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Register a new userThe impact of baryons on the direct detection of dark matter
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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.
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Detecting dark matter as it streams through detectors on Earth relies on knowledge of its phase space density on a scale comparable to the size of our solar system. Numerical simulations predict that our Galactic halo contains an enormous hierarchy of substructures, streams and caustics, the remnants of the merging hierarchy that began with tiny Earth mass microhalos. If these bound or coherent structures persist until the present time, they could dramatically alter signatures for the detection of weakly interacting elementary particle dark matter (WIMP). Using numerical simulations that follow the coarse grained tidal disruption within the Galactic potential and fine grained heating from stellar encounters, we find that microhalos, streams and caustics have a negligible likelihood of impacting direct detection signatures implying that dark matter constraints derived using simple smooth halo models are relatively robust. We also find that many dense central cusps survive, yielding a small enhancement in the signal for indirect detection experiments.
Traditional dark matter (DM) models, eg. WIMPs, assume dark matter is weakly coupled to the standard model so that elastic scattering between DM and baryons can be described perturbatively by Born approximation. Most direct detection experiments are analyzed according to that assumption. We show that when the interaction is attractive and strong, DM-nucleus scattering exhibits rich resonant behavior with a highly non-trivial dependence on atomic mass. The scattering is non-perturbative in much of the natural parameter range, and a full numerical calculation is needed. We also show that the extended rather than point-like nature of nuclei significantly impacts the cross sections and must therefore be properly taken into account. These effects are particularly important for dark matter with GeV-scale masses, near the boundary of exclusion regions from existing direct detection limits. They also affect the interpretation of CMB constraints, as we show. We report the corrected limits, which are in some respects weaker and in other respects stronger than previous bounds in the literature, which were based on perturbation theory and point-like sources and hence are now superceded. Sexaquark ($uuddss$) DM with mass $lesssim 2$ GeV, which exchanges QCD mesons with baryons, remains unconstrained for most of the parameter space of interest.
As part of the Snowmass process, the Cosmic Frontier WIMP Direct Detection subgroup (CF1) has drawn on input from the Cosmic Frontier and the broader Particle Physics community to produce this document. The charge to CF1 was (a) to summarize the current status and projected sensitivity of WIMP direct detection experiments worldwide, (b) motivate WIMP dark matter searches over a broad parameter space by examining a spectrum of WIMP models, (c) establish a community consensus on the type of experimental program required to explore that parameter space, and (d) identify the common infrastructure required to practically meet those goals.
The LUX experimental group has just announced the most stringent upper limits so far obtained on the cross section of WIMP-nucleon elastic scattering [1]. This result is a factor of two to five below the previous best upper limit [2] and effectively rules out earlier suggestions of low mass WIMP detection signals. The experimental expertise exhibited by this group is extremely impressive, but the fact of continued negative results raises the more basic question of whether or not this is the right approach to solving the dark matter problem. Here I comment upon this question, using as a basis the final chapter of my book on dark matter [3], somewhat revised and extended. I muse on dark matter and its alternative, modified Newtonian dynamics, or MOND.
The dark matter spike induced by the adiabatic growth of a massive black hole in a cuspy environment, may explain the thermal dark matter density required to fit the cut-off in the HESSJ1745-290 gamma-ray spectra as TeV dark matter signal with a background component. The spike extension appears comparable with the HESS angular resolution.
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