Prospects of WIMP Direct  Dark Matter  Introduction: Basics of Non-Baryonic

Searches for Non-baryonic Dark Matter. Recent Highlights

Particle Dark Matter can be detected indirectly by searching in cosmic ray experiments for particles produced in the WIMP annihilation in the galactic halos, like antiprotons, positrons or photons, or looking in deep underground detectors (like MACRO, Baksan, Kamiokande and Soudan), or in the projected or ongoing underwater neutrino telescopes (like AMANDA, ANTARES, Baikal, or NESTOR) for the neutrinos emerging as final products of WIMPs annihilation in the Sun or the Earth. MACRO in Gran Sasso has searched for such neutrinos (in the GeV-TeV range) by looking for neutrino-induced, upward-going muons coming from the direction of the Sun or the Earth core. The muon flux limit from non-atmospheric origin (in the window) is (from the Earth) and of (from the Sun), after exposures of 1250 m² yr and 380 m² yr respectively. Indirect searches of neutralinos have been carried out also at Baksan (at 850 m.w.e.) with the scintillator telescope. In 10.55 years of data, the upper bound on the muon fluxes are and (from the Earth and the Sun respectively). Predictions for the fluxes of such muons in supersymmetric models range in the case of the Earth from about 10^-14 cm^-2 s^-1 to 10^-17 cm^-2 s^-1 (and somehow higher for the Sun case), and so bounds on the neutralino annihilation rate as a function of the neutralino mass have been obtained and regions of the neutralino parameter space excluded. The above mentioned underwater neutrino telescopes project to enlarge the exposure areas up to 3000 m² or higher and plan to reach the 10^-15 cm^-2 s^-1 flux limit. Surfaces larger than 10⁵ m² would be suitable devices to search for neutralinos in wide zones of their parameter space and so projects for larger underwater neutrino telescopes (1 km³) are being considered. Particle Dark Matter can also be detected through its direct interaction with ordinary baryonic dark matter. WIMPs forming the galactic haloes could interact with the nuclei of a detector producing a measurable recoil. Semiconductor detectors of Ge and Si, and scintillators -both solids (NaI, CaF₂, LiF) and liquids (Xe)-have been used so far. The size of the expected signal and the event rate depend on the halo model and on the type of CDM particle. WIMPs, slow moving ( ) and heavy (10 GeV to 1 TeV) could produce a nuclear recoil of a few keV. In the case of WIMP with spin-independent interaction, the typical coherent rates range from 10 to 10³ events per day per kilogram of detector. Even at this rate, the background (due to microphonics, electronic noise and natural or induced radioactivity) can hide the signal in the low energy region of the spectrum where it is expected, and so the results obtained are constrained by the level of background achieved so far in such region. Heavy Dirac neutrinos have been excluded in that way with Ge detectors. Other heavy WIMPs interacting through axial, spin dependent interactions with detector nuclei having non-zero spin (like NaI scintillators), with expected rates in the range from 0.01 to 1 count per kg and day, have been explored to provide also valuable exclusions. More appealing candidates, like the neutralino, are essentially not yet reachable with these detectors at the present stage of development. The tiny energy delivered and the small rates expected in the WIMP nucleus interaction settle the strategy of the DM direct searches: To use detectors of very low energy threshold and very low background (intrinsic and environmental). On the other hand, only a small fraction of the energy delivered by the WIMP goes to ionization, the main part being released as heat and so, superconductors or thermal detectors should be employed. Such cryogenic detectors will achieve lower energies threshold, almost 100% efficiency and have better energy resolution than the conventional detectors. A further step in the background reduction is the use of mechanisms distinguishing for instance those events due to electron recoils (tracers of the background) from those due to nuclear recoils. Hybrid detectors measuring at the same time the ionization and heat (or the light and heat) produced in the detector, or the use of pulse shape discrimination techniques are preliminary successful attempts in the quest for the background reduction. The non observance, up to now, of a DM signal in direct spectra results in a exclusion plot. Distinctive DM features, not shared by the background or by instrumental artefacts, would be the way to unambiguously identify the DM, like the annual modulation due to seasonal June-December variation in the relative velocity Earth- halo or the forward-backward asymmetry in the direction of the nuclear recoil due to the Earth motion through the halo. Various experimental groups have searched for CDM with Ge detectors: USC/PNL/Zaragoza in Homestake (USA) and Canfranc (Spain) ; USCB/UCB/LBL in Oroville (USA); Caltech/PSI/Neuchatel in