The Identification of Dark Matter

The following sections are included:

• Preface

• Contents

Over the past three years, our confidence in the inferred values of cosmological parameters has increased dramatically, confirming that the flat matter dominated Universe that dominated cosmological model building for the past 20 years does not correspond to the Universe in which we live. I review recent developments here and quote best fit current values for fundamental cosmological parameters.

Some of the arguments which support the strong concensus for an Ω_o = 0.3, λ_o = 0.7 model are reexamined. Corrections for Malmquist bias, local flow and metallicity suggest a revised value for H_o of 63 ± 6 km/s/Mpc, improving the age problems for an Ω_o = 1 universe. The latest CMB results may require a high baryon density and hence new physics, for example a strong lepton asymmetry. Difficulties for the Ω_o = 1 model with cluster evolution, the baryon content of clusters, and the evidence from Type Ia supernovae favouring low Ω_o, Λ > 0 models, are discussed critically.

We review the use of Type Ia supernovae for cosmological distance determinations. Low-redshift SNe Ia (z ≲ 0.1) demonstrate that the Hubble expansion is linear, that H_o = 65 ± 2 (statistical) km S^-1 Mpc^-1, and that the properties of dust in other galaxies are similar to those of dust in the Milky Way. We find that the light curves of high-redshift (z = 0.3-1) SNe Ia are stretched in a manner consistent with the expansion of space; similarly, their spectra exhibit slower temporal evolution (by a factor of 1 + z) than those of nearby SNe Ia. The luminosity distances of our first set of 16 high-redshift SNe Ia are, on average, 10–15% farther than expected in a low massdensity (Ω_M = 0.2) universe without a cosmological constant. Preliminary analysis of our second set of 9 SNe Ia is consistent with this. Our work supports models with positive cosmological constant and a current acceleration of the expansion. We address the main potential sources of systematic error; at present, none of them appears to reconcile the data with Ω_Λ = 0 and q₀ ≥ 0. The dynamical age of the Universe is estimated to be 14.2 ± 1.7 Cyr, consistent with the ages of globular star clusters.

We discuss the meaning of a time dependent "cosmological constant" and give a set of conditions to recover asymptotic de Sitter behaviour for a class of cosmological models in the framework of extended gravity theories. To this purp0tie we introduce a time–dependent (effective) quantity which asymptotically becomes the true cosmological constant. We deal with scalar–tensor theories. Furthermore the existence of the cosmological constant can be connected to a nonminimal derivative coupling, in the action of gravity, between the geometry and the kinetic part of a given scalar field without introducing any effective potential of scalar fields.

The mechanisms leading to anisotropies in the CMB are reviewed in the context of CDM models, and in relation to large-scale structure and the latest anisotropy data. The CMB has a geometrical degeneracy in the parameters that can be estimated from the power spectrum, but this is largely broken if the Hubble constant is known. Models with purely scalar fluctuations fit the data quite well, and are consistent with large-scale structure, although it is necessary to assume a higher baryon density than required by nucleosynthesis. Adding a significant tensor component does not eliminate this inconsistency.

Clusters of galaxies are strong emitters of EUV and soft X-rays substantially in excess of the contribution from the hot intra–cluster medium at X–ray temperatures. After the initial discovery of EUV excess emission in the Virgo cluster, the phenomenon was reported also for the Coma, A1795 and A2199 clusters. Recent reobservations confirmed the finding, and further searches on a large sample of nearby galaxy clusters are clearly revealing the cosmological impact of the discovery.

Unhappily, there has been a maelstrom of problems for dark matter theories over the last few years and many serious difficulties still have no resolution in sight. This article reviews the evidence for dark matter in galaxies. Judged on the data from galactic scales alone, the case for dark matter is weak. Non-Newtonian theories of gravity have their own problems, but not on galactic scales.

The hierarchical cold dark matter (CDM) model for structure formation is a well defined and testable model. Direct detection is the best technique for confirming the model yet predictions for the energy and density distribution of particles on earth remain inadequate. Axially symmetric collapse of collisionless dark matter can leave observable caustic rings in phase space and this model is frequently used to make experimental predictions (Sikivie 1999). Such cold collapses inevitably suffer from radial orbit instabilities that produce unrealistic bar-like halos. Moreover, this model bears no relation to the hierarchical formation of CDM galactic halos via mergers and subsequent violent relaxation. In this case symmetry is broken by large scale tidal fields and caustics are phase wrapped on a scale set by the first objects to collapse. Axions can cluster on microscopic scales whereas free streaming of neutralinos erases structure smaller than ~ 100(GeV/m_CDM) A.U., therefore the dynamical effects of caustics in the CDM model are negligible.

