Dark Matter Velocity Distribution Research Articles

Complementarity of dark matter direct detection targets

We investigate the reconstruction capabilities of Dark Matter mass and spin-independent cross-section from future ton-scale direct detection experiments using germanium, xenon or argon as targets. Adopting realistic values for the exposure, energy threshold and resolution of Dark Matter experiments which will come online within 5 to 10 years, the degree of complementarity between different targets is quantified. We investigate how the uncertainty in the astrophysical parameters controlling the local Dark Matter density and velocity distribution affects the reconstruction. For a 50 GeV WIMP, astrophysical uncertainties degrade the accuracy in the mass reconstruction by up to a factor of $\sim 4$ for xenon and germanium, compared to the case when astrophysical quantities are fixed. However, combination of argon, germanium and xenon data increases the constraining power by a factor of $\sim 2$ compared to germanium or xenon alone. We show that future direct detection experiments can achieve self-calibration of some astrophysical parameters, and they will be able to constrain the WIMP mass with only very weak external astrophysical constraints.

Interpreting dark matter direct detection independently of the local velocity and density distribution

We demonstrate precisely what particle physics information can be extracted from a single direct detection observation of dark matter while making absolutely no assumptions about the local velocity distribution and local density of dark matter. Our central conclusions follow from a very simple observation: the velocity distribution of dark matter is positive definite, $f(v)\ensuremath{\ge}0$. We demonstrate the utility of this result in several ways. First, we show a falling deconvoluted recoil spectrum (deconvoluted of the nuclear form factor), such as from ordinary elastic scattering, can be ``mocked up'' by any mass of dark matter above a kinematic minimum. As an example, we show that dark matter much heavier than previously considered can explain the CoGeNT excess. Specifically, ${m}_{\ensuremath{\chi}}<{m}_{\mathrm{Ge}}$ can be in just as good agreement as light dark matter, while ${m}_{\ensuremath{\chi}}>{m}_{\mathrm{Ge}}$ depends on understanding the sensitivity of xenon to dark matter at very low recoil energies, ${E}_{R}\ensuremath{\lesssim}6\text{ }\text{ }\mathrm{keVnr}$. Second, we show that any rise in the deconvoluted recoil spectrum represents distinct particle physics information that cannot be faked by an arbitrary $f(v)$. As examples of resulting nontrivial particle physics, we show that inelastic dark matter and dark matter with a form factor can both yield such a rise.

Resonant dark matter

It is usually assumed that dark matter direct detection is sensitive to a large fraction of the dark matter (DM) velocity distribution. We propose an alternative form of dark matter-nucleus scattering which only probes a narrow range of DM velocities due to the existence of a resonance, a DM-nucleus bound state, in the scattering - resonant dark matter (rDM). The scattering cross section becomes highly element dependent, has increased modulation and as a result can explain the DAMA/LIBRA results whilst not being in conflict with other direct detection experiments. We describe a simple model that realizes the dynamics of rDM, where the DM is the neutral component of a fermionic weak triplet whose charged partners differ in mass by approximately 10 MeV.

Inelastic dark matter, non-standard halos and the DAMA/LIBRA results

The DAMA collaboration have claimed to detect particle dark matter (DM) via an annual modulation in their observed recoil event rate. This appears to be in strong disagreement with the null results of other experiments if interpreted in terms of elastic DM scattering, while agreement for a small region of parameter space is possible for inelastic DM (iDM) due to the altered kinematics of the collision. To date most analyses assume a simple galactic halo DM velocity distribution, the Standard Halo Model, but direct experimental support for the SHM is severely lacking and theoretical studies indicate possible significant differences. We investigate the dependence of DAMA and the other direct detection experiments on the local DM velocity distribution, utilizing the results of the Via Lactea and Dark Disc numerical simulations. We also investigate effects of varying the solar circular velocity, the DM escape velocity, and the DAMA quenching factor within experimental limits. Our data set includes the latest ZEPLIN-III results, as well as full publicly available data sets. Due to the more sensitive dependence of the inelastic cross section on the velocity distribution, we find that with Via Lactea the DAMA results can be consistent with all other experiments over an enlarged region of iDM parameter space, with higher mass particles being preferred, while Dark Disc does not lead to an improvement. A definitive test of DAMA for iDM requires heavy element detectors.

