Investigation of the gravitational-potential dependence of the fine-structure constant using atomic dysprosium

Radio-frequency E1 transitions between nearly degenerate, opposite-parity levels of atomic dysprosium (Dy) were monitored over an 8-month period to search for a variation in the fine-structure constant α. During this time period, data were taken at different points in the gravitational potential of the Sun. The data are fitted to the variation in the gravitational potential yielding a value of (−8.7±6.6)×10⁻⁶ for k_α, the variation of α in a changing gravitational potential. This value gives the first laboratory limit independent of assumptions regarding other fundamental constants. In addition, our value of k_α combined with other experimental constraints is used to extract the first limits on k_e and k_q. These coefficients characterize the variation of m_e∕m_p and m_q∕m_p in a changing gravitational potential, where m_e, m_p, and m_q are electron, proton, and quark masses. The results are k_e=(4.9±3.9)×10⁻⁵ and k_q=(6.6±5.2)×10⁻⁵. All these results indicate the absence of significant variation at the present level of sensitivity.