The upcoming ambitious projects aiming at finding Type Ia supernovae at high redshifts in order to measure cosmological parameters may also give important information on the distribution of matter in the universe. In particular, the distribution of observed luminosities for a sample of supernovae in a given redshift range may be used to constrain the fraction of the matter density of the universe which is in the form of point mass objects. Here we outline a method to simulate these luminosity distributions using numerical ray-tracing, and give some first results.

The late infall of cold dark matter onto an isolated galaxy such as our own produces flows with definite velocity vectors at any physical point in the galactic halo. It also produces caustics which are places where the dark matter density is very large. The outer caustics are topological spheres whereas the inner caustics are rings. The self-similar model of galactic halo formation predicts that the caustic ring radii a_n follow the approximate law a_n ~ 1/n. In a recent study of 32 extended and well-measured galactic rotation curves, we found evidence for this law.

We have studied the effects on the gamma-ray flux on Earth by neutralino annihilations in caustic rings of dark matter in the Galactic halo. The caustic ring model by Sikivie has been used where dark matter particles are assumed to possess angular momentum with respect to the Galactic centre. The computer code DarkSUSY was then used to examine the supersymmetric implications on the flux. We conclude that a small signal might be detectable under very optimistic assumptions, but under these only.

We generalize the spherical collapse model for the formation of dark matter halos to apply in a universe with arbitrary positive cosmological constant. We calculate the critical condition for collapse of an overdense region and give exact values of the characteristic densities and redshifts of its evolution.

In the past, theoretical predictions of the signal that dark matter detectors will observe have been based on the assumption of a smooth, featureless dark matter distribution. However, this assumption is strongly violated in the hierarchical clustering scenario where halos form in a random series of mergers. We look at the direct detection signatures expected by WIMP and axion detectors in a hierarchically-formed dark matter halo similar to that expected of the Milky Way. We find that the local solar neighbourhood is populated by numerous low density streams of dark matter which can have a significant impact on terrestrial dark matter detection experiments. These streams leave distinct signatures in the detection spectra which can be used to reconstruct the dynamics of the stream and test our understanding of hierarchical formation models.

Supersymmetric extensions of the Standard Model which incorporate the axion solution to the strong CP problem necessarily contain also the axino, the fermionic partner of the axion. In contrast to the neutralino and the gravitino, the axino mass is generically not of the order of the supersymmetry breaking scale and can be much smaller. The axino is therefore an intriguing candidate for a stable superpartner. It has been recently shown that axinos are a natural candidate for cold dark matter in the Universe when they are generated non-thermally through out-of-equilibrium neutralino decays. Here, we extend the study of non-thermal production and include a competing thermal production mechanism through scatterings and decays of particles in the plasma. We identify axino masses in the range of tens of MeV to several GeV (depending on a scenario) as corresponding to cold axino relics if the reheating temperature T_R is less than about 5 × 10⁴ GeV. At higher T_R and lower mass, axinos could constitute warm dark matter. In the scenario with axinos as relics the gravitino problem finds a natural solution. The lightest superpartner of the Standard Model spectrum will remain stable in high-energy detectors but may be either neutral or charged. The usual constraint Ω_χh² ≲1 on the relic abundance of the lightest neutralino becomes void.

A brief review is given of some of the recent developments in the theoretical analyses of supersymmetric dark matter. These include the effects of uncertainties in the wimp velocity and wimp density and of the effects of uncertainties in the quark densities of the proton. Also analyzed are the effects of non-universalities in the gaugino sector and their effects on determining the nature of cold dark matter, i.e., if the neutralino is bino like, higgsino like, or wino like. The maximum and the minimum elastic neutralino proton cross sections are discussed and a comparison of the direct and the indirect detection arising from the capture and annihilation of neutralinos in the core of the earth and the sun is given. Some of the other recent developments are summarized.

We study precise calculation of neutralino relic density in the minimal supergravity model. We compare the exact formula for a thermal average of the neutralino annihilation cross section times relative velocity with an expansion formula in terms of the temperature, including all the contributions to neutralino annihilation cross section. We confirm that the expansion formula fails badly near s-channel poles. We show that the expansion method causes 5-10 % error even far away from the poles.

