Annual Modulation of Dark Matter: A Review

TL;DR

This review discusses the physics, models, and implications of annual modulation signals in dark matter direct detection experiments, highlighting how these signals can reveal dark matter properties and Galactic halo structure.

Contribution

It provides a comprehensive overview of the theoretical understanding of annual modulation in dark matter detection and discusses how measurements can inform astrophysical and particle physics models.

Findings

Modulation follows a cosine pattern with maximum in June.

Generalized models can alter phase and amplitude of the modulation.

Measurement of modulation can reveal Galactic halo substructure.

Abstract

Direct detection experiments, which are designed to detect the scattering of dark matter off nuclei in detectors, are a critical component in the search for the Universe's missing matter. The count rate in these experiments should experience an annual modulation due to the relative motion of the Earth around the Sun. This modulation, not present for most known background sources, is critical for solidifying the origin of a potential signal as dark matter. In this article, we review the physics of annual modulation, discussing the practical formulae needed to interpret a modulating signal. We focus on how the modulation spectrum changes depending on the particle and astrophysics models for the dark matter. For standard assumptions, the count rate has a cosine dependence with time, with a maximum in June and a minimum in December. Well-motivated generalizations of these models, however, can affect both the phase and amplitude of the modulation. We show how a measurement of an annually modulating signal could teach us about the presence of substructure in the Galactic halo or about the interactions between dark and baryonic matter. Although primarily a theoretical review, we briefly discuss the current experimental situation for annual modulation and future experimental directions.