Distinguishing axions from generic light scalars using electric dipole moment and fifth-force experiments

We derive electric dipole moment (EDM) constraints on possible new macroscopic time-reversal and parity-violating (TVPV) spin-dependent forces. These constraints are compared to those derived from direct searches in fifth-force experiments and from combining laboratory searches with astrophysical bounds on stellar energy loss. For axion-mediated TVPV spin-dependent forces, EDM constraints dominate over fifth-force limits by several orders of magnitude. However, we show that for a generic light scalar, unrelated to the strong $CP$ problem, present bounds from direct fifth-force searches are more stringent than those inferred from EDM limits for the interaction ranges explored by fifth-force experiments. Thus, correlating observations in EDM and fifth-force experiments could help distinguish axions from more generic light scalar scenarios.

Figure 1

Representative diagrams of the contribution to nuclear EDMs arising from exchanges of the light scalar $\phi$ that mediates the macroscopic SD force. The first diagram corresponds to $\phi$ exchanges between nucleons in the nucleus. The second and third diagrams can be interpreted as an induced proton EDM and $CP$-odd pion-nucleon coupling due to $\phi$ exchange.

Figure 2

Leading contributions from a virtual $\phi$ loop that give rise to the shift in the $CP$-odd pion-nucleon coupling in Eq. (43).

Figure 3

The diagrams with the scalar coupling to nucleons. Each blob depicts the pseudoscalar coupling.

Figure 4

Two of the four wave-function renormalization diagrams. Each dark blob depicts the pseudoscalar coupling $g_p$, as defined in Eq. (2).

Figure 5

Diagrams with $NN\pi \phi$ and $NN\pi {\phi }^2$ vertices.