We propose that a model for dark matter based on the idea that there is a mirror sector to the universe with identical (but different) force and matter to ours can solve the three outstanding challenges of dark matter physics i.e. (i) why is Ω_DM of the same order as Ω_Baryons ? (ii) what are the near solar mass objects (~ 0.5M⊙) observed by the MACHO microlensingproject ? and (iii) what causes the shallow core density profile of the halos of dwarf as well as low surface brightness galaxies, which are supposed to be dark matter dominated? This model has therefore a certain advantage over the popular cold dark matter candidates such as the SUSY LSP and the axion. It is worth noting that the version of the mirror model under consideration was originally suggested to explain the neutrino anomalies and not to solve the dark matter problems. One of the key ingredients of our proposal is that the mirror baryons weigh 15-20 times the familiar baryons, and the mirror sector weak interactions be 225 to 400 times weaker than known weak processes.

Searches for pair production of sfermions and charginos and associated production of neutralinos have been performed using the data collected by the ALEPH detector at centre-of-mass energies ranging from 188.6 to 201.6 GeV. No evidence for any such signals is observed in a total of about 410 pb^-1 accumulated luminosity. The negative results of such searches are translated into exclusion domains in the space of the relevant Minimal Supersymmetric Model parameters, improving significantly on the previous constraints. Under the assumptions of gaugino and sfermion mass unification, these results allow a 95% confidence level lower limit of 36.8 GeV /c² to be set on the mass of the lightest supersymmetric particle, for any tan β and sfermion mass. Additional constraints on the MSSM parameter space have been derived by including the negative results of ALEPH searches for Higgs bosons. These raise the above limit to 38 GeV /c².

Searches for evidence of Supersymmetry (SUSY) using the D0 detector at the Fermilab Tevatron Collider are summarized in the context of the minimal supergravity scenario. Prospects for searches with the upgraded detector at Run II are discussed in relation to the lightest supersymmetric particle as a candidate for cold dark matter.

Cosmological nucleosynthesis calculations imply that many of the baryons in the Universe must be dark. We discuss the likelihood that some of these dark baryons may reside in galaxies as Massive Compact Halo Objects (MACHOs), the remnants of a first generation of pregalactic or protogalactic stars. Various candidates have been proposed for such remnants and we review the many types of observations which can be used to detect or exclude them. Claims to have found positive evidence for some of the candidates have generally turned out to be spurious or questionable, so the status of the MACHO scenario remains controversial. However, it would be premature to reject MACHOs altogether and further observations are likely to resolve the issue soon.

The nature of the dark matter in the Universe is one of the outstanding questions in astrophysics. In this talk, I address possible stellar baryonic contributions to the 50-90% of our Galaxy that is made of unknown dark matter. First I show that faint stars and brown dwarfs constitute only a few percent of the mass of the Galaxy. Next I show that stellar remnants, including white dwarfs and neutron stars, are also insufficient in abundance to explain all the dark matter of the Galaxy. High energy gamma-rays observed in HEGRA data place the most robust constraints, Ω_WD < 3 × 10^-3 h^-1, where h is the Hubble constant in units of 100 km s^-1 Mpc^-1. Overproduction of chemical abundances (carbon, nitrogen, and helium) provide the most stringent constraints, Ω_WD < 2 × 10^-4 h^-1. Comparison with recent updates of microlensing data are also made. According to the gamma-ray limit, all Massive Compact Halo Objects seen by the experiments (Machos) can be white dwarfs if one takes the extreme numbers; however, from chemical overproduction limits, NOT all Machos can be white dwarfs. Comments on recent observations of the infrared background and of white dwarfs are also made. In conclusion, a nonbaryonic component in the Halo seems to be required.

Brown dwarfs are now being discovered in significant numbers which means that at last it is possible to be quantitative about their number density and thus their contribution to the stellar neighbourhood. Overall the emerging picture is one of rising mass function into the brown dwarf regime though with a total brown dwarf mass contribution of around 10 percent that due to stars. Contrary to expectation, these stars too small to burn nuclear fuel, do not contribute significantly to our Galaxy's missing mass. However, their properties are more complex than expected and thus there are considerable uncertainties to overcome before a robust determination of the brown dwarf mass function is possible.

