Searching for relic neutralinos using neutrino telescopes

Neutrino telescopes of large area offer the possibility of searching for indirect signals of relic neutralinos in the galactic halo, due to annihilations in the Sun or the Earth. Here we investigate the sensitivity, using a supergravity scheme where the soft scalar mass terms are not constrained by universality conditions at the grand unification scale. We first discuss in which regions of the supersymmetric parameter space the neutralino may be considered as a good candidate for cold dark matter. The discovery potential of the search using neutrino telescopes is then compared to that of the direct search for relic neutralinos.     

Perspectives for detection of a higgsino-like relic neutralino

It has been conjectured by Ambrosanio, Kane, Kribs, Martin and Mrenna (AKM) that the CDF event is due to a decay chain involving two neutralino states (the lightest and the next-to-lightest ones). The lightest neutralino (χ ) has been further considered by Kane and Wells as a candidate for cold dark matter. In this paper we examine the properties of relic χ 's in their full parameter space, and examine the perspectives for detection by comparing theoretical predictions to sensitivities of various experimental searches. We find that for most regions of the parameter space the detectability of a relic χ would require quite substantial improvements in current experimental sensitivities. The measurements of neutrino fluxes from the center of the Earth and of an excess of in cosmic rays are shown to offer some favorable perspectives for investigating a region of the χ parameter space around the maximal tan β value allowed by the model.     

Inverse Compton Gamma Rays from Dark Matter Annihilation in the Dwarf Galaxies

Dwarf spheroidal (dSph) galaxies are thought to be good candidates for dark matter search due to their high mass-to-light (M/L) ratio. One of the most favored dark matter candidates is the lightest neutralino (neutral χ particle) as predicted in the Minimal Supersymmetric Standard Model (MSSM). In this study, we model the gamma ray emission from dark matter annihilation coming from the nearby dSph galaxies Draco, Segue 1, Ursa Minor and Willman 1, taking into account the contribution from prompt photons and photons produced from inverse Compton scattering off starlight and Cosmic Microwave Background (CMB) photons by the energetic electrons and positrons from dark matter annihilation. We also compute the energy spectra of electrons and positrons from the decay of dark matter annihilation products. Gamma ray spectra and fluxes for both prompt and inverse Compton emission have been calculated for neutralino annihilation over a range of masses and found to be in agreement with the observed data. It has been found that the ultra faint dSph galaxy Segue 1 gives the largest gamma ray flux limits while the lowest gamma ray flux limits has been obtained from Ursa Minor. It is seen that for larger M/L ratio of dwarf galaxies the intensity pattern originating from e ⁺ e ^? pairs scattering off CMB photons is separated by larger amount from that off the starlight photons for the same neutralino mass. As the e ⁺ e ^? energy spectra have an exponential cut off at high energies, this may allow to discriminate some dark matter scenarios from other astrophysical sources. Finally, some more detailed study about the effect of inverse Compton scattering may help constrain the dark matter signature in the dSph galaxies.     

Gamma-ray and synchrotron emission from neutralino annihilation in the Large Magellanic Cloud

We calculate the expected flux of γ-ray and radio emission from the LMC due to neutralino annihilation. Using rotation curve data to probe the density profile and assuming a minimum disk, we describe the dark matter halo of the LMC using models predicted by N-body simulations. We consider a range of density profiles including the NFW profile, a modified NFW profile proposed by Hayashi et al. (2003) to account for the effects of tidal stripping, and an isothermal sphere with a core. We find that the γ-ray flux expected from these models may be detectable by GLAST for a significant part of the neutralino parameter space. The prospects for existing and upcoming Atmospheric Cherenkov Telescopes (ACTs) are less optimistic, as unrealistically long exposures are required for detection. However, the effects of adiabatic compression due to the baryonic component may improve the chances for detection by ACTs. The maximum flux we predict is well below EGRET's measurements and thus EGRET does not constrain the parameter space. The expected synchrotron emission generally lies below the observed radio emission from the LMC in the frequency range of 19.7–8550 MHz. As long as σv<2×10⁻²⁶ cm³ s⁻¹ for a neutralino mass of 50 GeV, the observed radio emission is not primarily due to neutralinos and is consistent with the assumption that the main source is cosmic rays. We find that the predicted fluxes, obtained by integrating over the entire LMC, are not very strongly dependent on the inner slope of the halo profile, varying by less than an order of magnitude for the range of profiles we considered.     

