Additions to Standard Model and Atomic Physics

Recently, experiments at the ATOMKI laboratory on the possible existence of a

low-energy addition to the Standard Model have generated widespread attention.

While the experimental findings lack an independent confirmation, they give

incentive, from a more general point of view, to investigate the possible

detection of new particles with a rest mass of 15 to 20 MeV, in atomic physics

high-precision experiments. We find that, in this mass range, the effects will

be most pronounced in the spectra of muonic (not electronic) bound systems,

and, most notably, in the hyperfine structure. While it is a challenge to

separate the effects from nuclear-structure corrections to the hyperfine

splitting, this endeavor is definitely not hopeless. In general, in our work

[Phys.Rev.A 101 (2020) 062503], we derive the effective potentials

corresponding to the exchange of a pseudoscalar particle, and a conceivable

hitherto undetected vector particle, in two-body bound systems.

Comparison with the literature reveals

that the corresponding effective potentials may not have been treated

consistently in the past. Among other things, the role of the Coulomb gauge for

the massive vector propagator  and its suitability for bound-state calculations

are being discussed.