On the Detection of QCD Axion Dark Matter by Coherent Scattering

TL;DR

This paper explores the potential for detecting QCD axion dark matter through enhanced coherent scattering with nucleons, highlighting the significant theoretical cross section increases and the challenges in experimental detection.

Contribution

It introduces the idea that coherent effects can vastly increase the scattering cross section of QCD axions, suggesting new detection possibilities and analyzing the limitations due to cancellation effects.

Findings

Coherent scattering can enhance the cross section by over 10^25 times.

QCD axions may scatter in the Sun with about 1 probability for certain decay constants.

Stimulated emission effects could further increase the cross section, but detection remains challenging.

Abstract

The QCD axion is a promising candidate of the dark matter. In this paper, we discuss elastic scattering processes between nucleons and the QCD axion dark matter. We point out that the cross section can be enhanced by more than $\mathcal{O}(10^{25})$ by the coherent effect, compared to classical processes. As a result, for example, we show that QCD axions may scatters at the sun with $\mathcal{O}(1)$ probability for $f_a \lesssim 10^{11}\,\text{GeV}$. In addition, one may expect stimulated emission effects can also enhance the cross section because the number density of the axion DM is very large. The enhancement factor can be as large as another $\mathcal{O}(10^{25})$ and, if the factor exists, the force from the dark matter wind may be detected via e.g., torsion balance experiments. However, it is also found that there is a cancellation of the stimulated emission factor and the force is too small to be detected.