Astrophysical Consequences of an Electroweak Pseudo-Scalar

Recently, it has been suggested that the spectrum of physical states in the Standard Model may include an ultralight pseudo-scalar, denoted by ηw, in analogy with the η' state arising from the strong interactions. We find that typical expectations for the properties of ηw get challenged by astrophysical constraints on the couplings of ultralight bosons. Our strongest limit sets a lower bound of O (1000 TeV) on the decay constant of the hypothesized pseudo-scalar. We also briefly discuss whether ηw could be a dark matter candidate, or the origin of dark energy, but conclude that those identifications appear unlikely. Given the important implications of a potentially overlooked ηw state for a more complete understanding of the electroweak interactions and a fundamental description of Nature, further theoretical and phenomenological investigations of this possibility and its associated physics are warranted.