Weakly interacting massive particle annual modulation signal and nonstandard halo models

The spatial and velocity distributions of dark matter particles in the Milky Way Halo affect the signals expected to be observed in searches for dark matter. Results from direct detection experiments are often analyzed assuming a simple isothermal distribution of dark matter, the Standard Halo Model (SHM). Yet there has been skepticism regarding the validity of this simple model due to the complicated gravitational collapse and merger history of actual galaxies. In this paper we compare the SHM to the results of cosmological hydrodynamical simulations of galaxy formation to investigate whether or not the SHM is a good representation of the true WIMP distribution in the analysis of direct detection data. We examine two Milky Way-like galaxies from the MaGICC cosmological simulations (a) with dark matter only and (b) with baryonic physics included. The inclusion of baryons drives the shape of the DM halo to become more spherical and makes the velocity distribution of dark matter particles less anisotropic especially at large heliocentric velocities, thereby making the SHM a better fit. We also note that we do not find a significant disk-like rotating dark matter component in either of the two galaxy halos with baryons that we examine, suggesting that dark disks are not a generic prediction of cosmological hydrodynamical simulations. We conclude that in the Solar neighborhood, the SHM is in fact a good approximation to the true dark matter distribution in these cosmological simulations (with baryons) which are reasonable representations of the Milky Way, and hence can also be used for the purpose of dark matter direct detection calculations.

Section: Results For Velocity and Speed Distributions In Galactic Resmentioning

confidence: 88%

The impact of baryons on the direct detection of dark matter

Kelso

Savage

Valluri

et al. 2016

“…In order to predict the signal in such experiments for given dark matter particle physics properties it is necessary to specify also its local density and velocity distribution. Very little is known about the details of the local dark matter phase space density and this lack of knowledge introduces significant uncertainty in the interpretation of data from dark matter direct detection experiments [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. To overcome these problems halo independent methods have been developed and applied to data [15][16][17][18][19][20][21][22][23].…”

Section: Contentsmentioning

confidence: 99%

“…We verified that the gravitational potential obtained by solving the Poisson equation for Φ in the limit of spherical symmetry (as explained above), and the exact gravitational potential of our model are sufficiently close to each other. To this aim we calculated the baryonic contribution to the axisymmetric potential of our model, Φ bar (R, z), using the appropriate integral solution of Relative potential in units of c 2 Spherical symmetry Axial symmetry θ=0 Axial symmetry θ=π/4 Axial symmetry θ=π/2 Figure 3. Relative gravitational potential as a function of the galactocentric distance computed within the spherical approximation described in the text (blue line) and assuming axial symmetry according to Eq.…”

Section: Mass Model For the Milky Waymentioning

confidence: 99%

Anisotropic dark matter distribution functions and impact on WIMP direct detection

Bozorgnia

Catena

Schwetz

2013

Abstract. Dark matter N-body simulations suggest that the velocity distribution of dark matter is anisotropic. In this work we employ a mass model for the Milky Way whose parameters are determined from a fit to kinematical data. Then we adopt an ansatz for the dark matter phase space distribution which allows to construct self-consistent halo models which feature a degree of anisotropy as a function of the radius such as suggested by the simulations. The resulting velocity distributions are then used for an analysis of current data from dark matter direct detection experiments. We find that velocity distributions which are radially biased at large galactocentric distances (up to the virial radius) lead to an increased high velocity tail of the local dark matter distribution. This affects the interpretation of data from direct detection experiments, especially for dark matter masses around 10 GeV, since in this region the high velocity tail is sampled. We find that the allowed regions in the dark matter mass-cross section plane as indicated by possible hints for a dark matter signal reported by several experiments as well as conflicting exclusion limits from other experiments shift in a similar way when the halo model is varied. Hence, it is not possible to improve the consistency of the data by referring to anisotropic halo models of the type considered in this work.

“…In the SHM, the DM is distributed in an isothermal sphere and has a Maxwell-Boltzmann velocity distribution with the peak speed equal to the local circular speed, usually taken to be 220 km/s. The DM velocity distribution could, however, be different from the SHM and this could alter the exclusion limits derived from direct detection [6][7][8][9]. Departures from the SHM do not affect all the experiments in the same way, since, depending on the nuclear target and on the range of recoil energies that are analysed, different detectors probe different regions of the velocity distribution function.…”

Section: Introductionmentioning

confidence: 99%

On the correlation between the local dark matter and stellar velocities

Bozorgnia

Fattahi

Cerdeño

et al. 2019

The dark matter velocity distribution in the Solar neighbourhood is an important astrophysical input which enters in the predicted event rate of dark matter direct detection experiments. It has been recently suggested that the local dark matter velocity distribution can be inferred from that of old or metal-poor stars in the Milky Way. We investigate this potential relation using six high resolution magneto-hydrodynamical simulations of Milky Way-like galaxies of the Auriga project. We do not find any correlation between the velocity distributions of dark matter and old stars in the Solar neighbourhood. Likewise, there are no strong correlations between the local velocity distributions of dark matter and metalpoor stars selected by applying reasonable cuts on metallicity. In some simulated galaxies, extremely metal-poor stars have a velocity distribution that is statistically consistent with that of the dark matter, but the sample of such stars is so small that we cannot draw any strong conclusions.