Dark Matter Velocity Distribution Research Articles

Halo-independent direct detection of momentum-dependent dark matter

We show that the momentum dependence of dark matter interactions with nuclei can be probed in direct detection experiments without knowledge of the dark matter velocity distribution. This is one of the few properties of DM microphysics that can be determined with direct detection alone, given a signal of dark matter in multiple direct detection experiments with different targets. Long-range interactions arising from the exchange of a light mediator are one example of momentum-dependent DM. For data produced from the exchange of a massless mediator we find for example that the mediator mass can be constrained to be ≲ 10 MeV for DM in the 20-1000 GeV range in a halo-independent manner.

Analysis of the theoretical bias in dark matter direct detection

Fitting the model ''A'' to dark matter direct detection data, when the model that underlies the data is ``B'', introduces a theoretical bias in the fit. We perform a quantitative study of the theoretical bias in dark matter direct detection, with a focus on assumptions regarding the dark matter interactions, and velocity distribution. We address this problem within the effective theory of isoscalar dark matter-nucleon interactions mediated by a heavy spin-1 or spin-0 particle. We analyze 24 benchmark points in the parameter space of the theory, using frequentist and Bayesian statistical methods. First, we simulate the data of future direct detection experiments assuming a momentum/velocity dependent dark matter-nucleon interaction, and an anisotropic dark matter velocity distribution. Then, we fit a constant scattering cross section, and an isotropic Maxwell-Boltzmann velocity distribution to the simulated data, thereby introducing a bias in the analysis. The best fit values of the dark matter particle mass differ from their benchmark values up to 2 standard deviations. The best fit values of the dark matter-nucleon coupling constant differ from their benchmark values up to several standard deviations. We conclude that common assumptions in dark matter direct detection are a source of potentially significant bias.

A new halo-independent approach to dark matter direct detection analysis

Uncertainty in the local dark matter velocity distribution is a key difficulty in the analysis of data from direct detection experiments. Here we propose a new approach for dealing with this uncertainty, which does not involve any assumptions about the structure of the dark matter halo. Given a dark matter model, our method yields the velocity distribution which best describes a set of direct detection data as a finite sum of streams with optimised speeds and densities. The method is conceptually simple and numerically very efficient. We give an explicit example in which the method is applied to determining the ratio of proton to neutron couplings of dark matter from a hypothetical set of future data.

Impact of the dark matter velocity distribution on capture rates in the Sun

Dark matter could be captured in the Sun and self-annihilate, giving rise to an observable neutrino flux. Indirect searches for dark matter looking for this signal with neutrino telescopes have resulted in tight constraints on the interaction cross-section of dark matter with ordinary matter. We investigate how robust limits are against astro-physical uncertainties. We study the effect of the velocity distribution of dark matter in our Galaxy on capture rates in the Sun. We investigate four sources of uncertainties: orbital speed of the Sun, escape velocity of dark matter from the halo, dark matter velocity distribution functions and existence of a dark disc. We find that even extreme cases currently discussed do not decrease the sensitivity of indirect detection significantly because the capture is achieved over a broad range of the velocity distribution by integration over the velocity distribution. The effect of the uncertainty in the high-velocity tail of dark matter halo is very marginal as the capture process is rather inefficient at this region. The difference in capture rate in the Sun for various scenarios is compared to the expected change in event rates for direct detection.The possibility of co-rotating structure with the Sun can largely boost the signal and hence makes the interpretation of indirect detection conservative compared to direct detection.

A search for WIMPs and tests of local dark matter velocity distributions with the CoGeNT public dataset

A search for WIMPs and tests of local dark matter velocity distributions with the CoGeNT public dataset

Connecting direct dark matter detection experiments to cosmologically motivated halo models

Several direct detection experiments, including recently CDMS-II, have reported signals consistent with 5 to 10 GeV dark matter (DM) that appear to be in tension with null results from XENON and LUX experiments; these indicate a careful review of the theoretical basis, including the galactic DM velocity distribution function (VDF). We establish a VDF parameter space from DM-only cosmological simulations and illustrate that seemingly contradictory experimental results can be made consistent within this parameter space. Future experimental limits should be reported after they are marginalized over a range of VDF parameters.

