Standard Halo Model Research Articles

Direct detection of WIMPs: implications of a self-consistent truncated isothermal model of the Milky Way's dark matter halo

Direct detection of Weakly Interacting Massive Particle (WIMP) candidates of Dark Matter (DM) is studied within the context of a self-consistent truncated isothermal model of the finite-size dark halo of the Galaxy. The halo model, based on the ``King model'' of the phase space distribution function of collisionless DM particles, takes into account the modifications of the phase-space structure of the halo due to the gravitational influence of the observed visible matter in a self-consistent manner. The parameters of the halo model are determined by a fit to a recently determined circular rotation curve of the Galaxy that extends up to ∼ 60 kpc. Unlike in the Standard Halo Model (SHM) customarily used in the analysis of the results of WIMP direct detection experiments, the velocity distribution of the WIMPs in our model is non-Maxwellian with a cut-off at a maximum velocity that is self-consistently determined by the model itself. For our halo model that provides the best fit to the rotation curve data, the 90% C.L. upper limit on the WIMP-nucleon spin-independent cross section from the recent results of the CDMS-II experiment, for example, is ∼ 5.3 × 10−8 pb at a WIMP mass of ∼ 71 GeV. We also find, using the original 2-bin annual modulation amplitude data on the nuclear recoil event rate seen in the DAMA experiment, that there exists a range of small WIMP masses, typically ∼ 2–16 GeV, within which DAMA collaboration's claimed annual modulation signal purportedly due to WIMPs is compatible with the null results of other experiments. These results, based as they are on a self-consistent model of the dark matter halo of the Galaxy, strengthen the possibility of low-mass (≲10 GeV) WIMPs as a candidate for dark matter as indicated by several earlier studies performed within the context of the SHM. A more rigorous analysis using DAMA bins over smaller intervals should be able to better constrain the ``DAMA regions'' in the WIMP parameter space within the context of our model.

Form factor dark matter

We present a dynamical alternative to inelastic dark matter as a way of reconciling the modulating signal seen at DAMA with null results at other direct detection experiments.The essential ingredient is a new form factor which introduces momentum dependence inthe interaction of dark matter with nuclei. The role of the form factor is to suppress eventsat low momentum transfer. We find that a form factor approach is most likely not viable in the context of the standard halo model, however it is consistent with halo models suggested by recent Via Lactea simulations. As an example of possible form factors,we present a class of models where the necessary momentum dependence arises frominterference of GeV mass gauge bosons coupling the dark matter to nuclei. At energiesrelevant for direct detection experiments these models contain one or two additional parameters beyond the case of a standard WIMP.

Prospects for indirect detection of sneutrino dark matter with IceCube

We investigate the prospects for indirect detection of right-handed sneutrino dark matter at the IceCube neutrino telescope in a $U(1{)}_{B\ensuremath{-}L}$ extension of the minimal supersymmetric standard model. The capture and annihilation of sneutrinos inside the Sun reach equilibrium, and the flux of produced neutrinos is governed by the sneutrino-proton elastic scattering cross section, which has an upper bound of $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }\mathrm{pb}$ from the ${Z}^{\ensuremath{'}}$ mass limits in the $B\ensuremath{-}L$ model. Despite the absence of any spin-dependent contribution, the muon event rates predicted by this model can be detected at IceCube since sneutrinos mainly annihilate into leptonic final states by virtue of the fermion $B\ensuremath{-}L$ charges. These subsequently decay to neutrinos with 100% efficiency. The Earth muon event rates are too small to be detected for the standard halo model irrespective of an enhanced sneutrino annihilation cross section that can explain the recent PAMELA data. For modified velocity distributions, the Earth muon events increase substantially and can be greater than the IceCube detection threshold of 12 events ${\mathrm{km}}^{\ensuremath{-}2}\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$. However, this only leads to a mild increase of about 30% for the Sun muon events. The number of muon events from the Sun can be as large as roughly 100 events ${\mathrm{km}}^{\ensuremath{-}2}\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$ for this model.

