Weakly Interacting Massive Particles Detection Research Articles

Expected sensitivity to galactic/solar axions and bosonic super-WIMPs based on the axio-electric effect in liquid xenon dark matter detectors

We present systematic case studies to investigate the sensitivity of axion searches by liquid xenon detectors, using the axio-electric effect (analogue of the photoelectric effect) on xenon atoms. Liquid xenon is widely considered to be one of the best target media for detection of WIMPs (Weakly Interacting Massive Particles which may form the galactic dark matter) using nuclear recoils. Since these detectors also provide an extremely low radioactivity environment for electron recoils, very weakly-interacting low-mass particles (< 100keV/c2), such as the hypothetical axion, could be detected as well – in this case using the axio-electric effect. Future ton-scale liquid Xe detectors will be limited in sensitivity only by irreducible neutrino background (pp-chain solar neutrino and the double beta decay of 136Xe) in the mass range between 1 and 100keV/c2. Assuming one ton-year of exposure, galactic axions (as non-relativistic dark matter) could be detected if the axio-electric coupling gAe is greater than 10−14 at 1keV/c2 axion mass (or 10−13 at 100keV/c2). Below a few keV/c2, and independent of the mass, a solar axion search would be sensitive to a coupling gAe∼10-12. This limit will set a stringent upper bound on axion mass for the DFSZ and KSVZ models for the mass ranges mA<0.1 eV/c2 and <10eV/c2, respectively. Vector-boson dark matter could also be detected for a coupling constant α′/α>10-33 (for mass 1keV/c2) or >10-27 (for mass 100keV/c2).

Gamma rays from warm WIMP dark matter annihilation

The weakly interacting massive particle (WIMP) often serves as a candidate for the cold dark matter, however when produced non-thermally it could behave like warm dark matter. In this paper we study the properties of the $\gamma$-ray emission from annihilation of WIMP dark matter in the halo of our own Milky-Way Galaxy with high resolution $N$-body simulations of a Milky-Way like dark matter halo, assuming different nature of WIMPs. Due to the large free-streaming length in the scenario of warm WIMPs, the substructure contend of the dark matter halo is significantly different from that of the cold WIMP counterpart, resulting in distinct predictions of the $\gamma$-ray signals from the dark matter annihilation. We illustrate these by comparing the predicted $\gamma$-ray signals from the warm WIMP annihilation to that of cold WIMPs. Pronounced differences from the subhalo skymap and statistical properties between two WIMP models are demonstrated. Due to the potentially enhanced cross section of the non-thermal production mechanism in warm WIMP scenario, the Galactic center might be prior for the indirect detection of warm WIMPs to dwarf galaxies, which might be different from the cold dark matter scenario. As a specific example we consider the non-thermally produced neutralino of supersymmetric model and discuss the detectability of warm WIMPs with Fermi $\gamma$-ray telescope.

Background rejection in the DMTPC dark matter search using charge signals

The Dark Matter Time Projection Chamber (DMTPC) collaboration is developing a low pressure gas TPC for detecting Weakly Interacting Massive Particle (WIMP)-nucleon interactions. Optical readout with CCD cameras allows for the detection of the daily modulation of the direction of the dark matter wind. In order to reach sensitivities required for WIMP detection, the detector needs to minimize backgrounds from electron recoils. This paper demonstrates that a simplified CCD analysis achieves 7.3×10−5 rejection of electron recoils while a charge analysis yields an electron rejection factor of 3.3×10−4 for events with 241Am-equivalent ionization energy loss between 40keV and 200keV. A combined charge and CCD analysis yields a background-limited upper limit of 1.1×10−5 (90% confidence level) for the rejection of γ and electron events. Backgrounds from alpha decays from the field cage are eliminated by introducing a veto electrode that surrounds the sensitive region in the TPC. CCD-specific backgrounds are reduced more than two orders of magnitude when requiring a coincidence with the charge readout.