The timescale of quasar variability is widely expected to show the effects of time dilation. In this paper we analyse the Fourier power spectra of a large sample of quasar light curves to look for such an effect. We find that the timescale of quasar variation does not increase with redshift as required by time dilation, and conclude that the variations cannot be intrinsic to the quasars. The most likely cause of the variability would appear to be the microlensing effect of a large population of compact bodies along the line of sight. These bodies would constitute sufficient mass to account for the dark matter, and are most plausibly primordial black holes formed during the QCD epoch.

The EROS 2 microlensing experiment has completed the analysis of two years of SMC data, and three years of Galactic disk and LMC data. The Galactic disk data is in agreement with models including a bar. The SMC and LMC data strongly limit the amount of compact objects in the Galactic halo. Combining the results from EROS 2 and EROS 1 excludes in particular (at 95 % C.L.) that more than 10 % of the Halo be made of objects with a mass between 10^-6 M_⊙ and 10^-2 M_⊙, and more than 40 % of objects with a mass between 10^-7 M_⊙ and 1M_⊙. Analysis of the spatial distribution of LMC events, and comparison of LMC and SMC events in the near future should help discriminate between halo microlensing and other backgrounds like Magellanic Cloud self-lensing and unexpected variable stars.

The MACHO Collaboration has analyzed 5.7 years of photometry on approximately 11.7 million stars in the Large Magellanic Cloud (LMC) in a search for gravitational microlensing events. Two sets of cuts were used in the event selection process, and 13 and 17 events were found in the different sets. If the lenses are in the Galactic halo, then likelihood analysis for a standard halo model gives a halo mass fraction of about 0.2 with a 95% confidence interval of 0.08–0.5 and lens masses in the range 0.15–0.9M_⊙.

I give a review of the aims and actual state of the data analysis of the AGAPE collaboration concerning the observation of microlensing effects on unresolved stars in the M31 galaxy. On the three years of data taken at the TBL in the Pic du Midi from 1994 to 1996, we have detected 9 candidates that satisfy our selection criterias. If we include the data taken in 1998 at the MDM, in Kitt Peak, Arizona, we keep 2 candidates, with 2 more yet to be tested. The best candidate, unique in its short duration of ~ 5 days and large amplification at maximum m_R = 18, is called Z1 and was fully described in [Ansari et al., 1999]. We are now ready to apply our analysis tools described in [Le Du, 2000) to the data taken at the Isaac Newton Telescope, in the framework of the POINT-AGAPE collaboration, which provides a much larger statistics, cf. [Kerins et al., 2000].

We study in detail the trajectories followed by the images in binary microlensing events. First we give perturbative analytical resolutions of the lens equation in some special cases. Then we study the full non perturbative situation. We see that the images created during the caustic crossing can recombine with the others in many non trivial ways. This leads to a new classification of the microlensing events according to the behaviour of the images. We show that the images involved in these combinations depend on the folds that are crossed at the entry and at the exit from the caustic. Some consequences for the motion of the center of light of the source in astrometric measurements are also examined.

We propose a genuine, 3-dimensional offset between the "bar" and the disc components of the LMC, where the LMC's off-centered "bar" is an unvirialized structure slightly misaligned with, and offset from, the plane of the LMC disk perhaps due to recent tidal interactions of the LMC with the SMC and with the Galaxy (Zhao & Evans 2000). Such a model can account for most of the microlensing events to the LMC, with the lenses being faint stars in the nearer of the two components, be it the "bar" or the disc. This proposal, though radical, is consistent with the kinematics of the LMC and near-infrared star count maps from the DENIS and 2MASS surveys and in particular, the reported 25° –50° inclination range of the LMC and the east-west gradient of distance moduli of standard candles. Our model predicts preferential reddening and extinction of the source stars because the source stars are at the backside of the model LMC. This is a robust and generic signature, the detection of which could rule out competing models using exotic lenses such as blue halo white dwarfs.

POINT-AGAPE is a long-term, multi-colour, wide-field microlensing survey of the Andromeda Galaxy (M31), which commenced observations on the Isaac Newton Telescope (INT) in August 1999. We aim to detect or constrain the nature and abundance of massive compact halo objects (Machos) in both M31 and our own Galaxy, and also probe the structure and stellar mass function of the M31 bulge. I report here on the theoretical expectations for the survey, and on initial results.