Dark matter annihilation at cosmological redshifts: possible relic signal from annihilation of weakly interacting massive particles

We discuss the possibility of observing the products of the dark matter annihilation that was going on in the early Universe. Of all the particles that could be generated by this process, we consider only photons, as they are both uncharged and easily detectable. The younger the Universe was, the higher the dark matter concentration n and the annihilation rate (proportional to n ²) were. However, the emission from the very early Universe cannot reach us because of the opacity. The main part of the signal was generated at the moment the Universe had just become transparent for the photons produced by the annihilation. Thus, the dark matter annihilation in the early Universe should have created a sort of relic emission. We obtain its flux and the spectrum.
If weakly interacting massive particles (WIMPs) constitute dark matter, it is shown that we may expect an extragalactic gamma-ray signal in the energy range 0.5–20 MeV with a maximum near 8 MeV. We show that an experimentally observed excess in the gamma-ray background at 0.5–20 MeV could be created by the relic signal from the annihilation of WIMPs only if the dark matter structures in the Universe had appeared before the Universe became transparent for the annihilation products  ( z ≃ 300)  . We discuss in more detail physical conditions whereby this interpretation could be possible.     

用中子星限制暗物质粒子散射截面

中子星可以通过重子物质和暗物质的相互作用吸积暗物质,且在一定条件下, 中子星吸积的暗物质粒子可以引发自引力塌缩形成小型黑洞, 生成的黑洞可能会进一步吞噬中子星.依据文献已有模型, 基于以上物理过程给出了在暗物质粒子不同质量下对暗物质粒子--中子的散射截面的限制.使用弱相互作用大质量粒子(Weakly Interacting Massive Particle, WIMP)模型, 并考虑暗物质粒子是玻色子的情形, 讨论了暗物质粒子有无自相互作用以及有无湮灭等条件下对限制暗物质参数的影响.既考虑了已发现的两个中子星系统来给出对暗物质参数空间的限制,也考虑了两个可能存在的年老中子星来预测未来观测可能对暗物质参数空间的限制.对于考虑玻色--爱因斯坦凝聚(Bose-Einstein Condensate, BEC)的玻色子暗物质, 在无自相互作用或有弱自相互作用, 无湮灭或有很小湮灭截面的条件下,中子星给出的间接观测对暗物质粒子-中子散射截面的限制的强度比XENON1T直接探测实验来得更强.未来, 如果在银心附近能观测到年老中子星, 其观测结果可以提升模型给出的对暗物质粒子--中子散射截面的限制, 从而帮助人们进一步理解暗物质.     

The impact of subhalos on the gamma-ray signal from dark matter annihilation

Assuming the lightest neutralino in the minimal supersymmetric extension to the Standard Model (MSSM) as the main dark matter (DM) component, we estimate the cumulative enhancement of the neutralino-induced gamma-ray signal in the Galactic halo due to the presence of subhalos. A realistic semi-analytical model for the spatial mass function of subhalos is implemented, incorporating effects that may influence the distribution and the evolution of substructures, such as the mass loss due to the tidal stripping and the orbital decay due to the dynamical friction.     