Can the flyby anomaly be attributed to earth-bound dark matter?

We make preliminary estimates to assess whether the recently reported flyby anomaly can be attributed to dark matter interactions. We consider both elastic and exothermic inelastic scattering from dark matter constituents; for isotropic dark matter velocity distributions, the former decrease, while the latter increase, the final flyby velocity. The fact that the observed flyby velocity anomaly shows examples with both positive and negative signs, requires the dominance of different dark matter scattering processes along different flyby trajectories. The magnitude of the observed anomalies requires dark matter densities many orders of magnitude greater than the galactic halo density. Such a large density could result from an accumulation cascade, in which the solar system-bound dark matter density is much higher than the galactic halo density, and the earth-bound density is much higher than the solar system-bound density. We discuss a number of strong constraints on the hypothesis of a dark matter explanation for the flyby anomaly. These require dark matter to be non-self-annihilating, with the dark matter scattering cross section on nucleons much larger, and the dark matter mass much lighter, than usually assumed.

Investigation of possible dark matter direct detection in electron accelerators

We investigate a possibility of neutralino dark matter (DM) direct detection in the future electron accelerators. That is counting of high ${p}_{T}$ electron recoil events by neutralinos in halo. If selectron and neutralino masses would be precisely measured in future collider experiments, the beam energy could be tuned so that the scatterings are dominated by on-pole selectron exchange. When selectron and neutralino mass difference is smaller than $O(10)$ GeV, the elastic cross section exceeds over micro barn. Discovery of the high ${p}_{T}$ electron events would be a firm prove of the neutralino DM component in halo. In the experiment, the electron beam energy must be tuned within $O(10)$ MeV and the electron beam with high currents of $O(100)A$ is required for the detectors of the total length of a few hundred meters so that the sufficient event rate is obtained. The dependence of the event rate on the DM velocity distribution in halo is also discussed. This method might be applicable to other DM candidates.

Status of Experiments for Direct Detection of Galactic Dark Matter Particles

There is increasing evidence that the majority of dark matter is non-baryonic. Principal candidates are weakly interacting massive particles (WIMPS), axions, and neutrinos. There has been increasing effort on sensitive WIMP searches, motivated in particular by supersymmetry theory, which predicts a stable neutral particle in the mass range 10-1000 GeV. Interactions of these with normal matter would produce low energy nuclear recoils which could be observed by underground detectors capable of discriminating these from background. Current experimental progress is summarised, together with plans for more sensitive experiments. These include gaseous detectors with directional sensitivity, offering the prospect of a ‘dark matter telescope’ which would provide information on the dark matter velocity distribution. Axions could be detected by conversion to microwave photons, and experimental sensitivity is approaching the theoretically-required levels. Relic neutrinos could also form a component of the dark matter if any has a cosmologically significant mass, and the latter could be checked with a new detector able to detect the higher neutrino flavours from a Galactic supernova burst. More distant future possibilities are outlined for direct detection of relic neutrinos by coherent scattering.

Diurnal and annual modulation of cold dark matter signals

We calculate the diurnal and annual modulation of the signals in axion and weakly interacting massive particle (WIMP) dark matter detectors on Earth caused by a cold flow of dark matter in the solar neighborhood. The effects of the Sun's and the Earth's gravity, and of the orbital and rotational motions of the Earth are included. A cold flow on Earth produces a peak in the spectrum of microwave photons in cavity detectors of dark matter axions, and a plateau in the nuclear recoil energy spectrum in WIMP detectors. Formulas are given for the positions and heights of these peaks and plateaux as a function of time in the course of day and year, including all corrections down to the 0.1% level of precision. The results can be applied to an arbitrary dark matter velocity distribution $f(\stackrel{\ensuremath{\rightarrow}}{v})$ by integrating the one-flow results over velocities. We apply them to the set of flows predicted by the caustic ring model of the Galactic halo. The caustic ring model predicts the dark matter flux on Earth to be largest in December $\ifmmode\pm\else\textpm\fi{}$ one month. Nonetheless, because of the role of energy thresholds, the model is consistent with the annual modulation results published by the DAMA Collaboration provided the WIMP mass is larger than approximately 100 GeV.

