CAST constraints on the axion-electron coupling

TL;DR

This paper calculates the solar axion flux from non-hadronic models, including axio-recombination, and sets new limits on axion-electron and axion-photon couplings using CAST data, with implications for future experiments.

Contribution

First calculation of axio-recombination contribution to solar axion flux, leading to improved constraints on axion couplings from CAST data.

Findings

g_ag x g_ae< 8.1 x 10^-23 GeV^-1 for m_a<10 meV/c2 at 95% CL

Inclusion of axio-recombination significantly refines flux estimates

Next-generation helioscopes could surpass stellar energy-loss bounds

Abstract

In non-hadronic axion models, which have a tree-level axion-electron interaction, the Sun produces a strong axion flux by bremsstrahlung, Compton scattering, and axio-recombination, the "BCA processes." Based on a new calculation of this flux, including for the first time axio-recombination, we derive limits on the axion-electron Yukawa coupling g_ae and axion-photon interaction strength g_ag using the CAST phase-I data (vacuum phase). For m_a < 10 meV/c2 we find g_ag x g_ae< 8.1 x 10^-23 GeV^-1 at 95% CL. We stress that a next-generation axion helioscope such as the proposed IAXO could push this sensitivity into a range beyond stellar energy-loss limits and test the hypothesis that white-dwarf cooling is dominated by axion emission.