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High Energy Physics

The QCD axion in the O(10) MeV mass range and recent anomalies

The QCD axion is one of the most compelling solutions of the strong CP problem. There are major current efforts into searching for ultralight axion dark matter, 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. In this talk, I will revisit experimental constraints on QCD axions in the O(10 MeV) mass range and show that a specific variant of the QCD axion remains compatible with existing constraints. Specifically, the axion must (i) couple predominantly to the first generation of SM fermions; (ii) decay to e+eāˆ’ with a short lifetime of less than 10^āˆ’13 s; and (iii) have suppressed isovector couplings, i.e., if it must be piophobic. Remarkably, these are precisely the properties required to explain recently observed anomalies in nuclear de-excitations of the Be-8 and He-4 nuclei, as well as the 2āˆ’3 sigma anomaly in the measurement of the neutral pion branching ratio to e+e-. I will discuss a variety of low-energy axion signatures, such as rare meson decays, nuclear de-excitations via axion emission, and axion production in e+eāˆ’ annihilation and fixed target experiments.

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