THE DARK DISK OF THE MILKY WAY

Massive satellite accretions onto early galactic disks can lead to the deposition of dark matter in disk-like configurations that co-rotate with the galaxy. This phenomenon has potentially dramatic consequences for dark matter detection experiments. We utilize focused, high-resolution simulations of accretion events onto disks designed to be Galaxy analogues, and compare the resultant disks to the morphological and kinematic properties of the Milky Way's thick disk in order to bracket the range of co-rotating accreted dark matter. We find that the Milky Way's merger history must have been unusually quiescent compared to median LCDM expectations and therefore its dark disk must be relatively small: the fraction of accreted dark disk material near the Sun is about 20% of the host halo density or smaller and the co-rotating dark matter fraction near the Sun, defined as particles moving with a rotational velocity lag less than 50 km/s, is enhanced by about 30% or less compared to a standard halo model. Such a dark disk could contribute dominantly to the low energy (of order keV for a dark matter particle with mass 100 GeV) nuclear recoil event rate of direct dectection experiments, but it will not change the likelihood of detection significantly. These dark disks provide testable predictions of weakly-interacting massive particle dark matter models and should be considered in detailed comparisons to experimental data. Our findings suggest that the dark disk of the Milky Way may provide a detectable signal for indirect detection experiments, contributing up to about 25% of the dark matter self-annihilation signal in the direction of the center of the Galaxy, lending the signal a noticeably oblate morphology.

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.

Dark matter disc enhanced neutrino fluxes from the Sun and Earth

As disc galaxies form in a hierarchical cosmology, massive merging satellites are preferentially dragged towards the disc plane. The material accreted from these satellites forms a dark matter disc that contributes 0.25–1.5 times the non-rotating halo density at the solar position. Here, we show the importance of the dark disc for indirect dark matter detection in neutrino telescopes. Previous predictions of the neutrino flux from WIMP annihilation in the Earth and the Sun have assumed that Galactic dark matter is spherically distributed with a Gaussian velocity distribution, the standard halo model. Although the dark disc has a local density comparable to the dark halo, its higher phase space density at low velocities greatly enhances capture rates in the Sun and Earth. For typical dark disc properties, the resulting muon flux from the Earth is increased by three orders of magnitude over the SHM, while for the Sun the increase is an order of magnitude. This significantly increases the sensitivity of neutrino telescopes to fix or constrain parameters in WIMP models. The flux from the Earth is extremely sensitive to the detailed properties of the dark disc, while the flux from the Sun is more robust. The enhancement of the muon flux from the dark disc puts the search for WIMP annihilation in the Earth on the same level as the Sun for WIMP masses ≲100 GeV.

Annual Modulation of Dark Matter in the Presence of Streams

In addition to a smooth component of Weakly Interaction Massive Particle (WIMP) dark matter in galaxies, there may be streams of material; the effects of WIMP streams on direct detection experiments is examined in this paper. The contribution to the count rate due to the stream cuts off at some characteristic energy. Near this cutoff energy, the stream contribution to the annual modulation of recoils in the detector is comparable to that of the thermalized halo, even if the stream represents only a small portion (∼5% or less) of the local halo density. Consequently the total modulation may be quite different than would be expected for the standard halo model alone: it may not be cosine-like and can peak at a different date than expected. The annual modulation of this cutoff energy together with the annual modulation of the overall signal provide a “smoking gun” for WIMP detection.

Annual modulation of dark matter in the presence of streams

In addition to a smooth component of weakly interacting massive particle (WIMP) dark matter in galaxies, there may be streams of material; the effects of WIMP streams on direct detection experiments is examined in this paper. The contribution to the count rate due to the stream cuts off at some characteristic energy. Near this cutoff energy, the stream contribution to the annual modulation of recoils in the detector is comparable to that of the thermalized halo, even if the stream represents only a small portion ($\ensuremath{\sim}5%$ or less) of the local halo density. Consequently the total modulation may be quite different than would be expected for the standard halo model alone: it may not be cosinelike and can peak at a different date than expected. The effects of speed, direction, density, and velocity dispersion of a stream on the modulation are examined. We describe how the observation of a modulation can be used to determine these stream parameters. Alternatively, the presence of a dropoff in the recoil spectrum can be used to determine the WIMP mass if the stream speed is known. The annual modulation of the cutoff energy together with the annual modulation of the overall signal provide a ``smoking gun'' for WIMP detection.

Cryogenic Dark Matter Search: limits and projections

The results from the second run of the Cryogenic Dark Matter Search experiment in the Soudan Underground Laboratory yields new exclusion limits on the coherent WIMPnucleon scalar cross-sections for WIMP masses above 15 GeV/c2. Two towers, each consisting of six detectors, were operated in the mine for 74.5 live days, doubling the previous Ge exposure to 34 kg-d after cuts for recoil energies of 10-100 keV. When combined with the data from the first run at Soudan, the new upper limit is 1.6 × 10−43 cm2 for a 60 GeV WIMP using the standard dark-matter halo and nuclear physics WIMP model. This is a factor of 2.5 lower than our previous limit and a factor of 10 lower than any other experiment. Limits from the Si data (12 kg-d) are less stringent, but extend the excluded region for low mass WIMPs. CDMS also has sensitivity to spin-dependent limits.

