Testing standard-model extensions with isotope shifts in few-electron ions

Abstract

When collecting spectroscopic data on at least four isotopes, nonlinearities in the King plot are a possible sign of physics beyond the standard model. In this work, an improved approach to the search for hypothetical new interactions with isotope-shift spectroscopy of few-electron ions is presented. Careful account is taken of the small nuclear corrections to the energy levels and the gyromagnetic factors, which cause deviations from King linearity within the standard model and are hence a possible source of confusion. In this approach, the experimental King nonlinearity is not compared to the vanishing prediction of the standard model at the leading order, but to the calculated full standard model contribution to King nonlinearity. This makes searching for beyond-the-standard-model physics with King linearity analysis possible in a high-precision experimental regime, avoiding confusion. The bounds which can be set on beyond-the-standard-model parameters remain limited by the uncertainties on the small standard model nuclear corrections which cause King nonlinearity. Direct comparison between theory and experiment on a single pair of isotopes is advocated as a more suitable approach for few-electron ions.