Selective enhancement of the QCD axion couplings

Understand

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 are 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.