Astrophysical consequences of an electroweak η w pseudoscalar

Recently, it has been suggested that the spectrum of physical states in the Standard Model may include an ultralight pseudoscalar, denoted by η w , in analogy with the η ′ state arising from the strong interactions. We find that typical expectations for the properties of η w get challenged by astrophysical constraints on the couplings of ultralight bosons. Our strongest limit sets a lower bound of O ( 100 TeV ) on the decay constant of the hypothesized pseudoscalar. We also briefly discuss whether η w 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 η w 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.