Interactions of exotic particles with ordinary matter

Weakly interacting massive particles (WIMPs) and strangelets are two classes of “exotic” particles not yet discovered, and in agreement with theoretical scenarios most probably produced in different early stages of evolution of the Universe. Some peculiarities of their energy loss in the electronic and nuclear interactions with ordinary matter are investigated. For the direct detection of WIMPs the signals produced by the stopping of recoils in matter are used for their identification. The influence of the orientation of the recoil in respect to crystal axes for crystalline silicon (as material for detectors) is analysed as average quantities: energy loss, and as transient thermal effects. For strangelets, the mechanisms of picking-up neutrons during their penetration into matter and the effects on electronic and nuclear stopping are considered. The clarification of the aspects related to the stopping of these hypothetical particles in matter will permit a better interpretation of some experimental results and could also contribute to the search for new techniques or materials for their detection, if they exist.

Evaluation of the neutron background in a direct WIMP detector with neutron vetosystem based on Gd-doped liquid scintillator

A direct WIMP (Weakly Interacting Massive Particle) detector with aneutron veto system is designed to better reject neutrons. Twoexperimental configurations are studied in the present paper: one isfor an Xe detector enclosed in a Gd-loaded scintillator and theother one is for an Xe detector placed inside a reactor neutrinodetector. The Gd-doped liquid scintillator (or the neutrinodetector) is used as a neutron veto device. The neutron backgroundsfor the two experimental designs have been estimated using Geant4simulations. The results show that the neutron backgrounds candecrease to O(0.1) events per year per tonne of liquid Xenon. Wecalculate the sensitivities to spin-independent WIMP-nucleon elasticscattering. An exposure of one tonne × year could reach across-section of about 6 × 10−11 pb.

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.

Monitoring of the thermal neutron flux in the EDELWEISS II dark matter direct search experiment

The results from measurements of thermal neutron flux in the EDELWEISS II experiment aimed at the direct detection of WIMPs (weakly interacting massive particles) by means of cryogenic germanium bolometers are described. Detailed knowledge of the neutron background is of crucial importance for the experiment, since neutrons with the MeV energy range of scattering seem to be hard to distinguish from the expected WIMP signal within the bolometers. Monitoring of the thermal neutron flux is performed using a mobile detection system with a low background proportional 3He counter. The neutron flux measurements were performed both outside and inside the device’s shielding, in the direct proximity of a cryostat with built-in germanium detectors. The sensitivity of the created thermal neutron detection system is on the level of 10−9 neutron (cm2 s)−1.

Direct dark matter search with CRESST and EURECA

The current status of the direct Dark Matter experiments CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) and the planned EURECA (European Underground Rare Event Calorimeter Array) is presented. Both experiments are aimed at the direct detection of WIMPs (Weakly Interacting Massive Particles), potential candidates for the Dark Matter in the universe. New design developments of the cryogenic detectors operated at mK temperatures are investigated to optimize detector performance and to simplify mass production. Thus, CRESST is also providing a basis for the EURECA project, aimed at a ton of cryogenic detectors with a multi-material target.

Cryogenic composite detectors for the dark matter experiments CRESST and EURECA

Weakly interacting massive particles (WIMPs) are candidates for non-baryonic dark matter. WIMPs are supposed to interact with baryonic matter via scattering off nuclei producing a nuclear recoil with energies up to a few 10 keV with a very low interaction rate of ∼10 −6 events per kg of target material and day in the energy region of interest. The dark matter experiment cryogenic rare event search with superconducting thermometers (CRESST) and the European underground rare event calorimeter array (EURECA) project are aimed at the direct detection of WIMPs with the help of very sensitive modularised cryogenic detectors that basically consist of a transition edge sensor (TES) in combination with a massive absorber crystal. In the CRESST experiment the search for coherent WIMP-nucleon scattering events is validated by the detection of two processes. In the scintillating absorber single crystal, CaWO 4, heat (phonons) and scintillation light are produced and detected with two independent cryogenic detectors: a phonon channel and a separate light channel. The development of such cryogenic detectors and the potential ton-scale production are investigated in this paper. To decouple the TES production from the choice of the target material in order to avoid heating cycles of the absorber crystal and to allow pretesting of the TESs, a composite detector design (CDD) for the detector production has been developed and studied. An existing thermal detector model has been extended to the CDD, in order to investigate, understand, and optimize the performance of composite detectors. This extended model, which has been worked out in detail, can be expected to provide a considerable help when tailoring composite detectors to the requirements of various experiments.

