Selective enhancement of the QCD axion couplings

TL;DR

This paper introduces a mechanism to selectively enhance QCD axion couplings to nucleons, photons, or electrons without affecting its mass, with implications for astrophysics and experimental searches.

Contribution

It proposes a novel nucleophilic axion model that enhances specific couplings, and demonstrates its potential to explain astrophysical phenomena and guide future experiments.

Findings

Enhanced axion-nucleon and photon couplings can explain neutron star X-ray emissions.

The model remains consistent with existing constraints and can be tested in upcoming experiments.

Selective coupling enhancement does not increase the axion mass, preserving its role as a solution to the strong CP problem.

Abstract

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.