📝 SummarXiv

Abstract

Recently, it has been suggested that the spectrum of physical states in the Standard Model may include an ultralight pseudo-scalar, denoted by $\eta_w$, in analogy with the $\eta'$ state arising from the strong interactions. We find that typical expectations for the properties of $\eta_w$ 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 $\eta_w$ 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 $\eta_w$ 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.