WIMP Direct Detection Research Articles

Direct WIMP detection with cryogenic detectors.

Cryogenic detectors have been developed since the mid 1980s and have been applied successfully to dark-matter searches since the mid 1990s. Among the advantages of cryogenic detectors are their high sensitivity to nuclear recoil, their low detection thresholds, the wide choice of target materials and the possibility of implementing event type recognition on an event-by-event basis. I explain the basics of cryogenic detectors, review various implementations, discuss advantages and drawbacks and give an overview of current dark-matter-search experiments based upon cryogenic detectors.

Light relic neutralinos

The relic abundance and the scalar cross-section off nucleon for light neutralinos (of mass below about 45 GeV) are evaluated in an effective MSSM model with R-parity conservation and without GUT-inspired relations among gaugino masses. It is shown that these neutralinos may provide a sizeable contribution to the matter density in the Universe and produce measurable effects in WIMP direct detection experiments. A lower bound on the neutralino mass m x ≳ 6 GeV, based on the new WMAP data on Ω CDM, is worked out

Direct detection of WIMPs with conventional (non-cryogenic) detectors. Experimental review

An overview of the current status of WIMP direct searches with conventional detectors is presented, emphasizing strategies, achievements and prospects.

Study of the internal background of CsI(Tℓ) crystal detectors for dark matter search

A search for particle cold dark matter with CsI(Tℓ) crystal is being prepared at the Cheong-Pyeong underground laboratory in Korea. The background spectra of CsI(Tℓ) crystal detectors in a prototype shield were obtained. The lowest background count rate of the test sample of crystals is measured to be 64.7±5.1 counts /keV/kg/day in the energy range of 5– 20 keV . Quantitative estimation of residual radioactive isotope in CsI(Tℓ) was made using the GEANT4 Monte Carlo simulation. Analysis results show that CsI(Tℓ) crystal could be a good candidate for direct detection of WIMPs when the contamination level of cesium radioisotopes is reduced to under a few mBq/kg.

Modulation analysis methods for WIMP direct detection: merits and restrictions

The recent trend of proposing dark matter direct detection experiments using detector masses of the order of one ton initiated this study about its merits and restrictions for WIMP signature detection. It is shown that measurements of the modulation signature would become a sensitive method to study WIMP halo properties using detector masses of the order of one ton in case the DAMA evidence is confirmed. On the other hand, as soon as experiments improve limits by roughly one order of magnitude one will have to test signatures different from the WIMP modulation signature. This paper discusses data analysis techniques to reveal the modulation signature for WIMP direct detection experiments. A periodogram approach, adopted from astronomy, least squares and maximum likelihood techniques are applied to the problem of detecting a weak periodic signal in unevenly sampled noisy data. The systematics of WIMP modulation analysis is discussed and a comparison of evidence regions to limits for selected target materials is carried out.

Effect of halo modeling on weakly interacting massive particle exclusion limits

WIMP direct detection experiments are just reaching the sensitivity required to detect galactic dark matter in the form of neutralinos. Data from these experiments are usually analyzed under the simplifying assumption that the Milky Way halo is an isothermal sphere with Maxwellian velocity distribution. Observations and numerical simulations indicate that galaxy halos are in fact triaxial and anisotropic. Furthermore, in the cold dark matter paradigm galactic halos form via the merger of smaller subhalos, and at least some residual substructure survives. We examine the effect of halo modeling on WIMP exclusion limits, taking into account the detector response. Triaxial and anisotropic halo models, with parameters motivated by observations and numerical simulations, lead to significant changes which are different for different experiments, while if the local WIMP distribution is dominated by small scale clumps then the exclusion limits are changed dramatically.

Dark Matter search

Some general arguments on the particle Dark Matter search are addressed. The WIMP direct detection technique is mainly considered and recent results obtained by exploiting the annual modulation signature are summarized.

Background discrimination capabilities of a heat and ionization germanium cryogenic detector

The discrimination capabilities of a 70 g heat and ionization Ge bolometer are studied. This first prototype has been used by the EDELWEISS dark matter experiment, installed in the Laboratoire Souterrain de Modane, for direct detection of WIMPs. Gamma and neutron calibrations demonstrate that this type of detector is able to reject more than 99.6% of the background while retaining 95% of the signal, provided that the background events distribution is not biased towards the surface of the Ge crystal. However, the 1.17 kg day of data taken in a relatively important radioactive environment show an extra population slightly overlapping the signal. This background is likely due to interactions of low energy photons or electrons near the surface of the crystal, and is somewhat reduced by applying a higher charge-collecting inverse bias voltage (−6 V instead of −2 V) to the Ge diode. Despite this contamination, more than 98% of the background can be rejected while retaining 50% of the signal. This yields a conservative upper limit of 0.7 event day −1 kg −1 keV recoil −1 at 90% confidence level in the 15–45 keV recoil energy interval; the present sensitivity appears to be limited by the fast ambient neutrons. Upgrades in progress on the installation are summarized.

Relic neutralinos — update on neutralino-nucleon scalar cross-section

We discuss the effect induced on the neutralino-nucleon scalar cross-section by the present uncertainties in the values of the quark masses and of the quark scalar densities in the nucleon. We examine the implications of this aspect on the determination of the neutralino cosmological properties, as derived from measurements of WIMP direct detection. We show that, within current theoretical uncertainties, the DAMA annual modulation data are compatible with a neutralino as a major dark matter component, to an extent which is even larger than the one previously derived.