The existence of dark matter in the Universe is well established and various techniques are applied to either directly or indirectly detect this dark matter. Supersymmetry provides a well-motivated candidate, the WIMP, which many direct searches seek to detect. Some of the present dark matter experiments use cryogenic detectors, operating in the milli-Kelvin temperature range. This article introduces the general concept underlying these detectors, discusses the different types of detectors in use and shows how some of the experiments achieve suppression of background by being able to distinguish between nuclear recoil and electron recoil.

WIMP direct searches probe regions of the supersymmetric parameter space that may entail relic neutralinos of cosmological interest. This result, which depends on the specific theoretical scenario, is obtained when the main astrophysical and particle physics uncertainties, relevant for a proper comparison between theory and experimental data, are taken into account.

A method is proposed for extracting limits on spin-dependent WIMP-nucleon interaction cross sections from direct detection dark matter experiments. The new method has the advantage that the limits on individual WIMP-proton and WIMP-neutron cross sections for a given WIMP mass can be combined in a simple way to give a model-independent limit on the properties of WIMPs scattering from both protons and neutrons in the target nucleus. Extension of the technique to the case of a target material consisting of several different species of nuclei is discussed.

We investigate the evolution of the global(axionic) string network in the radiation dominated universe by use of numerical simulations in 3+1 dimensions. We find that the global string network settles down to the scaling regime where the energy density of global strings, ρ_s, is given by ρ_s = ξµ/t² with µ the string tension per unit length and the scaling parameter, ξ ~ (1.00 ± 0.08), irrespective of the cosmic time. We also find that the loop distribution function can be fitted with that predicted by the so-called one scale model. Concretely, the number density, n_l(t), of the loop with the length, l, is given by n_l(t) = v/[t^3/2(l + Kt)^5/2] where v ~ 0.0865 and K is related with the Nambu-Goldstone(NG) boson(axion) radiation power from global(axionic) strings, P, as P = Kµ with K ~ 0.535 . Therefore, the loop production function also scales and the typical scale of produced loops is nearly the horizon distance. Thus, the evolution of the global(axionic) string network in the radiation dominated universe can be well described by the one scale model in contrast with that of the local string network. Furthermore, the power spectrum of axions radiated from axionic strings is calculated from the simulation data, which is found to be highly peaked around the Hubble scale, and a more accurate constraint on the Peccei-Quinn breaking scale is obtained.

A new Solar System population of Weakly Interacting Massive Particle (WIMP) dark matter has been proposed to exist. We investigate the implications of this population on indirect signals in neutrino telescopes (due to WIMP annihilations in the Earth) for the case when the WIMP is the lightest neutralino of the MSSM, the minimal supersymmetric extension of the standard model. The velocity distribution and capture rate of this new population is evaluated and the flux of neutrino-induced muons from the center of the Earth in neutrino telescopes is calculated. We show that the effects of the new population can be crucial for masses around 60–120 GeV, where enhancements of the predicted muon flux from the center of the Earth by up to a factor of 100 compared to previously published estimates occur. As a result of the new WIMP population, neutrino telescopes should be able to probe a much larger region of parameter space in this mass range.

A massive black hole is present at the centre of our galaxy and inevitably accretes dark matter particles, creating a region of very high particle density. The annihilation rate is enhanced with a large number of e⁺e^- pairs produced either directly or by sucessive decays of mesons. We evaluate the synchrotron emission (and self-absorption) associated with the propagation of these particles through the galactic magnetic field and give constraints on the values of mass and cross section of the dark matter particles.

The question of the nature of the dark matter in the Universe remains one of the most outstanding unsolved problems in basic science. One of the best motivated particle physics candidates is the lightest supersymmetric particle, assumed to be the lightest neutralino. We here describe DarkSUSY, an advanced numerical FORTRAN package for supersymmetric dark matter calculations which we release for public use. With the help of this package, the masses and compositions of various supersymmetric particles can be computed, for given input parameters of the minimal supersymmetric extension of the Standard Model (MSSM). For the lightest neutralino, the relic density is computed, using accurate methods which include the effects of resonances, pair production thresholds and coannihilations. Accelerator bounds are checked to identify viable dark matter candidates. Finally, detection rates are computed for a variety of detection methods, such as direct detection and indirect detection through antiprotons, gamma-rays and positrons from the Galactic halo or neutrinos from the center of the Earth or the Sun.