Dark stars at the Galactic Centre – the main sequence

In regions of very high dark matter density such as the Galactic Centre, the capture and annihilation of WIMP dark matter by stars has the potential to significantly alter their evolution. We describe the dark stellar evolution code D ark S tars , and present a series of detailed grids of WIMP-influenced stellar models for main-sequence stars. We describe the changes in stellar structure and main-sequence evolution which occur as a function of the rate of energy injection by WIMPs, for masses of  0.3–2.0 M_⊙  and metallicities   Z = 0.0003–0.02  . We show what rates of energy injection can be obtained using realistic orbital parameters for stars at the Galactic Centre, including detailed consideration of the velocity and density profiles of dark matter. Capture and annihilation rates are strongly boosted when stars follow elliptical rather than circular orbits. If there is a spike of dark matter induced by the supermassive black hole at the Galactic Centre, single solar mass stars following orbits with periods as long as 50 yr and eccentricities as low as 0.9 could be significantly affected. Binary systems with similar periods about the Galactic Centre could be affected on even less eccentric orbits. The most striking observational effect of this scenario would be the existence of a binary consisting of a low-mass protostar and a higher mass evolved star. The observation of low-mass stars and/or binaries on such orbits would either provide a detection of WIMP dark matter, or place stringent limits on the combination of the WIMP mass, spin-dependent nuclear-scattering cross-section, halo density and velocity distribution near the Galactic Centre. In some cases, the derived limits on the WIMP mass and spin-dependent nuclear-scattering cross-section would be of comparable sensitivity to current direct-detection experiments.     

The survival and disruption of cold dark matter microhaloes: implications for direct and indirect detection experiments

If the dark matter particle is a neutralino, then the first structures to form are cuspy cold dark matter (CDM) haloes collapsing after redshifts   z ≈ 100  in the mass range  10⁻⁶–10⁻³ M_⊙  . We carry out a detailed study of the survival of these microhaloes in the Galaxy as they experience tidal encounters with stars, molecular clouds, and other dark matter substructures. We test the validity of analytic impulsive heating calculations using high-resolution N -body simulations. A major limitation of analytic estimates is that mean energy inputs are compared to mean binding energies, instead of the actual mass lost from the system. This energy criterion leads to an overestimate of the stripped mass and an underestimate of the disruption time-scale, since CDM haloes are strongly bound in their inner parts. We show that a significant fraction of material from CDM microhaloes can be unbound by encounters with Galactic substructure and stars; however, the cuspy central regions remain relatively intact. Furthermore, the microhaloes near the solar radius are those which collapse significantly earlier than average and will suffer very little mass-loss. Thus, we expect a fraction of surviving bound microhaloes, a smooth component with narrow features in phase space, which may be uncovered by direct detection experiments, as well as numerous surviving cuspy cores with proper motions of arcminutes per year, which can be detected indirectly via their annihilation into gamma-rays.     

The graininess of dark matter haloes

We use the recently completed one billion particle Via Lactea II Λ cold dark matter simulation to investigate local properties like density, mean velocity, velocity dispersion, anisotropy, orientation and shape of the velocity dispersion ellipsoid, as well as the structure in velocity space of dark matter haloes. We show that at the same radial distance from the halo centre, these properties can deviate by orders of magnitude from the canonical, spherically averaged values, a variation that can only be partly explained by triaxiality and the presence of subhaloes. The mass density appears smooth in the central relaxed regions but spans four orders of magnitude in the outskirts, both because of the presence of subhaloes as well as of underdense regions and holes in the matter distribution. In the inner regions, the local velocity dispersion ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in the outer parts where the orientation becomes more isotropic. The clumpy structure in local velocity space of the outer halo cannot be well described by a smooth multivariate normal distribution. Via Lactea II also shows the presence of cold streams made visible by their high 6D phase space density. Generally, the structure of dark matter haloes shows a high degree of graininess in phase space that cannot be described by a smooth distribution function.     

