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

When collecting spectroscopic data on at least four isotopes, nonlinearitiesin the King plot are a possible sign of Physics beyond the Standard Model. Inthis work, an improved approach to the search for hypothetical new interactionswith isotope shift spectroscopy of few-electron ions is presented. Very carefulaccount is taken of the small nuclear corrections to the energy levels and thegyromagnetic factors, which cause deviations from King linearity within theStandard Model and are hence a possible source of confounds. In this newapproach, the experimental King nonlinearity is not compared to the vanishingprediction of the Standard Model at the leading order, but to the calculatedfull Standard Model contribution to King nonlinearity. This makes searching forbeyond-the-Standard-Model physics with King linearity analysis possible in avery-high-precision experimental regime, avoiding confounds. The bounds whichcan be set on beyond-the-Standard-Model parameters remain limited by theuncertainties on the small Standard Model nuclear corrections which cause Kingnonlinearity. Direct comparison between theory and experiment on a single pairof isotopes is advocated as a more suitable approach for few-electron ions.