Selective enhancement of the QCD axion couplings

We present a mechanism wherein the QCD axion coupling to nucleons, photons, or electrons can be enhanced selectively without increasing the axion mass. We focus in particular on the axion-nucleon couplings that arc generally considered to be largely model independent, and we show how nucleophilic axion models can be constructed. We discuss the implications of a nucleophilic axion for astrophysics, cosmology, and laboratory searches. We present a model with enhanced axion couplings to nucleons and photons that can provide an excellent fit to the anomalous emission of hard x-rays recently observed from a group of nearby neutron stars, and we argue that such a scenario can be thoroughly tested in forthcoming axion-search experiments.

Automated summary

The study introduces a mechanism for enhancing the coupling of QCD axions to nucleons, photons, or electrons without increasing the axion mass, focusing on the model-independent axion-nucleon couplings and proposing nucleophilic axion models. They discuss the implications for astrophysics, cosmology, and laboratory searches, presenting a model with enhanced axion couplings that can explain the anomalous hard X-ray emissions from nearby neutron stars and suggesting future testing in upcoming axion-search experiments.