Echo mapping of Swift J1753.5−0127

Caustics are a generic feature of the non-linear growth of structure in the dark matter distribution. If the dark matter were absolutely cold, its mass density would diverge at caustics, and the integrated annihilation probability would also diverge for individual particles participating in them. For realistic dark matter candidates, this behaviour is regularized by small but non-zero initial thermal velocities. We present a mathematical treatment of evolution from hot, warm or cold dark matter initial conditions which can be directly implemented in cosmological N -body codes. It allows the identification of caustics and the estimation of their annihilation radiation in fully general simulations of structure formation.     

Nonthermal production of baryon and dark matter

We study nonthermal production of baryon and dark matter. If we extend the MSSM by introducing some singlet chiral superfields so as to enlarge the conserved global symmetry, the abundance of the baryon and the dark matter in the universe may be explained as the charge asymmetry of that symmetry. In such a case, the baryon energy density and the dark matter energy density in the present universe can be correlated each other and take the similar order values naturally.     

Sommerfeld enhancement of invisible dark matter annihilation in galaxies and galaxy clusters

Recent observations indicate that core-like dark matter structures exist in many galaxies, while numerical simulations reveal a singular dark matter density profile at the center. In this article, I show that if the annihilation of dark matter particles gives invisible sterile neutrinos, the Sommerfeld enhancement of the annihilation cross-section can give a sufficiently large annihilation rate to solve the core-cusp problem. The resultant core density, core radius, and their scaling relation generally agree with recent empirical fits from observations. Also, this model predicts that the resultant core-like structures in dwarf galaxies can be easily observed, but not for large normal galaxies and galaxy clusters.     

Consequences of dark matter self-annihilation for galaxy formation

Galaxy formation requires a process that continually heats gas and quenches star formation in order to reproduce the observed shape of the luminosity function of bright galaxies. To accomplish this, current models invoke heating from supernovae, and energy injection from active galactic nuclei. However, observations of radio-loud active galactic nuclei suggest that their feedback are likely not to be as efficient as required, signaling the need for additional heating processes. We propose the self-annihilation of weakly interacting massive particles that constitute dark matter as a steady source of heating. In this paper, we explore the circumstances under which this process may provide the required energy input. To do so, dark matter annihilations are incorporated into a galaxy formation model within the Millennium cosmological simulation. Energy input from self-annihilation can compensate for all the required gas cooling and reproduce the observed galaxy luminosity function only for what appear to be extreme values of the relevant key parameters. The key parameters are: the slope of the inner density profile of dark matter haloes and the outer spike radius. The inner density profile needs to be steepened to slopes of −1.5 or more and the outer spike radius needs to extend to a few tens of parsecs on galaxy scales and a kpc or so on cluster scales. If neutralinos or any thermal relic Weakly Interacting Massive Particle with s-wave annihilation constitute dark matter, their self-annihilation is inevitable and could provide enough power to modulate galaxy formation. Energy from self-annihilating neutralinos could be yet another piece of the feedback puzzle along with supernovae and active galactic nuclei.     

Searching for neutrinos from WIMP annihilations in the Galactic stellar disc

Weakly interacting massive particles (WIMPs) are a viable candidate for the relic abundance of dark matter (DM) produced in the early universe. So far, WIMPs have eluded direct detection through interactions with baryonic matter. Neutrino emission from accumulated WIMP annihilations in the solar core has been proposed as a signature of DM, but has not yet been detected. These null results may be due to small-scale DM density fluctuations in the halo with the density of our local region being lower than the average  (∼0.3 GeV cm⁻³)  . However, the accumulated neutrino signal from WIMP annihilations in the Galactic stellar disc would be insensitive to local density variations. Inside the disc, DM can be captured by stars causing an enhanced annihilation rate and therefore a potentially higher neutrino flux than what would be observed from elsewhere in the halo. We estimate a neutrino flux from the WIMP annihilations in the stellar disc to be enhanced by more than an order of magnitude compared to the neutrino fluxes from the halo. We offer a conservative estimate for this enhanced flux, based on the WIMP–nucleon cross-sections obtained from direct-detection experiments by assuming a density of  ∼0.3 GeV cm⁻³  for the local DM. We also compare the detectability of these fluxes with a signal of diffuse high-energy neutrinos produced in the Milky Way by the interaction of cosmic rays with the interstellar medium. These comparative signals should be observable by large neutrino detectors.     

