Dark-Matter Harmonics Beyond Annual Modulation

TL;DR

This paper demonstrates that higher-frequency modulations beyond the annual cycle, such as daily variations, can be significant in dark matter detection and serve as a tool to identify dark matter signals.

Contribution

It provides new analytical expressions for harmonic modes in dark matter detection signals, incorporating an updated Earth's velocity model and exploring their dependence on astrophysical parameters.

Findings

Higher-frequency modulations can be nearly as strong as annual modulation.

Ratios of harmonic modes are approximately constant for isotropic halos.

Anisotropic velocity distributions produce observable features in the harmonic spectrum.

Abstract

The count rate at dark-matter direct-detection experiments should modulate annually due to the motion of the Earth around the Sun. We show that higher-frequency modulations, including daily modulation, are also present and in some cases are nearly as strong as the annual modulation. These higher-order modes are particularly relevant if (i) the dark matter is light, O(10) GeV, (ii) the scattering is inelastic, or (iii) velocity substructure is present; for these cases, the higher-frequency modes are potentially observable at current and ton-scale detectors. We derive simple expressions for the harmonic modes as functions of the astrophysical and geophysical parameters describing the Earth's orbit, using an updated expression for the Earth's velocity that corrects a common error in the literature. For an isotropic halo velocity distribution, certain ratios of the modes are approximately constant as a function of nuclear recoil energy. Anisotropic distributions can also leave observable features in the harmonic spectrum. Consequently, the higher-order harmonic modes are a powerful tool for identifying a potential signal from interactions with the Galactic dark-matter halo.