Dark Atoms and the Positron-Annihilation-Line Excess in the Galactic Bulge

It was recently proposed that stable particles of charge −2,O--, can exist and constitute dark matter after they bind with primordial helium in O-helium (OHe) atoms. We study here in detail the possibility that this model provides an explanation for the excess of gamma radiation in the positron-annihilation line from the galactic bulge observed by INTEGRAL. This explanation assumes that OHe, excited to a 2s state through collisions in the central part of the Galaxy, deexcites to its ground state via anE0transition, emitting an electron-positron pair. The cross-section for OHe collisions with excitation to 2s level is calculated and it is shown that the rate of such excitations in the galactic bulge strongly depends not only on the mass of O-helium, which is determined by the mass ofO--, but also on the density and velocity distribution of dark matter. Given the astrophysical uncertainties on these distributions, this mechanism constrains theO--mass to lie in two possible regions. One of these is reachable in the experimental searches for stable multicharged particles at the LHC.

Signatures of dark matter scattering inelastically off nuclei

Direct dark matter detection focuses on elastic scattering of dark matter particles off nuclei. In this study, we explore inelastic scattering where the nucleus is excited to a low-lying state of 10-100 keV, with subsequent prompt de-excitation. We calculate the inelastic structure factors for the odd-mass xenon isotopes based on state-of-the-art large-scale shell-model calculations with chiral effective field theory WIMP-nucleon currents. For these cases, we find that the inelastic channel is comparable to or can dominate the elastic channel for momentum transfers around 150 MeV. We calculate the inelastic recoil spectra in the standard halo model, compare these to the elastic case, and discuss the expected signatures in a xenon detector, along with implications for existing and future experiments. The combined information from elastic and inelastic scattering will allow to determine the dominant interaction channel within one experiment. In addition, the two channels probe different regions of the dark matter velocity distribution and can provide insight into the dark halo structure. The allowed recoil energy domain and the recoil energy at which the integrated inelastic rates start to dominate the elastic channel depend on the mass of the dark matter particle, thus providing a potential handle to constrain its mass.

Anisotropic dark matter distribution functions and impact on WIMP direct detection

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.

PROPERTIES OF GALACTIC DARK MATTER: CONSTRAINTS FROM ASTRONOMICAL OBSERVATIONS

The distributions of normal matter and of dark matter in the Galaxy are coupled to each other as they both move in the common gravitational potential. In order to fully exploit this interplay and to derive the various properties of dark matter relevant to their direct and indirect detection, we have comprehensively reviewed the astronomical observations of the spatial and velocity distributions of the components of normal matter. We then postulate that the phase-space distribution of dark matter follows a lowered-isothermal form and self-consistently solve Poisson's equation to construct several models for the spatial and velocity distributions of dark matter. In this paper, we compute the total gravitational potential of the normal and dark matter components and investigate their consistency with current observations of the rotation curve of the Galaxy and of the spatial and velocity distributions of blue horizontal-branch and blue straggler stars. Even with this demand of consistency, a large number of models with a range of parameters characterizing the dark matter distribution remain. We find the best choice of parameters, within the range of allowed values, for the surface density of the disk 55 M$_\odot$ pc$^{-2}$, are the following: the dark matter density at the Galactic center $\rho_{DM,c}\approx 100-250$ GeV cm$^{-3}$, the local dark matter density $\rho_{DM}(R_0) \approx 0.56-0.72$ GeV cm$^{-3}$, and the root-mean-speed of dark matter particles $< v_{DM}^{2}(R_0)> ^{1/2}\approx490-550$ km s$^{-1}$. We also discuss possible astronomical observations that may further limit the range of the allowed models.

The impact of the phase-space density on the indirect detection of dark matter

We study the indirect detection of dark matter when the local dark matter velocity distribution depends upon position, as expected for the Milky Way and its dwarf spheroidal satellites, and the annihilation cross-section is not purely s-wave. Using a phase-space distribution consistent with the dark matter density profile, we present estimates of cosmic and gamma-ray fluxes from dark matter annihilations. The expectations for the indirect detection of dark matter can differ significantly from the usual calculation that assumes that the velocity of the dark matter particles follows a Maxwell-Boltzmann distribution.

