Heat flow of the Earth and resonant capture of solar 57-Fe axions

TL;DR

This paper estimates an upper limit on the mass of hadronic axions based on Earth's heat flow assumptions involving resonant capture of solar 57-Fe axions, providing bounds that are less restrictive than dedicated experiments but more restrictive than some other methods.

Contribution

It introduces a conservative method to constrain axion mass using Earth's heat flow and resonant capture, improving existing astrophysical bounds.

Findings

Upper limit on axion mass: m_a<1.8 keV without considering radioactive decay heat.

Refined limit: m_a<1.6 keV when including decay heat contributions.

Limits are less restrictive than dedicated axion search experiments but more restrictive than some other astrophysical bounds.

Abstract

In a very conservative approach, supposing that total heat flow of the Earth is exclusively due to resonant capture inside the Earth of axions, emitted by 57-Fe nuclei on Sun, we obtain limit on mass of hadronic axion: m_a<1.8 keV. Taking into account release of heat from decays of 40-K, 232-Th, 238-U inside the Earth, this estimation could be improved to the value: m_a<1.6 keV. Both the values are less restrictive than limits set in devoted experiments to search for 57-Fe axions (m_a<216-745 eV), but are much better than limits obtained in experiments with 83-Kr (m_a<5.5 keV) and 7-Li (m_a<13.9-32 keV).