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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 of the strong CP problem, we revisit experimental constraints on QCD axions in the 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, production in $e^+e^-$ annihilation and fixed target experiments. This reopens the possibility of solving the strong CP problem at the GeV scale.
The QCD axion remains experimentally viable in the mass range of O(10 MeV) if (i) it couples predominantly to the first generation of SM fermions; (ii) it decays to $e^+ e^-$ with a short lifetime $tau_alesssim 10^{-13},$s; and (iii) it has suppresse
A detailed discussion is given of the analysis of recent data to obtain improved upper bounds on the couplings $|U_{e4}|^2$ and $|U_{mu 4}|^2$ for a mainly sterile neutrino mass eigenstate $ u_4$. Using the excellent agreement among ${cal F}t$ values
Improved upper bounds are presented on the coupling $|U_{e4}|^2$ of an electron to a sterile neutrino $ u_4$ from analyses of data on nuclear and particle decays, including superallowed nuclear beta decays, the ratios $R^{(pi)}_{e/mu}=BR(pi^+ to e^+
The QCD axion is one of the most appealing candidates for the dark matter in the Universe. In this article, we discuss the possibility to predict the axion mass in the context of a simple renormalizable grand unified theory where the Peccei-Quinn sca
Axion models with generation-dependent Peccei-Quinn charges can lead to flavor-changing neutral currents, thus motivating QCD axion searches at precision flavor experiments. We rigorously derive limits on the most general effective flavor-violating c