Limits on WIMP–nucleon interactions from the Cryogenic Dark Matter Search at the Soudan Underground Laboratory

We present the results of the first two data runs of the Cryogenic Dark Matter Search at Soudan Underground Laboratory. These data exclude substantial new parameter space for both spin-independent and spin-dependent WIMP–nucleon interactions within the standard halo model.

A galaxy halo model of large-scale structure

We present a new, galaxy halo model of large-scale structure, in which the galaxies entering a given sample are the fundamental objects. Haloes attach to galaxies, in contrast to the standard halo model, in which galaxies attach to haloes. The galaxy halo model pertains mainly to the relationships between the power spectra of galaxies and mass, and their cross-power spectrum. With surprisingly little input, an intuition-aiding approximation to the galaxy-matter cross-correlation coefficient R(k) emerges in terms of the halo mass dispersion. This approximation seems valid to mildly non-linear scales (k ≤ 3 h Mpc -1 ), allowing the measurement of the bias and the matter power spectrum from measurements of the galaxy and galaxy-matter power spectra (or correlation functions). This is especially relevant given the recent advances in precision in measurements of the galaxy-matter correlation function from weak gravitational lensing. The galaxy halo model also addresses the issue of interpreting the galaxy-matter correlation function as an average halo density profile, and provides a simple description of galaxy bias as a function of scale.

The Physics of Galaxy Clustering. I. A Model for Subhalo Populations

We present a semianalytic model for cold dark matter halo substructure that can be used as a framework for studying the physics of galaxy formation and as an ingredient in halo models of galaxy clustering. The model has the following main ingredients: (1) extended Press-Schechter mass accretion histories, (2) host halo density profiles computed according to the trends observed in cosmological simulations, (3) distributions of initial orbital parameters of accreting subhalos measured in a high-resolution simulation of three Milky Way-size halos, and (4) integration of the orbital evolution of subhalos including the effects of dynamical friction and tidal mass loss. We perform a comprehensive comparison of the model calculations to the results of a suite of high-resolution cosmological simulations. The comparisons show that subhalo statistics such as the velocity and mass functions, the radial distributions, and the halo occupation distributions agree well over 3 orders of magnitude in host halo mass and at various redshifts. We find that both in the simulations and in our model the radial distributions of subhalos are significantly shallower than that of the dark matter density. The abundance of subhalos in a host is set by competition between tidal disruption and new accretion. Halos of high mass and halos at high redshift tend to host more subhalos because the subhalos have, on average, been accreted more recently. Similarly, at a fixed mass and epoch, halos that formed more recently host a larger number of subhalos. Observed fossil groups may represent an extreme tail of this correlation. We find a related correlation between host halo concentration and satellite abundance at fixed host mass, Nsat ∝ c, where a changes with redshift and host-to-subhalo mass ratio. Lastly, we use our substructure model to populate host halos in one of the high-resolution cosmological simulations, replacing the actual subhalos resolved in this simulation and using the host mass as the only input for the model calculation. We show that the resulting correlation function of such a hybrid halo ensemble is indistinguishable from that measured directly in the simulation. This supports one of the key tenets of the standard halo model, i.e., the assumption that the halo occupation distribution is statistically independent of the host halo environment.

First Results from the Cryogenic Dark Matter Search in the Soudan Underground Laboratory

We report the first results from a search for weakly interacting massive particles (WIMPs) in the Cryogenic Dark Matter Search experiment at the Soudan Underground Laboratory. Four Ge and two Si detectors were operated for 52.6 live days, providing 19.4 kg d of Ge net exposure after cuts for recoil energies between 10 and 100 keV. A blind analysis was performed using only calibration data to define the energy threshold and selection criteria for nuclear-recoil candidates. Using the standard dark-matter halo and nuclear-physics WIMP model, these data set the world's lowest exclusion limits on the coherent WIMP-nucleon scalar cross section for all WIMP masses above 15 GeV/c2, ruling out a significant range of neutralino supersymmetric models. The minimum of this limit curve at the 90% C.L. is 4 x 10(-43) cm2 at a WIMP mass of 60 GeV/c2.