Brighter galaxy bias: underestimating the velocity dispersions of galaxy clusters

We study the systematic bias introduced when selecting the spectroscopic redshifts of brighter cluster galaxies to estimate the velocity dispersion of galaxy clusters from both simulated and observational galaxy catalogues. We select clusters with Ngal > 50 at five low redshift snapshots from a semi-analytic model galaxy catalogue, and from a catalogue of SDSS DR8 groups and clusters across the redshift range 0.021<z<0.098. We employ various selection techniques to explore whether the velocity dispersion bias is simply due to a lack of dynamical information or is the result of an underlying physical process occurring in the cluster, for example, dynamical friction. The velocity dispersions and stacked particle velocity distributions of the parent dark matter (DM) halos are compared to the corresponding cluster dispersions and galaxy velocity distribution. We find a clear bias between the halo and the semi-analytic galaxy cluster velocity dispersion on the order of sigma gal / sigma DM = 0.87-0.95 and a distinct difference in the stacked galaxy and DM particle velocity distribution. We identify a systematic underestimation of the velocity dispersions when imposing increasing absolute I-band magnitude limits. This underestimation is enhanced when using only the brighter cluster members for dynamical analysis on the order of 5-35%, indicating that dynamical friction is a serious source of bias when using galaxy velocities as tracers of the underlying gravitational potential. In contrast to the literature we find that the resulting bias is not only halo mass-dependent but that the nature of the dependence changes according to the galaxy selection strategy. We make a recommendation that, in the realistic case of limited availability of spectral observations, a strictly magnitude-limited sample should be avoided to ensure an unbiased estimate of the velocity dispersion.

Complementarity of direct and indirect dark matter detection experiments

We investigate the prospects for reconstructing the mass, spin-independent and spin-dependent cross sections of dark matter particles with a combination of future direct detection experiments such as XENON1T, and the IceCube neutrino telescope in the 86-string configuration including the DeepCore array. We quantify the degree of complementarity between the two experiments by adopting realistic values for their exposure, energy threshold and resolution. Starting from benchmark models arising from a supersymmetric model with 25 free parameters, we show that despite the stringent constraints set by the run with 79 strings, IceCube can help break the degeneracies in the dark matter cross section parameter space, even in the unfortunate case where it fails to discover high energy neutrinos from the Sun. We also discuss how the reconstruction of the dark matter particle parameters from the combined data sets is affected by the uncertainties associated with the nuclear structure of the target material in case of spin-dependent scattering and those associated with astrophysical quantities such as the dark matter density and velocity distribution.

Integrating in dark matter astrophysics at direct detection experiments

We study the capabilities of the Majorana Demonstrator, a neutrinoless double-beta decay experiment currently under construction at the Sanford Underground Laboratory, as a light WIMP detector. For a cross section near the current experimental bound, the Majorana Demonstrator should collect hundreds or even thousands of recoil events. This opens up the possibility of simultaneously determining the physical properties of the dark matter and its local velocity distribution, directly from the data. We analyze this possibility and find that allowing the dark matter velocity distribution to float considerably worsens the WIMP mass determination. This result is traced to a previously unexplored degeneracy between the WIMP mass and the velocity dispersion. We simulate spectra using both isothermal and Via Lactea II velocity distributions and comment on the possible impact of streams. We conclude that knowledge of the dark matter velocity distribution will greatly facilitate the mass and cross section determination for a light WIMP.

Analyzing direct dark matter detection data with unrejected background events by the AMIDAS website

In this talk I have presented the data analysis results of extracting properties of halo WIMPs: the mass and the (ratios between the) spin-independent and spin-dependent couplings/cross sections on nucleons by the AMIDAS website by taking into account possible unrejected background events in the analyzed data sets. Although non-standard astronomical setup has been used to generate pseudodata sets for our analyses, it has been found that, without prior information/assumption about the local density and velocity distribution of halo Dark Matter, these WIMP properties have been reconstructed with ∼ 2% to ≲ 30% deviations from the input values.

Bayes and present dark matter direct search status

Recently there has been a huge activity in the dark matter direct detection field, with the report of an excess from CoGeNT and CRESST along with the annual modulated signal of DAMA/LIBRA and the strong exclusion bound from XENON100. We analyse these results within the framework of Bayesian inference and evidence. Indeed Bayesian methods are well suited for marginalizing over experimental systematics and background. We present the results for spin-independent interaction on nuclei with particular attention to the low dark matter mass region and the compatibility between experiments. In the same vein we also investigate the impact of astrophysical uncertainties on the WIMP preferred parameter space within the class of isotropic dark matter velocity distributions.

