Probing imperfect axions with nuclear clocks

Abstract

We show that ${}^{229}$Th nuclear clocks can probe departures from the Peccei-Quinn (PQ) solution to the strong CP problem. Explicit PQ breaking or additional CP-violating interactions can shift the minimum of the axion potential and generate CP-violating couplings to matter, inducing a clock response linear in the axion field. We investigate explicit PQ breaking and quark chromo-electric dipole operators as representative realizations of these \textit{imperfect axion} scenarios, consistent with electric dipole moment constraints. For axion dark matter, the linear signal oscillates at the Compton frequency and can dominate the quadratic response. Even without axion dark matter, the Earth can source a static axion field, enabling fifth-force searches with nuclear clocks.

arXiv comment5+2 pages, 1+3 figures