Differences between Axions and Generic Light Scalars in Laboratory Experiments

TL;DR

This paper compares axions and generic light scalars, highlighting how experimental bounds differ significantly due to their distinct theoretical properties, with EDM constraints being dominant for axions but not for generic scalars.

Contribution

It clarifies the contrasting experimental constraints on axions versus generic light scalars, emphasizing the role of their theoretical origins in determining bound strengths.

Findings

EDM bounds are strongest for axions due to their link to the Strong CP problem.

Fifth-force and astrophysical bounds are more restrictive for generic scalars.

The constraints vary significantly depending on the scalar's relation to the Strong CP problem.

Abstract

It is well-known that electric dipole moment (EDM) constraints provide the most stringent bounds on axion-mediated macroscopic spin-dependent (SD) and time reversal and parity violating (TVPV) forces. These bounds are several orders of magnitude stronger than those arising from direct searches in fifth-force experiments and combining astrophysical bounds on stellar energy loss with Eotvos tests of the weak equivalence principle (WEP). This is a consequence of the specific properties of the axion, invoked to solve the Strong CP problem. However, the situation is quite different for generic light scalars that are unrelated to the strong CP problem. In this case, bounds from fifth-force experiments and astrophysical processes are far more stringent than the EDM bounds, for the mass range explored in direct searches.