Dark Matter in the Earth I - Diurnal Modulation

In collaboration with Chris Kouvaris.

The fundamental assumption and hope of direct detection of dark matter (DM) is the idea, that dark matter particles interact with ordinary matter (such as the detector’s target material) in some way apart from gravity. If this interaction is sufficiently strong, DM - matter scatterings occur of course not just in the detector, but also in the Earth’s bulk mass, as can be seen in the animation. Terrestrial scatterings deflect and decelerate the incoming DM particles, which can change their distribution. If particles are likely to scatter before entering the detector, this can change the expected signal at that experiment in different ways, depending on the interaction strength. The possibility of strong interactions between DM and ordinary atoms is certainly ruled out for the classical WIMP with GeV scale mass. But for lighter masses, and especially light mediators, it is still a viable option.

One aspect, which we studied in 2016/17, is the resulting diurnal modulation, an increase and decrease of the signal rate during a day. As the Earth moves through the DM wind of the halo while rotating, incoming DM particles need to pass different amounts of the Earth’s mass, as visible in the second video. Therefore pre-detection scatterings are more likely during some times during the day than others, leading to variations in the signal rate. We can predict the amplitude and phase using the DaMaSCUS code, which we published alongside the paper.