The paper considers the Dark Matter (DM) in the Newtonian approach of a gaseous compressible medium. This kinetic and phenomenologic approach bases on the classic (non-relativistic) principles and extended linear and nonlinear Maxwell's electromagnetic theory on the compressible medium case. The Newtonian Dark Matter (NDM) is non-barionic gaseous type matter forming galaxy halos and uniform distributed background of non-observed substance in the universe. This identification allows to calculate all major performances of NDM in a free space. A few special experimental tests for direct NDM detection are observed.

The ≃ 100 kg highly radiopure NaI(Tl) DAMA set-up is running deep underground in the Gran Sasso National Laboratory of the I.N.F.N. since several years. Results on various rare event processes have already been obtained and others are under investigation. The main purpose of this experiment is the investigation of the WIMP annual modulation signature. Results obtained during four annual cycles have been released so far and they have been summarized here.

The status of dark matter searches with inorganic scintillator detectors at Boulby mine is reviewed. Results of a test experiment with CsI(Tl) crystal are presented. The objectives of this experiment were to study anomalous fast events and ways to remove this background. We found clear indications that these events were due to surface contamination of crystals by alphas, probably from radon decay. A new array of unencapsulated NaI(Tl) crystals immersed in liquid paraffin or nitrogen atmosphere is under construction at Boulby. Such an approach allows us to control the surface of the crystals. Preliminary results from the first module are presented.

The cold dark matter search has been carried out at Oto Cosmo Observatory with the large volume NaI(Tl) scintillators of ELEGANT V. The new limits on WIMPs could be obtained by the analysis of the annual modulation.

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for WIMPs via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS I data, from the Stanford Underground Facility shallow site, give limits on the spin-independent WIMP-nucleon elastic-scattering cross-section that exclude unexplored parameter space above 10 GeV c^-2 WIMP mass and, at > 75% CL, the entire 3σ allowed region for the WIMP signal reported by the DAMA experiment. The move to a deep site at Soudan, Minnesota, should improve the experiment's sensitivity by ~ 100x. "First Dark" for CDMS II will be in 2001.

In its WIMP search the CDMS collaboration deploys cryogenic detectors utilizing ionization and phonon sensors for discriminating between electron and nuclear recoil events. Experience from CDMS I demonstated the importance of identifying surface (electron) events. Thus, for CDMS II we are deploying more advanced detectors utilizing both athermal phonon sensors, which permit active surface event rejection with a phonon risetime cut, and ionization electrodes incorporating a layer of amorphous Si. Initial results from operating these Si and Ge Z-sensitive Ionization and Phonon (ZIP) detectors at the Stanford Underground Facility are reported.

ROSEBUD (Rare Objects SEarch with Bolometers UndergrounD) is an experiment to search for low mass Weakly Interacting Massive Particles (WIMPs) with massive bolometers, which is being carried out at the Canfranc Underground Laboratory. We report in this paper the results of the four runs of Phase 1 (performed along 1999) and the progressive background reduction obtained. The plans for Phase 2, just started in November 2000, are also presented.

Two germanium detectors are currently operating in the Canfranc Underground Laboratory at 2450 m.w.e looking for WIMP dark matter. One is a 2 kg ⁷⁶Ge IGEX detectors (RG-2) which has an energy threshold of 4 keV and a low-energy background rate of about 0.3 c/keV/kg/day. The other is a small (234 g) natural abundance Ge detector (COSME), of low energy threshold (2.5 keV) and an energy resolution of 0.4 keV at 10 keV which is looking for WIMPs and for solar axions. The analysis of 73 kg-days of data taken by COSME in a search for solar axions via their photon Primakoff conversion and Bragg scattering in the Ge crystal yields a 95% C.L. limit for the axion-photon coupling g_aγγ < 2.8 × 10^-9 GeV^-1. These data, analyzed for WIMP searches provide an exclusion plot for WIMP-nucleon spin-independent interaction which improves previous plots in the low mass region. On the other hand, the σ(m) exclusion plot derived from the 60 kg-days of data from the RG-2 IGEX detector improves the exclusion limits derived from other ionization (non thermal) germanium detector experiments in the region of WIMP masses from 30 to 100 GeV recently singled out by the reported DAMA annual modulation effect.