Probing the local velocity distribution of WIMP dark matter with directional detectors

We explore the ability of directional nuclear-recoil detectors to constrain the local velocity distribution of weakly interacting massive particle (WIMP) dark matter by performing Bayesian parameter estimation on simulated recoil-event data sets. We discuss in detail how directional information, when combined with measurements of the recoil-energy spectrum, helps break degeneracies in the velocity-distribution parameters. We also consider the possibility that velocity structures such as cold tidal streams or a dark disk may also be present in addition to the Galactic halo. Assuming a CF4 detector with a 30-kg-yr exposure, a 50-GeV WIMP mass, and a WIMP-nucleon spin-dependent cross-section of 10−3pb, we show that the properties of a cold tidal stream may be well constrained. However, measurement of the parameters of a dark-disk component with a low lag speed of ∼ 50km/s may be challenging unless energy thresholds are improved.

Breit-Wigner enhancement considering the dark matter kinetic decoupling

In this paper we study the Breit-Wigner enhancement of dark matter (DM) annihilation considering the kinetic decoupling in the evolution of DM freeze-out at the early Universe. Since the DM temperature decreases much faster (as $1/{R}^{2}$) after kinetic decoupling than that in kinetic equilibrium (as $1/R$), we find the Breit-Wigner enhancement of the DM annihilation rate after the kinetic decoupling will affect the DM relic density significantly. Focusing on the model parameters that try to explain the anomalous cosmic positron/electron excesses observed by PAMELA/Fermi/ATIC, we find the elastic scattering $Xf\ensuremath{\rightarrow}Xf$ is not efficient in keeping dark matter in kinetic equilibrium, and the kinetic decoupling temperature ${T}_{\mathrm{kd}}$ is comparable to the chemical decoupling temperature ${T}_{f}\ensuremath{\sim}O(10)\text{ }\text{ }\mathrm{GeV}$. The reduction of the relic density after ${T}_{\mathrm{kd}}$ is significant and leads to a limited enhancement factor $\ensuremath{\sim}O({10}^{2})$. Therefore, it is difficult to explain the anomalous positron/electron excesses in cosmic rays by DM annihilation and give the correct DM relic density simultaneously in the minimal Breit-Wigner enhancement model. We also mention that if the nonthermal DM velocity distribution after kinetic decoupling or new scattering processes are considered, this constraint could be changed.

Long-range forces in direct dark matter searches

We discuss the positive indications of a possible dark matter signal in direct detection experiments in terms of a mechanism of interaction between the dark matter particle and the nuclei occurring via the exchange of a light mediator, resulting in a long-range interaction. We analyze the annual modulation results observed by the DAMA and CoGeNT experiments and the observed excess of events of CRESST. In our analysis, we discuss the relevance of uncertainties related to the velocity distribution of galactic dark matter and to the channeling effect in NaI. We find that a long-range force is a viable mechanism, which can provide full agreement between the reconstructed dark matter properties from the various experimental data sets, especially for masses of the light mediator in the 10--30 MeV range and a light dark matter with a mass around 10 GeV. The relevant bounds on the light mediator mass and scattering cross section are then derived, should the annual modulation effects be due to this class of long-range forces.

Effects ofp-wave annihilation on the angular power spectrum of extragalactic gamma-rays from dark matter annihilation

We present a formalism for estimating the angular power spectrum of extragalactic gamma-rays produced by dark matter annihilating with any general velocity-dependent cross section. The relevant density and velocity distribution of dark matter is modeled as an ensemble of smooth, universal, rigid, disjoint, spherical halos with distribution and universal properties constrained by simulation data. We apply this formalism to theories of dark matter with $p$-wave annihilation, for which the relative-velocity-weighted annihilation cross section is $\ensuremath{\sigma}v=a+b{v}^{2}$. We determine that this significantly increases the gamma-ray power if $b/a\ensuremath{\gtrsim}{10}^{6}$. The effect of $p$-wave annihilation on the angular power spectrum is very similar for the sample of particle physics models we explored, suggesting that the important effect for a given $b/a$ is largely determined by the cosmic dark matter distribution. If the dark matter relic from strong $p$-wave theories is thermally produced, the intensities of annihilation gamma-rays are strongly $p$-wave suppressed, making them difficult to observe. If an angular power spectrum consistent with a strong $p$ wave were to be observed, it would likely indicate nonthermal production of dark matter in the early Universe.