Nonlinear Calcium King Plot Constrains New Bosons and Nuclear Properties

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Abstract
Nonlinearities in King plots (KP) of isotope shifts (IS) can reveal the existence of beyond-standard-model (BSM) interactions that couple electrons and neutrons. However, it is crucial to distinguish higher-order standard model (SM) effects from BSM physics. We measure the IS of the transitions P 3 0 → P 3 1 in Ca 14 + and S 2 1 / 2 → D 5 / 2 2 in Ca + with sub-Hz precision as well as the nuclear mass ratios with relative uncertainties below 4 × 10 − 11 for the five stable, even isotopes of calcium ( Ca 40 , 42 , 44 , 46 , 48 ). Combined, these measurements yield a calcium KP nonlinearity with a significance of ∼ 10 3 σ . Precision calculations show that the nonlinearity cannot be fully accounted for by the expected largest higher-order SM effect, the second-order mass shift, and identify the little-studied nuclear polarization as the only remaining SM contribution that may be large enough to explain it. Despite the observed nonlinearity, we improve existing KP-based constraints on a hypothetical Yukawa interaction for most of the new boson masses between 10 eV / c 2 and 10 7 eV / c 2 .
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Funding Information
  • Office of Isotope Research and Development and Production
  • Max-Planck-Gesellschaft
  • Deutsche Forschungsgemeinschaft (SCHM2678/5-2, SU 658/4-2, 544815538, 390837967, 390783311)
  • Munich Institute for Astro- and Particle Physics
  • Horizon 2020 Framework Programme (101019987-FunClocks, 832848-FunI)
  • H2020 European Research Council
  • Alexander von Humboldt-Stiftung
  • H2020 Marie Skłodowska-Curie Actions (795121)
  • Intelligence Advanced Research Projects Activity
  • Army Research Office (W911NF-16-1-0070)

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