Galactic Halo Models and Particle Dark-Matter Detection

TL;DR

The paper addresses how uncertainties in the Galactic halo's spatial and velocity distributions beyond the standard isothermal sphere affect WIMP direct-detection predictions. It adopts self-consistent Evans-based axisymmetric halo models, varying flattening $q$ and bulk rotation (via rotating distribution functions) while keeping the same local and asymptotic rotation speeds, to quantify impacts on $\rho_0$, $f_1(v)$, and the recoil spectrum. The main findings are that flattening can raise the local density by up to about a factor of 2, but changes to the velocity distribution have negligible effects on detection rates, whereas bulk rotation can modify rates by up to ~10% for realistic spins (up to ~40% in extreme cases); importantly, the recoil-energy spectrum could constrain halo shape and rotation. The results provide conservative uncertainty bounds for direct-detection predictions, suggesting that rate calculations are robust to plausible halo-model variations, and also indicate that indirect-detection fluxes may be more sensitive to central-density differences, while the recoil spectrum offers a potential window into Galactic halo structure.

Abstract

Rates for detection of weakly-interacting massive-particle (WIMP) dark matter are usually carried out assuming the Milky Way halo is an isothermal sphere. However, it is possible that our halo is not precisely spherical; it may have some bulk rotation; and the radial profile may differ from that of an isothermal sphere. In this paper, we calculate detection rates in observationally consistent alternative halo models that produce the same halo contributions to the local and asymptotic rotation speeds to investigate the effects of theoretical uncertainty of the WIMP spatial and velocity distribution. We use self-consistent models to take into account the effects of various mass distributions on the local velocity distribution. The local halo density may be increased up to a factor of 2 by flattening or by an alternative radial profile (which may also decrease the density slightly). However, changes in the WIMP velocity distribution in these models produce only negligible changes in the WIMP detection rate. Reasonable bulk rotations lead to only an $O(10%)$ effect on event rates. We also show how the nuclear recoil spectrum in a direct-detection experiment could provide information on the shape and rotation of the halo.

Figures (2)

• Figure 1: Halo isodensity contours for the Evans models for $q=1,0.85,0.707$, where the $z$ and $R$ axes are in kpc.)
• Figure 2: Local speed distributions $f_1(v)$ for nonrotating, maximally corotating, and maximally counterrotating models with $q=1$ (solid curves), $q=0.85$ (dashed curves), and $q=0.707$ (dotted curves) and with $v_s=220$ km/s.