Constraints on the axion-electron coupling for solar axions produced by Compton process and bremsstrahlung

TL;DR

This paper sets new experimental limits on the axion-electron coupling and axion mass by searching for solar axions produced via Compton and bremsstrahlung processes, using nuclear excitation detection techniques.

Contribution

It provides the first model-independent constraints on axion-electron and axion-nucleon couplings from solar axion searches involving nuclear excitation.

Findings

New upper limit on axion-electron and axion-nucleon couplings: $g_{Ae}|g_{AN}| \,\leq 2.1\times10^{-14}$

Upper limit on axion mass: 105 eV for DFSZ and 1.3 keV for KSVZ models

Constraints improve previous bounds on solar axion properties.

Abstract

The search for solar axions produced by Compton ($\gamma+e^-\rightarrow e^-+A$) and bremsstrahlung-like ($e^-+Z \rightarrow Z+e^-+A$) processes has been performed. The axion flux in the both cases depends on the axion-electron coupling constant. The resonant excitation of low-lying nuclear level of $^{169}\rm{Tm}$ was looked for: $A+^{169}$Tm $\rightarrow ^{169}$Tm$^*$ $\rightarrow ^{169}$Tm $+ \gamma$ (8.41 keV). The Si(Li) detector and $^{169}$Tm target installed inside the low-background setup were used to detect 8.41 keV $\gamma$-rays. As a result, a new model independent restriction on the axion-electron and the axion-nucleon couplings was obtained: $g_{Ae}\times|g^0_{AN}+ g^3_{AN}|\leq 2.1\times10^{-14}$. In model of hadronic axion this restriction corresponds to the upper limit on the axion-electron coupling and on the axion mass $g_{Ae}\times m_A\leq3.1\times10^{-7}$ eV (90% c.l.). The limits on axion mass are $m_A\leq$ 105 eV and $m_A\leq$ 1.3 keV for DFSZ- and KSVZ-axion models, correspondingly (90% c.l.).