Dependence of direct detection signals on the WIMP velocity distribution

TL;DR

This paper investigates how uncertainties in the local dark matter velocity distribution affect signals in WIMP direct detection experiments, highlighting the impact on event rates, modulation signals, and WIMP mass determination.

Contribution

It provides a detailed analysis of how different velocity distributions influence detection signals and the uncertainties in WIMP parameter estimation.

Findings

Uncertainties in local circular speed and dark disc presence significantly alter event rates.

Shape uncertainties in the velocity distribution cause about 5% systematic error in WIMP mass.

Direction-dependent signals are sensitive to velocity distribution but require similar event numbers for detection.

Abstract

The signals expected in WIMP direct detection experiments depend on the ultra-local dark matter distribution. Observations probe the local density, circular speed and escape speed, while simulations find velocity distributions that deviate significantly from the standard Maxwellian distribution. We calculate the energy, time and direction dependence of the event rate for a range of velocity distributions motivated by recent observations and simulations, and also investigate the uncertainty in the determination of WIMP parameters. The dominant uncertainties are the systematic error in the local circular speed and whether or not the MW has a high density dark disc. In both cases there are substantial changes in the mean differential event rate and the annual modulation signal, and hence exclusion limits and determinations of the WIMP mass. The uncertainty in the shape of the halo velocity distribution is less important, however it leads to a 5% systematic error in the WIMP mass. The detailed direction dependence of the event rate is sensitive to the velocity distribution. However the numbers of events required to detect anisotropy and confirm the median recoil direction do not change substantially.