DAMA and WIMP dark matter

We study whether spin-independent scattering of weakly-interacting massive particles (WIMPs) with nuclei can account for the annual modulation signal reported by DAMA. We consider both elastic and inelastic scattering processes. We find that there is a region of WIMP parameter space which can simultaneously accommodate DAMA and the null results of CDMS, CRESST, and XENON. This region corresponds to an ordinary, elastically-scattering WIMP with a standard Maxwell-Boltzmann distribution, a mass 3mDM8, and a spin-independent cross section with nucleons 3 × 10−412σpSI5 × 10−392. This new region of parameter space depends crucially on the effect of channeling on the energy threshold for WIMP detection in the DAMA experiment; without the inclusion of this effect, the DAMA allowed region is essentially closed by null experiments. Such low-mass WIMPs arise in many theories of Beyond the Standard Model physics, from minimal extensions of the MSSM to solutions of the baryon-dark matter coincidence problem. We find that inelastic scattering channels do not open up a significant parameter region consistent with all experimental results. Future experiments with low energy thresholds for detecting nuclear recoils, such as CDMSII-Si and those utilizing ultra-low energy germanium detectors, will be able to probe the DAMA region of parameter space.

The ArDM, a ton-scale liquid argon experiment for direct dark matter detection

The ArDM project aims at developing and operating large noble liquid detectors to search for direct evidence of Weakly Interacting Massive Particles (WIMP) as Dark Matter in the Universe. The initial goal is to design, assemble and operate an approximately 1 ton liquid argon prototype based on the double-phase detection principle to demonstrate the feasibility of a ton-scale experiment with the required performance to efficiently detect and sufficiently discriminate backgrounds for a successful WIMP detection. The detector will independently measure the primary scintillation light and the ionization charge. This paper will mainly describe the concept, R&D results and status of the charge read out system.

Identification of Weakly Interacting Massive Particles Through a Combined Measurement of Axial and Scalar Couplings

We study the prospects for detecting weakly interacting massive particles (WIMPs) in a number of phenomenological scenarios, with a detector composed of a target simultaneously sensitive to both spin-dependent and spin-independent couplings, as is the case of COUPP (Chicagoland Observatory for Underground Particle Physics). First, we show that sensitivity to both couplings optimizes chances of initial WIMP detection. Second, we demonstrate that, in case of detection, a comparison of the signal on two complementary targets, such as in COUPP CF3I and C4F10 bubble chambers, allows a significantly more precise determination of the dark matter axial and scalar couplings. This strategy would provide crucial information on the nature of the WIMPs and possibly allow discrimination between neutralino and Kaluza-Klein dark matter.

Direct dark matter searches with CDMS and XENON

The cryogenic dark matter search (CDMS) and XENON experiments aim to directly detect dark matter in the form of weakly interacting massive particles (WIMPs) via their elastic scattering on the target nuclei. The experiments use different techniques to suppress background event rates to the minimum, and at the same time, to achieve a high WIMP detection rate. The operation of cryogenic Ge and Si crystals of the CDMS-II experiment in the Soudan mine reported spectrum-weighted exposures of 34 (12) kg-d for the Ge (Si) targets after cuts, over the recoil energies 10–100 keV for a WIMP mass of 60 GeV/ c 2. It gives an upper limit (90% C.L.) of spin-independent WIMP-nucleon cross-section at 1.6 × 10 −43 cm 2 for a WIMP mass of 60 GeV/ c 2, starting to constrain predications in supersymmetry models. The two-phase xenon detector of the XENON10 experiment is currently taking data in the Gran Sasso underground lab and promising preliminary results were recently reported. Both experiments are expected to increase their WIMP sensitivity by a one order of magnitude in the scheduled science runs for 2007.

