Speaker
Description
New physics beyond the Standard Model (SM) could be searched for via fifth force experiments which could have interesting outcomes for atomic physics observables, such as the ground state hyperfine splitting in muonium. With that in mind, we consider in full generality the effective interactions of new particles with scalar, pseudo-scalar, vector and axial couplings with SM fermions. First, we update the bounds on new physics coming from recent experimental data. Then, we venture beyond the tree-level to understand how quantum effects may be relevant. Unlike calculations based on the Bethe-Salpeter equation, we rely on the combination of dispersion techniques with non-relativistic expansions to directly link the Feynman amplitudes to the hyperfine splittings. Specifically, we account for renormalization effects and the additional contributions coming from triangular and box Feynman diagrams for each type of interaction. As a result, we observe a significant improvement provided by the boxes which may constrain better the parameter spaces of light new physics. Additionally, we also study how these effects may contaminate the electron-to-muon mass ratio in the 1S − 2S transition frequency, by entering the ground state hyperfine splitting through corrections to the Fermi energy.
