Searching for a Fifth Force with Atomic and Nuclear Clocks

TL;DR

This paper explores how atomic and nuclear clocks can detect a hypothetical fifth force mediated by an ultralight scalar field, potentially violating the equivalence principle, and assesses current and future experimental sensitivities.

Contribution

It introduces a framework for using atomic and nuclear clocks to detect equivalence principle violating fifth forces and evaluates their sensitivity compared to existing constraints.

Findings

Annual modulation in clock frequencies can probe fifth forces.

Orbiting clocks can achieve greater sensitivity than laboratory experiments.

Current clock technology can match existing limits on electron-coupled fifth forces.

Abstract

We consider the general class of theories in which there is a new ultralight scalar field that mediates an equivalence principle violating, long-range force. In such a framework, the sun and the earth act as sources of the scalar field, leading to potentially observable location dependent effects on atomic and nuclear spectra. We determine the sensitivity of current and next-generation atomic and nuclear clocks to these effects and compare the results against the existing laboratory and astrophysical constraints on equivalence principle violating fifth forces. We show that in the future, the annual modulation in the frequencies of atomic and nuclear clocks in the laboratory caused by the eccentricity of the earth's orbit around the sun may offer the most sensitive probe of this general class of equivalence principle violating theories. Even greater sensitivity can be obtained by placing a precision clock in an eccentric orbit around the earth and searching for time variation in the frequency, as is done in anomalous redshift experiments. In particular, an anomalous redshift experiment based on current clock technology would already have a sensitivity to fifth forces that couple primarily to electrons at about the same level as the existing limits. Our study provides well-defined sensitivity targets to aim for when designing future versions of these experiments.