Fine-structure constant sensitivity of the Th-229 nuclear clock transition
Kjeld Beeks, Georgy A. Kazakov, Fabian Schaden, I. Morawetz, L. Toscani De Col, Thomas Riebner, Michael Bartokos, Tomáš Šikorský, Thorsten Schumm, Chuankun Zhang, Tian Ooi, Jacob S. Higgins, Jack F. Doyle, Jun Ye, M. S. Safronova
Abstract
Nuclear laser spectroscopy at the 10−12 precision level (Nature 633.8028 (2024): 63–70) determined the fractional change in the nuclear quadrupole moment of 229Th upon excitation, ΔQ0/Q0 = 1.791(2)%. Such high-accuracy nuclear parameters enable stringent tests and refinement of 229Th nuclear models. Using a semi-classical prolate-spheroid model, we quantify the transition frequency’s sensitivity to fine-structure constant variations as K = 5900(2300), with uncertainty dominated by the measured charge-radius change Δ〈r2〉. This supports the predicted higher α-sensitivity of nuclear clocks over atomic clocks, important for new-physics searches. We find ΔQ0 strongly dependent on nuclear volume, challenging the constant-volume approximation. The deviation between measured and predicted ΔQ0/Q0 underlines the need for improved modeling and measurement of additional nuclear parameters. We explicitly assess the octupole contribution to α-sensitivity. The authors report on new developments on the sensitivity of the nuclear clock transition in Th229 for new physics searches involving variations of the fine-structure constant. This highlights the need for developing of advanced nuclear models and parameter searches relating to experimental measurements.