Fine-structure constant sensitivity of the Th-229 nuclear clock transition

Area of Science:

• Nuclear Physics
• Atomic Physics
• Spectroscopy

Background:

• Precise nuclear structure information is crucial for testing fundamental physics theories.
• Nuclear clocks offer unique sensitivity to variations in fundamental constants like the fine-structure constant (α).
• The Thorium-229 (²²⁹Th) nucleus presents a unique case for nuclear structure studies due to its low-lying nuclear states.

Purpose of the Study:

• To precisely measure the fractional change in the nuclear quadrupole moment (ΔQ₀/Q₀) of ²²⁹Th upon excitation.
• To quantify the sensitivity of ²²⁹Th nuclear transitions to variations in the fine-structure constant (α).
• To test and refine nuclear models of ²²⁹Th and assess the role of nuclear volume effects.

Main Methods:

• Employed nuclear laser spectroscopy with 10⁻¹² precision.
• Utilized a semi-classical prolate-spheroid model for calculations.
• Measured the charge-radius change (Δ〈r²〉) and nuclear quadrupole moment change (ΔQ₀/Q₀).

Main Results:

• Determined ΔQ₀/Q₀ = 1.791(2)%, a high-accuracy measurement.
• Quantified the sensitivity to fine-structure constant variations as K = 5900(2300).
• Observed a strong dependence of ΔQ₀ on nuclear volume, challenging the constant-volume approximation.

Conclusions:

• The high-accuracy measurement enables stringent tests of ²²⁹Th nuclear models.
• Nuclear clocks show enhanced sensitivity to α variations compared to atomic clocks, beneficial for new physics searches.
• Discrepancies highlight the need for improved nuclear modeling and measurement of additional nuclear parameters, including octupole contributions.