Phase of the annual modulation as a tool for determining the mass of the weakly interacting massive particle

The count rate of weakly interacting massive particle (WIMP) dark matter candidates in direct detection experiments experiences an annual modulation due to the Earth's motion around the Sun. In the standard isothermal halo model, the signal peaks near June 2nd at high recoil energies; however, the signal experiences a phase reversal and peaks in December at low energy recoils. We show that this phase reversal may be used to determine the WIMP mass. If an annual modulation were observed with the usual phase (i.e., peaking on June 2nd) in the lowest accessible energy recoil bins of the DAMA, CDMS-II, CRESST-II, EDELWEISS-II, GENIUS-TF, ZEPLIN-II, XENON, or ZEPLIN-IV detectors, one could immediately place upper bounds on the WIMP mass of 103, 48, 6, 97, 10, 52, 29, and 29 GeV, respectively. In addition, detectors with adequate energy resolution and sufficiently low recoil energy thresholds may determine the crossover recoil energy at which the phase reverses, thereby obtaining an independent measurement of the WIMP mass. We study the capabilities of various detectors, and find that CRESST-II, ZEPLIN-II, and GENIUS-TF should be able to observe the phase reversal in a few years of runtime, and can thus determine the mass of the WIMP if it is $O(100\mathrm{GeV}).$ Xenon based detectors with 1000 kg (XENON and ZEPLIN-IV) and with energy recoil thresholds of a few keV require 25 kg yr exposure, which will be readily attained in upcoming experiments.

Statistical interpretation of Large Magellanic Cloud microlensing candidates

After a decade of gravitational microlensing experiments, a dozen microlensing candidates in the direction of the stars of the Large Magellanic Cloud (LMC) have been detected by the EROS and MACHO groups. Recently it was shown that the distribution of the duration of the observed LMC microlensing events is significantly narrower than what is expected from the standard halo model. In this article we make the same comparison, using non-standard halo models and considering the contribution of non-halo components of the Milky Way such as the disc, spheroid and the LMC itself in the microlensing events. Comparing the theoretical and experimental widths of the distribution of the duration of events shows that neither standard nor non-standard halo models are compatible with the microlensing data at least with 95 per cent confidence. These results may be explained if (i) the MACHOs in the Galactic halo reside in clumpy structures or (ii) the durations of events have been underestimated due to the blending effect.

Testing the halo model against the SDSS photometric survey

We present halo model predictions for the expected angular clustering and associated errors from the completed Sloan Digital Sky Survey (SDSS) photometric galaxy sample. These results are used to constrain halo model parameters under the assumption of a fixed ΛCDM cosmology using standard Fisher matrix techniques. Given the ability of the five-colour SDSS photometry to separate galaxies into subpopulations by intrinsic colour, we also use extensions of the standard halo model formalism to calculate the expected clustering of red and blue galaxy subpopulations as a further test of the galaxy evolution included in the semi-analytical methods for populating dark matter haloes with galaxies. The extremely small sample variance and Poisson errors from the completed SDSS should result in very impressive constraints (∼1–10 per cent) on the halo model parameters for a simple magnitude-limited sample and should provide an extremely useful check on the behaviour of current and future N-body simulations and semi-analytical techniques. We also show that similar constraints are possible using a narrow selection function, as would be possible using photometric redshifts, without making linear assumptions regarding the evolution of the underlying power spectra. In both cases, we explore the effects of uncertainty in the selection function on the resulting constraints and the degeneracies between various combinations of parameters.

Declining Rotation Curve and Brown Dwarf MACHOs

If the Galactic rotation speed at the solar circle is ~200 km s-1 or smaller, as is supported by several recent studies, the rotation curve of the Galaxy could be declining in the outermost region. Motivated by this, we investigate the effect of such a declining rotation curve on the estimate of the MACHO mass and the fractional contribution of the MACHOs to the Galactic dark halo. Using Hernquist and Plummer halo models instead of the standard halo model, we find that the MACHO mass could be significantly smaller than that for the standard halo case. In particular, there exists a certain set of halo parameters for which the MACHO mass is 0.1 M☉ or less and at the same time the MACHO contribution to the total mass of the halo is almost 100%. This result indicates that a halo that consists solely of brown dwarfs can be consistent with both the observed microlensing properties and the constraints from the rotation curve, provided that the outer rotation curve is indeed declining.

Microlensing Rates from Self-consistent Galactic Models

We study different models of dark matter distribution for the halo of our galaxy. In particular, we consider Eddington and King-Michie models, which [C[C, which include anisotropy in the velocity space, and compute in a self-consistent way the amount of dark matter present in the halo. Assuming that the dark matter is in form of MACHOs, we find for each model the expected number of microlensing events and their average time duration for an experiment monitoring stars in the LMC. The main effect of including anisotropy is to reduce the microlensing rate by about 30% and to increase, but only slightly, the mean event duration as compared to the standard halo model. Consideration of different luminous models for the visible part of the galaxy also induce variations in the microlensing results by roughly the same amount as mentioned above. The main uncertainty, in order to be able to discriminate between different dark matter distributions and to estimate the fraction of it in form of MACHOs is due to the poor knowledge of the rotation velocity at large galactocentric distances.