Abstract
The QCD axion is one of the most compelling solutions of the strong CP problem. There are major current efforts into searching for an ultralight, invisible axion, which is believed to be the only phenomenologically viable realization of the QCD axion. Visible axions with decay constants at or below the electroweak scale are believed to have been long excluded by laboratory searches. Considering the significance of the axion solution to the strong CP problem, we revisit experimental constraints on QCD axions in the mathcal{O} (10 MeV) mass window. In particular, we find a variant axion model that remains compatible with existing constraints. This model predicts new states at the GeV scale coupled hadronically, and a variety of low-energy axion signatures, such as rare meson decays, nuclear de-excitations via axion emission, and production in e+e− annihilation and fixed target experiments. This reopens the possibility of solving the strong CP problem at the GeV scale.
Highlights
Such as mass and decay constant, span several orders of magnitude, its phenomenology changes dramatically, with implications ranging from hadronic physics to astrophysics and cosmology
5 Constraints from charged kaon decays. It was widely claimed in the literature [54, 55] that bounds from K+ → π+a ruled out all QCD axion variants in the MeV mass range
Because of much confusion in the literature, we review the differences between the PQ current, the Bardeen-Tye current, and the current associated with the axion mass eigenstate. Identifying the latter, in particular, is a critical part of working out the proper phenomenology of axions, since it is the physical axion current that determines the coupling of the axion to the chiral U(3)χ currents that mediate meson decays, nuclear de-excitations, as well as the electromagnetic anomaly that induces the axion coupling to two photons
Summary
Generic constraints on QCD axions in the MeV mass window can be broadly classified into three categories: (i) amenable to model-building, (ii) plagued by large hadronic uncertainties, and (iii) evaded only by pion-phobia. The third category encompasses the strongest constraints, which can only be avoided by a special class of axion variants which are pion-phobic, i.e., which have suppressed mixing with the neutral pion. While this can be achieved by model-building to some degree, the extreme pion-phobia needed to avoid exclusion depends critically on the light quark mass ratio being close to a ratio of Peccei-Quinn charges The most up-to-date determinations of mu/md indicate that it is very close 1/2, making extreme pion-phobia a realistic possibility
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