Astrophysical Consequences of an Electroweak ${\eta }_{\mathrm{w}}$ Pseudo-Scalar

Abstract

Recently, it has been suggested that the spectrum of physical states in the Standard Model may include an ultralight pseudo-scalar, denoted by ${\eta }_{\mathrm{w}}$, in analogy with the ${\eta }^{\prime }$ state arising from the strong interactions. We find that typical expectations for the properties of ${\eta }_{\mathrm{w}}$ get challenged by astrophysical constraints on the couplings of ultralight bosons. Our strongest limit sets a lower bound of $\mathcal{O}(100\mathrm{TeV})$ on the decay constant of the hypothesized pseudo-scalar. We also briefly discuss whether ${\eta }_{\mathrm{w}}$ 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 ${\eta }_{\mathrm{w}}$ 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.

Comments: Revtex4-2, 4 pages, 1 figure. Error in ${\eta }_{\mathrm{w}}$ coupling to photons corrected. New material and references added