does not explain the origin of dark matter, dark energy, the baryon asymmetry in the universe, or include gravity.
Muonium, an exotic atom composed of an anti-muon and an electron, is a unique system for addressing these outstanding questions. Unlike normal atoms, which have a nucleus made of neutrons and protons with a complicated substructure that cannot be calculated from first principles, the energy spectra of muonium, composed of two elementary point-like particles, can be predicted to a very high accuracy using quantum electrodynamics.
Therefore, if one can measure its properties very precisely by means of spectroscopy using microwaves or lasers, muonium becomes an ideal object to search for new physics beyond the Standard Model. In fact, if a difference between the theoretical predictions and experimental results is found, it could hint at new processes not considered in the calculations.
A notable example that is being scrutinized is the long-standing discrepancy between the Standard Model prediction and the Brookhaven National Laboratory measurement of the anomalous muon magnetic moment (g-2), which has recently been confirmed at Fermilab.
High-precision spectroscopy of muonium is the main objective of the Mu-MASS experiment, funded through this ERC grant. From the measurement of its spectroscopic properties, one can also extract improved values for fundamental constants such as the muon mass and an independent determination of the Rydberg constant and the fine structure constant, free of nuclear finite-size effects.