Gothard (Switzerland); MPI-Heidelberg/Moscow in Gran Sasso (Italy) and TANDAR /USC/PNL/Zaragoza in Sierra Grande (Argentina), with energy thresholds ranging from 1.6 keV to 12 keV, and backgrounds of 0.2 to 2 c/keV.kg.day. Summing up the results of these experiments, it is concluded that the heavy Dirac neutrino is excluded as a DM component for masses ranging from 9-10 GeV (Canfranc, San Gothard), up to 4 TeV (Homestake, Gran Sasso). In the case of NaI scintillators, the level of background achieved has been steadily approaching and even improving that of the traditionally ultralow background Ge diodes by using selected components of very low radioactivity and incorporating statistical techniques of background discrimination (BPRS Beijing/Paris/Rome/Saclay, UKDMC Imperial College/Oxford/Rutherford and Roma II groups). The exclusion plots obtained by UKDMC and Rome are essentially similar to that obtained with Ge detectors (spin independent interactions case) and are one to two orders of magnitude more stringent than that of the Ge for spin-dependent couplings. It can be concluded, , roughly speaking, that the present experimental lowest bounds on the rate stand about event/kg day (integrated over a  2 keV window around threshold), both with Ge and NaI. Searches for annual modulation signals have been or are being carried out by various groups with NaI and liquid Xe scintillators [Zaragoza (Canfranc), Roma II (Gran Sasso)]. Experiments with large masses of scintillators are in preparation or starting in Boulby, Gran Sasso and Canfranc. As far as the use of cryogenic detectors for dark matter is concerned, two types of devices are being developed: bolometers/thermal detectors and superconducting superheated grains (SSG). Thermal detectors measure the increase of temperature due to the energy deposition of the WIMPs at very low T. On the other hand, the signal produced in the metastable, superheated grains as response to the WIMP interaction is due to the disappearance of the Meissner effect when the heat delivered by the particle energy deposition is able to trigger the superconducting to normal phase transistion of the grains. The first bolometer operating underground (Gran Sasso) was that of the Milan group dedicated to double beta decay searches of Tellurium. Large bolometers of TeO₂ (340 g.) with NTD Ge-sensors, although optimised for decay searches, are also producing data for DM (resolution of 1% at 60 keV and background of c./keV.kg.day at threshold, keV), which have lead to encouraging exclusion plots for spin-independent WIMPs couplings to Tellurium and Oxygen. This group has proved the sensitivity of these bolometers to nuclear recoils. A French Collaboration (EDELWEISS) has performed at Frejus a bolometer experiment with a 24 g. sapphire crystal endowed with an NTD Ge thermistor. Recent improvements of this experiment, have produced backgrounds of 25 c/keV.kg.day at low energy. Small sapphire bolometers of keV are being explored in Canfranc within a French-Spanish Collaboration, ROSEBUD (IAS/IAP/Zaragoza). The Munich/Garching/Oxford Collaboration has developed thermal detectors with superconducting phase transition thermometers (in indium, indium/gold, tungsten) in various absorbers (mainly sapphire Al₂O₃) with the purpose of a DM search experiment (CRESST) in Gran Sasso. This Collaboration got, with a 31 g. sapphire bolometer (at 15 mK), an energy threshold of 0.3 keV and a energy resolution of keV for 1.5 keV X-rays FWHM, which is the best resolution obtained so far per unit of detector mass. CRESST is being currently installed in Gran Sasso for a first series of sapphire experiments with 4x250 g. crystals. Low temperature hybrid devices have been developed by the CfPA/Berkeley/Stanford/Santa Barbara Collaboration with Ge bolometers which also collect electron-holes pairs (CDMS experiment). The proof-of-principle of a 70 g. hybrid Ge detector was successful and good discrimination efficiency between nuclear recoils and Compton background was obtained. Also resolutions of 0.7 keV for heat and of 1.6 keV for ionisation at 60 keV were achieved. Background performances (at shallow depth) were 2 c/keV.kg.day in the relevant low energy region. The experiment is producing its first results of a search for WIMPs at shallow depth, and will be soon moved underground to Soudan (USA) with a set of hybrid detectors of Ge and Si. In much the same way, the French Collaboration EDELWEISS has obtained recently very encouraging results with hybrid Ge detectors. The Bern/PSI/Annecy group (ORPHEUS experiment) and the Lisbon-Zaragoza-Paris Collaboration (SALOPARD experiment) are constructing Superconducting Superheated Grains (SSG) prototype detectors with micrograins of tin for WIMP searches. Other type of Superconducting detectors, like the Superconducting Tunnel Junction (STJ), made very important R+D progresses following the developments of the quasiparticle trapping technique of the Oxford group, but there is not yet any planned STJ dark matter experiment.