Preliminary results obtained with 320g bolometers with simultaneous ionization and heat measurements are described. After a few weeks of data taking, data accumulated with one of these detectors are beginning to exclude the upper part of the DAMA region. Prospects for the present run and the second stage of the experiment, EDELWEISS-II, using an innovative reversed cryostat allowing data taking with 100 detectors, are briefly described.

A detector system, which consists primarily of CaF₂ scintillators, is developed to search for dark matters. The ¹⁹F nucleus in the CaF₂ detector is the best nucleus for the study of spin coupled dark matters which is the most promising candidate for the cold dark matters at present. In this article characteristics of the detector are described. It showed good performance at our laboratory (sea level). The system was moved at Oto Cosmo Observatory which has 1300 water equivalent shield. Current status and future prospect of the detector system are described.

Thanks to very high energy resolutions , low energy thresholds and very high sensitivity to nuclear recoils, cryogenic thermal detectors my play an important role in study and search dark matter particles of the universe. Milano group works with this type of detectors since many years and, at presents, operates the bigger cryogenic thermal detector with a total mass of 6.8 kg of TeO₂ crystals segmented in 20 active channels. This detector is in operation since 1998 and has realized to search for double beta decay of ¹³⁰Te. As a by product of this measurement also dark matter study will be conducted and, especially in the last year, a strong effort was made to further reduce threshold and spurious background in low energy region . The preliminary data on WIMPs searches with this detector is here reported . The improvement of this experiment will be the realization of the CUORICINO detector. Some preliminary tests on this new detector will be presented and briefly discussed.

Tokyo group started the underground measurement for dark matter search in January 2000 at Kamioka mine (2700 m.w.e). An array of eight LiF bolometers with a mass of 21 grams each is used aiming at a direct detection of spin-dependently interacting supersymmetric neutralinos. We present a preliminary result obtained with the measurement at Kamioka mine.

SIMPLE (Superheated Instrument for Massive ParticLE searches) employs superheated droplet detectors (SDDs) to search for Weakly Interacting Massive Particle (WIMP) dark matter. As a result of the intrinsic SDD insensitivity to minimum ionizing particles and high fluorine content of target liquids, competitive WIMP limits were already obtained at the early prototype stage. We comment here on the expected immediate increase in sensitivity of the program and on future plans to exploit this promising technnique.

We present the current status as well as future developments for the CRESST (Cryogenic Rare Event Search using Superconducting Thermometers) experiment, aiming at the direct detection of WIMP dark matter. After describing the extensive test-phase to acquire a complete control over all experimental parameters of the setup, we show first results of the measurements using Sapphire calorimeters as absorbers, being particularly sensitive to light WIMPs.

The first phase of the CRESST dark matter search has reached a low radioactive background of 1 count/d/kg/keV in the 15–20 keV region using sapphire calorimeters. Further progress will require being able to reject the γ and β backgrounds. We are therefore developing scintillating calorimeters in which photons emitted from the main calorimeter are read in a secondary calorimeter, allowing us to distinguish between luminous electron recoils and darker nuclear recoils. Progress has been made in scaling up a small but promising CaWO₄ proof-of-principle experiment. Based on the background presently observed, a 300 g CaWO₄ device exposed for three months would have a sensitivity on a par with current best limits. Thirty three such devices are planned to run in the next phase of the CRESST experiment.

The status of dark matter search with the HDMS experiment is reviewed. After one year of running the HDMS prototype detector in the Gran Sasso Underground Laboratory, the inner crystal of the detector has been replaced with a HPGe crystal of enriched ⁷³Ge. The results of the operation of the HDMS prototype detector are discussed.

MACHe3 (MAtrix of Cells of superfluid ³He) is a project of a new detector for direct Dark Matter Search. The idea is to use superfluid ³He as a sensitive medium. The existing device, the superfluid ³He cell, will be briefly introduced. Then a description of the MACHe3 project will be presented, in particular the background rejection and the neutralino event rate that may be achieved with such a device.

PICASSO is a new approach for the detection of cold dark matter candidates based on the metastability of moderately superheated liquids. The detection method relies on the fact that for suitable operating parameters, a liquid to vapour phase transition can be triggered exclusively by heavily ionising nuclear recoils and not by minimally ionizing particles.