Design of the Prototype Readout System of BGO Calorimeter in the Space Dark Matter Detector of Purple Mountain Observatory

A space dark matter detector is proposed by the Key Laboratory of Dark Matter And Space Astronomy of Chinese Academy of Sciences for detecting the high-energy electrons and Gamma particles produced by the annihilation of dark matter in space. The whole detector is mainly composed of the BGO (bismuth germanium oxide) high-energy image calorimeter and scintillation hodoscope. The energy range of the detector will cover the high-energy electrons and Gamma particles of 10 Gev∼10 TeV, in which the energies of high-energy particles are mainly deposited in the BGO calorimeter. For verifying the scheme of the detector, we have designed a prototype readout system for the BGO calorimeter of the space dark matter detector, and made a preliminary test on it.     

Statistical issues in astrophysical searches for particle dark matter

In this review statistical issues appearing in astrophysical searches for particle dark matter, i.e. indirect detection (dark matter annihilating into standard model particles) or direct detection (dark matter particles scattering in deep underground detectors) are discussed. One particular aspect of these searches is the presence of very large uncertainties in nuisance parameters (astrophysical factors) that are degenerate with parameters of interest (mass and annihilation/decay cross sections for the particles). The likelihood approach has become the most powerful tool, offering at least one well motivated method for incorporation of nuisance parameters and increasing the sensitivity of experiments by allowing a combination of targets superior to the more traditional data stacking. Other statistical challenges appearing in astrophysical searches are to large extent similar to any new physics search, for example at colliders, a prime example being the calculation of trial factors. Frequentist methods prevail for hypothesis testing and interval estimation, Bayesian methods are used for assessment of nuisance parameters and parameter estimation in complex parameter spaces. The basic statistical concepts will be exposed, illustrated with concrete examples from experimental searches and caveats will be pointed out.     

Massive particle decay and cold dark matter abundance

The decoupling of a cold relic during a decaying-particle-dominated cosmological evolution is analyzed, the relic density is calculated both numerically and semi-analytically and the results are compared with each other. Using plausible values (from the point of view of supersymmetric models) for the mass and the thermal-averaged cross-section times the velocity of the cold relic, we investigate scenaria of equilibrium or non-equilibrium production. In both cases, acceptable results for the dark matter abundance can be obtained, by constraining the reheat temperature of the decaying particle, its mass and the averaged number of the produced cold relic. The required reheat temperature is in any case lower than about 20 GeV.     

Mass Distribution and Kinematics of the Barred Galaxy NGC 2336

For the barred galaxy NGC 2336, stationary models are constructed which reproduce in a consistent manner the observed NIR surface brightness distribution and the observed HII kinematics in those regions affected by the bar. This procedure can answer the question whether the observed kinematics of the galaxy agree with the model predictions derived from the observed distribution of visible matter (assuming that plausible corrections for the presence of dark matter can be applied). It turns out that the parameter values from the morphological decomposition and those needed to fit the HII rotation curves of NGC 2336 best are in excellent agreement. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detecting WIMPs in the microwave sky

The hierarchical clustering observed in cold dark matter simulations results in highly clumped galactic halos. If the dark matter in our halo is made of weakly interacting massive particles (WIMPs), their annihilation products should be detectable in high density and nearby clumps. We consider WIMPs to be neutralinos and calculate the synchrotron flux from their annihilation products in the presence of the Galactic magnetic field. We derive a self-consistent emission spectrum including pair annihilation, synchrotron self-absorption, and synchrotron self-Compton reactions. The resulting radiation spans microwave frequencies that can be observed over the anisotropies in the cosmic microwave background. These synchrotron sources should be identifiable as WIMP clumps by their spatial structure and their distinctive radio spectrum.