Implications for relic neutralinos of the theoretical uncertainties in the neutralino–nucleon cross section

We discuss the effect induced on the neutralino–nucleon cross section by the present uncertainties in the values of the quark masses and of the quark scalar densities in the nucleon. We examine the implications of this aspect on the determination of the neutralino cosmological properties, as derived from measurements of WIMP direct detection. We show that, within current theoretical uncertainties, the DAMA annual modulation data are compatible with a neutralino as a major dark matter component, to an extent which is even larger than the one previously derived. We also comment on implications of the mentioned uncertainties for experiments of indirect dark matter detection.

Extending the DAMA annual-modulation region by inclusion of the uncertainties in astrophysical velocities

The original annual-modulation region, singled out by the DAMA/NaI experiment for direct detection of WIMPs, is extended by taking into account the uncertainties in the galactic astrophysical velocities. Also the effect due to a possible bulk rotation for the dark matter halo is considered. We find that the range for the WIMP mass becomes 30 GeV < m_chi < 130 GeV at 1-sigma C.L. with a further extension in the upper bound, when a possible bulk rotation of the dark matter halo is taken into account. We show that the DAMA results, when interpreted in the framework of the Minimal Supersymmetric extension of the Standard Model, are consistent with a relic neutralino as a dominant component of cold dark matter (on the average in our universe and in our galactic halo). It is also discussed the discovery potential for the relevant supersymmetric configurations at accelerators of present generation.

Calibration of a CsI(Tl) crystal with nuclear recoils and pulse shape measurements for dark matter detection

The quenching factor of cesium and iodine nuclei recoiling in a CsI(Tl) scintillator is measured by scattering of 3 to 6 MeV neutrons. This factor increases when recoil energy decreases, from 7% at 150 keV to 15% at 25 keV. This relatively high efficiency could be useful in experiments dealing with very low recoil energies like the WIMP direct detection. These values are well explained by the Birks model. Pulse shape discrimination between electron and nuclei recoils is also investigated. Results are sufficiently good to allow a significant statistical rejection of radioactive background. This rejection capability is shown to be better than for NaI(Tl), at the same electron equivalent energy.

Neutralino properties in the light of a further indication of an annual modulation effect in WIMP direct search

We demonstrate that the further indication of a possible annual modulation effect, singled out by the DAMA/NaI experiment for WIMP direct detection is widely compatible with an interpretation in terms of a relic neutralino as the major component of dark matter in the Universe. We discuss the supersymmetric features of this neutralino in the minimal supersymmetric extension of the standard model and their implications for searches at accelerators.

Compatibility of the new DAMA/NaI data on an annual modulation effect in WIMP direct search with a relic neutralino in supergravity schemes

Recent results of the DAMA/NaI experiment for WIMP direct detection point to a possible annual modulation effect in the detection rate. We show that these results, when interpreted in terms of a relic neutralino, are compatible with supergravity models. Together with the universal SUGRA scheme, we also consider SUGRA models where the unification condition in the Higgs mass parameters at GUT scale is relaxed.

Combining the data of annual modulation effect in WIMP direct detection with measurements of WIMP indirect searches

In previous papers we showed that the data of the DAMA/NaI experiment for WIMP direct detection, which indicate a possible annual modulation effect, are widely compatible with an interpretation in terms of a relic neutralino as the major component of dark matter in the Universe. In the present note we discuss the detectability of the relevant supersymmetric neutralino configurations by two of the most promising methods of indirect search for relic particles: measurement of cosmic-ray antiprotons and measurement of neutrino fluxes from Earth and Sun.

Direct detection of WIMPs with the HDMS-experiment and new WIMP-limits from the Heidelberg–Moscow experiment

The Heidelberg-Dark-Matter-Search (HDMS) experiment consists of two HPGe-detectors which operate in a unique configuration. The anticoincidence between the detectors will act as an effective background reduction method allowing to reach a sensitivity comparable to those of planned cryogenic experiments. The experiment is situated in the Gran Sasso Underground Laboratory and starts to take data now. New upper limits on WIMP-nucleon cross sections are obtained after 0.53 kg yr of measurement with one of the enriched 76Ge detectors of the Heidelberg–Moscow experiment. An energy threshold of 8.8 keV and a background level of 0.05 events/kg yr keV in the energy region between 8.8 and 100 keV was reached. The derived limits are at present the most stringent ones for spin independent interaction obtained by using only raw data without background subtraction.

GENIUS: a new dark matter project

GErmanium in liquid NItrogen Underground Setup (GENIUS) is a proposal for a high mass germanium experiment with a much larger sensitivity for direct WIMP detection relative to existing experiments. It would operate 1 ton of “naked” Ge-detectors in a liquid nitrogen shielding of very low level radioactivity. Already in a first 100 kg of natural Ge version, GENIUS would be able to reach a sensitivity of the order 0.01 counts/d kg and additionally to look for the annual modulation WIMP-signature. Operating 1 t of enriched 76Ge-detectors, GENIUS could search for the neutrinoless double beta decay, probing neutrino masses down to 0.02 eV.

Effects of galactic dark halo rotation on WIMP direct detection

The effects of a possible rotation of the galactic dark halo on the calculation of the direct detection rates for particle dark matter are analyzed, with special attention to the extraction of the upper limits on the WIMP-nucleon scalar cross section from the experimental data. We employ a model of dark halo rotation which describes the maximal possible effects. For WIMP masses above 50 GeV, the upper limit exclusion plot is modified by less than a factor of two when rotation is included. For lighter masses the effect can be stronger, suggesting the necessity to develop specific models of halo rotation in order to provide more accurate conclusions.

Limitations of modulation analysis for dark matter direct detection experiments

We examine the limitations of the data analysis scheme for annual modulations as a signature of WIMP direct detection. The consequences for existing and planned experiments are pointed out. These include determinations of optimum recoil energy regions to be analyzed for modulations and the minimum target mass needed to detect them. Calculations for 23Na, nat. Ge, and 129Xe as example nuclei have been performed.