Probing new spin-independent interactions through precision spectroscopy in atoms with few electrons

TL;DR

This paper uses high-precision spectral measurements in few-electron atoms to search for new spin-independent forces, setting new bounds on exotic interactions and dark-photon models.

Contribution

It demonstrates the potential of atomic spectroscopy to probe new physics beyond the Standard Model, especially in the MeV to keV mass range.

Findings

Current data probe new force-carrier couplings around MeV mass.

Spectroscopy sensitivity below keV is comparable to electron magnetic moment measurements.

Helium transitions provide strong bounds on dark-photon models above 100keV.

Abstract

The very high precision of current measurements and theory predictions of spectral lines in few-electron atoms allows to efficiently probe the existence of exotic forces between electrons, neutrons and protons. We investigate the sensitivity to new spin-independent interactions in transition frequencies (and their isotopic shifts) of hydrogen, helium and some helium-like ions. We find that present data probe new regions of the force-carrier couplings to electrons and neutrons around the MeV mass range. We also find that, below few keV, the sensitivity to the electron coupling in precision spectroscopy of helium and positronium is comparable to that of the anomalous magnetic moment of the electron. Finally, we interpret our results in the dark-photon model where a new gauge boson is kinetically mixed with the photon. There, we show that helium transitions, combined with the anomalous magnetic moment of the electron, provide the strongest indirect bound from laboratory experiments above 100keV.