Abstract
Besides CP-preserving interactions, axions and axion-like particles may also have small CP-violating scalar Yukawa interactions with nucleons and electrons. Any such interaction will generate macroscopic monopole-dipole forces which can be searched for experimentally. When the best experimental limits on scalar interactions are combined with stellar energy-loss arguments constraining pseudoscalar interactions, strong bounds can be set on CP-violating axion couplings which almost intersect the expectation for QCD models. Over the years, both astrophysical and laboratory tests have improved. We provide a much-needed up-to-date compilation of these constraints, showing improvements in some regions of parameter space by factors between 40 and 130. We advocate experimental opportunities, without astrophysical or dark-matter assumptions, to track down the axion in the lesser-explored corners of its parameter space.
Highlights
The axion is a beyond-the-Standard-Model (SM) pseudoscalar, originally appearing as a consequence of Peccei and Quinn’s solution to the strong CP problem of quantum chromodynamics (QCD) [1,2,3,4,5]
III we present an up-to-date summary of astrophysical bounds on those couplings, and the axion mass
The spins of astrophysical black holes can be used to rule out the existence of bosonic fields in a manner that is mostly independent of how strongly they couple to the Standard Model [119,120,121,122,123,124,125]
Summary
The axion is a beyond-the-Standard-Model (SM) pseudoscalar, originally appearing as a consequence of Peccei and Quinn’s solution to the strong CP problem of quantum chromodynamics (QCD) [1,2,3,4,5]. The possibility of scalar axion-nucleon interactions is an intriguing prospect from an experimental standpoint They would mediate both monopole-monopole (spin-independent) and monopole-dipole forces (between spin-polarized and unpolarized bodies, sometimes called spin-mass forces).
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