Search for Weakly Interacting Massive Particles with CDMS and XENON

The Cryogenic Dark Matter Search (CDMS) and XENON experiments aim to directly detect dark matter in the form of weakly interacting massive particles (WIMPs) via their elastic scattering on the target nuclei. The experiments use different techniques to suppress background event rates to the minimum, and at the same time, to achieve a high WIMP detection rate. The operation of cryogenic Ge and Si crystals of the CDMS-II experiment in the Soudan mine has yielded the most stringent spin-independent WIMP-nucleon crosssection (∼ 10-43 cm2) at a WIMP mass of 60 GeV/c2. The two-phase xenon detector of the XENON10 experiment is currently taking data in the Gran Sasso underground lab and promising preliminary results were recently reported. Both experiments are expected to increase their WIMP sensitivity by a one order of magnitude in the scheduled science runs for 2007.

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.

ArDM: a ton-scale liquid Argon experiment for direct detection of Dark Matter in the Universe

The ArDM project aims at developing and operating large noble liquid detectors to search for direct evidence of Weakly Interacting Massive Particle (WIMP) as Dark Matter in the Universe. The initial goal is to design, assemble and operate a ≈1 ton liquid Argon prototype to demonstrate the feasibility of a ton-scale experiment with the required performance to efficiently detect and sufficiently discriminate backgrounds for a successful WIMP detection. Our design addresses the possibility to detect independently ionization and scintillation signals. In this paper, we describe this goal and the conceptual design of the detector.

MOON for double beta decays and X-rays from WIMP nuclear interactions

Neutrino-less double beta decays ( 0 ν β β ) and direct observation of cold dark matter (DM) are of great interest for studying fundamental properties of neutrinos and weakly interacting massive particles (WIMPs). These are crucial for particle physics and cosmology beyond the standard electro-weak theory. The present seminar in the Erice neutrino school reports briefly (1) the effective neutrino mass studied by 0 ν β β , (2) the unique features and the present status of MOON (Molybdenum Observatory Of Neutrinos) for high-sensitivity 0 ν β β studies with 100Mo in the quasi-degenerate and inverted hierarchy regions, and (3) the direct detection of WIMPs by measuring atomic X-rays following inner-shell ionization by WIMPs nuclear interactions. The MOON project is carried out in collaboration with the MOON collaboration, and the X-ray work from WIMPs is done with Ch.C. Moustakidis and J.D. Vergados.

The dark side of the universe

I will begin by reviewing the evidence for dark matter in the Universe, as well as the candidates for dark matter. At most 20% of the dark matter in galaxies can be in the form of MACHOs (Massive Compact Halo Objects); the remainder appears to be some unknown exotic component. The most sensible candidates from the point of view of particle physics are axions and WIMPs (Weakly Interacting Massive Particles), where WIMPs may be supersymmetric particles. Three recent claims of possible detection of WIMP dark matter are tantalizing and will be discussed: the DAMA annual modulation, the HEAT positron excess, and gamma-rays from the Galactic Center. In addition, I will discuss the dependence of signals in detectors on the mass distribution in the Galactic Halo. In particular, the Sagittarius stream can be a smoking gun for WIMP detection.

Indirect detection of WIMPs

There is compelling evidence for the existence of dark matter in the Universe. One of the favourite candidates is a Weakly Interacting Massive Particle (WIMP). We will here focus on indirect ways to search for WIMPs and compare their advantages and disadvantages, the emphasis though will be on searches via neutrinos from the Earth/Sun. As a concrete WIMP example, we will focus on the neutralino that arises in supersymmetric extensions of the standard model.

CRESST-II: dark matter search with scintillating absorbers

In the CRESST-II experiment, scintillating CaWO4 crystals are used as absorbers for direct WIMP (weakly interacting massive particles) detection. Nuclear recoils can be discriminated against electron recoils by measuring phonons and scintillation light simultaneously. The absorber crystal and the silicon light detector are read out by tungsten superconducting phase transition thermometers (W-SPTs). Results on the sensitivity of the phonon and the light channel, radiopurity, the scintillation properties of CaWO4, and on the WIMP sensitivity are presented.