A Cosmological Kinetic Theory for the Evolution of Cold Dark Matter Halos with Substructure: Quasi‐Linear Theory

We present a kinetic theory for the evolution of the phase-space distribution of dark matter particles in galaxy halos in the presence of a cosmological spectrum of fluctuations. This theory introduces a new way to model the formation and evolution of halos, which traditionally have been investigated by analytic gravitational infall models or numerical N-body methods. Unlike the collisionless Boltzmann equation, our kinetic equation contains nonzero terms on the right-hand side arising from stochastic fluctuations in the gravitational potential due to substructures in the dark matter mass distribution. Using statistics for constrained Gaussian random fields in standard cosmological models, we show that our kinetic equation to second order in perturbation theory is of the Fokker-Planck form, with one scattering term representing drift and the other representing diffusion in velocity space. The drift is radial, and the drift and diffusion coefficients depend only on positions and not velocities; our relaxation process in the quasi-linear regime is therefore different from the standard two-body relaxation. We provide explicit expressions relating these coefficients to the linear power spectrum of mass fluctuations and present results for the currently favored cold dark matter model with a nonzero cosmological constant. Solutions to this kinetic equation will provide a complete description of the cold dark matter spatial and velocity distributions for the average halo during the early phases of galaxy halo formation.

The Halo Occupation Distribution: Toward an Empirical Determination of the Relation between Galaxies and Mass

We investigate galaxy bias in the framework of the halo occupation distribution (HOD), which defines the bias of a population of galaxies by the conditional probability P(N|M) that a dark matter halo of virial mass M contains N galaxies, together with prescriptions that specify the relative spatial and velocity distributions of galaxies and dark matter within halos. By populating the halos of a cosmological N-body simulation using a variety of HOD models, we examine the sensitivity of different galaxy clustering statistics to properties of the HOD. The galaxy correlation function responds to different aspects of P(N|M) on different scales. Obtaining the observed power-law form of ξg(r) requires rather specific combinations of HOD parameters, implying a strong constraint on the physics of galaxy formation; the success of numerical and semianalytic models in reproducing this form is entirely nontrivial. Other clustering statistics such as the galaxy-mass correlation function, the bispectrum, the void probability function, the pairwise velocity dispersion, and the group multiplicity function are sensitive to different combinations of HOD parameters and thus provide complementary information about galaxy bias. We outline a strategy for determining the HOD empirically from redshift survey data. This method starts from an assumed cosmological model, but we argue that cosmological and HOD parameters will have nondegenerate effects on galaxy clustering, so that a substantially incorrect cosmological model will not reproduce the observations for any choice of HOD. Empirical determinations of the HOD as a function of galaxy type from the Two-Degree Field (2dF) and Sloan Digital Sky Survey (SDSS) redshift surveys will provide a detailed target for theories of galaxy formation, insight into the origin of galaxy properties, and sharper tests of cosmological models.

Experimental identification of nonpointlike dark-matter candidates.

We show that direct dark-matter detection experiments can distinguish between pointlike and nonpointlike dark-matter candidates. The shape of the nuclear recoil-energy spectrum from pointlike dark-matter particles, e.g., neutralinos, is determined by the velocity distribution of dark matter in the galactic halo and by nuclear form factors. Typical cross sections of nonpointlike dark matter, for example, Q-balls, have a new form factor, which decreases rapidly with the recoil energy. A signal from nonpointlike dark matter is expected to peak near the experimental threshold and to fall off rapidly at higher energies.

Effect of elastic scattering in the Earth on cold dark matter experiments.

The diurnal modulation of fluxes and velocity distributions of cold dark matter (CDM) candidate particles, due to elastic scattering on the constituent nuclei of the Earth, is predicted. The geology, nuclear physics, the Earth's orientation, rotation, and trajectory through the galactic halo are included as well as the isotropic Maxwellian velocity distribution of CDM in the rest frame of the galactic halo. Observable modulations in detection rates and energy spectra are predicted for some interesting theoretical and experimental scenarios. The effects are seen to be maximal at southern latitudes. The differences in the sensitivity of experiments at southern and northern latitudes are explored. It is shown that it would be possible to detect Dirac neutrinos with densities well below those heretofore experimentally excluded.