Directional recoil rates for WIMP direct detection

TL;DR

This paper analyzes how directional recoil rates in WIMP detection vary with different dark matter velocity models, introducing a folded rate to account for detector limitations and assessing detection requirements.

Contribution

It introduces a folded directional recoil rate to handle head-tail ambiguity and compares detection sensitivities across various dark matter velocity distributions.

Findings

Folded recoil rate effectively distinguishes signals for standard halo models.

Detection thresholds depend on the dark matter velocity distribution.

Some anisotropic models are less distinguishable with folded rates.

Abstract

New techniques for the laboratory direct detection of dark matter weakly interacting massive particles (WIMPs) are sensitive to the recoil direction of the struck nuclei. We compute and compare the directional recoil rates ${dR}/{d\cos\theta}$ (where $\theta$ is the angle measured from a reference direction in the sky) for several WIMP velocity distributions including the standard dark halo and anisotropic models such as Sikivie's late-infall halo model and logarithmic-ellipsoidal models. Since some detectors may be unable to distinguish the beginning of the recoil track from its end (lack of head-tail discrimination), we introduce a ``folded'' directional recoil rate ${dR}/{d|\cos\theta|}$, where $|\cos\theta|$ does not distinguish the head from the tail of the track. We compute the CS$_2$ and CF$_4$ exposures required to distinguish a signal from an isotropic background noise, and find that ${dR}/{d|\cos\theta|}$ is effective for the standard dark halo and some but not all anisotropic models.