The coherent photon scattering background in sub-GeV/$c^2$ direct dark matter searches

TL;DR

Future low-threshold dark matter detectors must address the significant background from coherent photon scattering, especially at sub-10 eV nuclear recoil energies, by using low-Z materials or photon rejection techniques.

Contribution

This paper analyzes the coherent photon scattering background in sub-GeV/c^2 dark matter searches and discusses strategies to mitigate it in future detector designs.

Findings

Coherent photon scattering dominates the background below ~10 eVnr.

Using low-Z target materials like helium can reduce this background.

Photon rejection methods are essential for next-generation detectors.

Abstract

Proposed dark matter detectors with eV-scale sensitivities will detect a large background of atomic (nuclear) recoils from coherent photon scattering. This background climbs steeply below $\sim10$~eVnr, far exceeding the declining rate of low-energy Compton recoils. The upcoming generation of dark matter detectors will not be limited by this background, but further development of eV-scale and sub-eV detectors will require the use of low-$Z$ target materials, such as helium, to avoid a large rate of coherent photon scattering, or highly efficient methods to reject photons when they scatter.