The UK Dark Matter Collaboration is developing a series of liquid Xe detectors to search for the hypothetical weakly interacting massive particles (WIMPs) which may comprise a significant component of the Galactic dark matter. These detectors will be operated at a depth of 1100 m in the Boulby salt mine. The first of these detectors, ZEPLIN-I is a 1.7 litre single phase liquid Xe scintillation detector which employs pulse shape discrimination analysis to distinguish nuclear recoils due to WIMPs from electron recoils from background gamma interactions.

The current status of ZEPLIN-I is presented in this paper. The detector design will be described and results discussed of tests on the detector performance using various types of radioactive sources in the 10 keV-1MeV energy range. The light yield at the level of 1 photoelectron per 1 keV of deposited energy has been achieved. Clear discrimination between gamma initiated electron recoils and neutron initiated nuclear recoils has been observed. We are going to use this fact to recognize the type of interacting particle once the detector is installed undeground in the Boulby mine.

A pure liquid Xenon scintillator, filled by Kr-free Xenon enriched at 99.5% in ¹²⁹Xe, is operating at Gran Sasso since time. Several upgradings have been performed to improve its sensitivity during last years. The obtained results are summarized.

Results are presented of tests performed with Liquid Xenon (LXe) detector prototypes. In order to study their response to WIMP elastic scattering, they have been exposed to a neutron beam. Nuclear recoil events have been measured and compared to electron recoils. A preliminary analysis indicates a relative scintillation efficiency of 0.22 in the energy range of 40 to 70 keV. Pulse-shape properties have also been studied: under the assumption of a single dominant decay component, a value of 21.0 ± 0.5 ns has been measured for the log-normal mean-parameter T₀ of nuclear recoil pulses within the visible energy interval from 6 to 30 keV_ee; for electron recoil, T₀ is ~ 30 ns at ~ 15 keV_ee, rising slowly. Comparison with predictions and similar measurements is discussed.

The full potential of xenon two-phase discriminating dark matter detectors will only be realised by moving to high electric field operation allowing both scintillation and ionisation signals to be recovered for both electron recoil type background events and nuclear recoil event. Here we present designs for a 7kg xenon detector in which this can be achieved and which will have a sensitivity at the 0.01 dru level. This detector is now being constructed. This paper presents the design parameters and performance predictions. In a companion paper results from a laboratory prototype are presented.

First measurements from a two-phase 3kg Xe prototype cell have been taken. Signals have been observed from a variety of radiation sources and results characterised as a function of applied electric field. Electron drift times in excess of 60µs have been measured. Signals are observed from both phases - primary scintillation in the liquid phase and secondary scintillation due to ionisation drift into the gas phase. The two-phase output from alpha particles and gamma photons are presented. Future prospects for the detection of nuclear recoil ionisation and the fabrication of a 9kg two-phase detector - Zeplin III, are discussed.

The Directional Recoil Identification From Tracks (DRIFT) project is an endeavor to build and operate a 1 m³ low pressure Negative Ion TPC to search for WIMP dark matter. This paper will focus on a neutron calibration of a DRIFT prototype and a Monte Carlo to simulate these events.

The scintillation efficiency of carbon and hydrogen nuclear recoils in an organic liquid scintillator was measured for a possible dark matter detector. The recoil energies from 50 keV to ~ 1 MeV were explored for both nuclei. The carbon recoil efficiency, of particular interest for a dark matter detector, was observed to increase from of the electron recoil efficiency at 500 keV to at 46 keV. Such an enhancement is very encouraging for the purpose of dark matter searches as well as other similar low-energy experiments.

The PVLAS experiment is designed to study photon-photon interactions in the low energy region using optical techniques. In the PVLAS apparatus photons from a laser beam interact in vacuum with an external magnetic field, and the two-photon interaction results in the possible production of neutral, nearly massless, scalar/pseudoscalar particles. Evidence of particle production is extracted from the study of the light polarisation state after traversing a region where a the magnetic field is provided by a superconducting dipole magnet. Polarisation measurements are conducted using a very sensitive ellipsometer based on a highfinesse (~100000). 6.4 m long, Fabry-Perot optical resonator. The PVLAS apparatus is now fully integrated and operational: the first commissioning run with a 4.0 T field has been successfully completed, the first data taking runs have been conducted, yielding a total integration time of about 20 hours, and data analysis is now in progress. We will present details from the commissioning run and preliminary results from the data runs.

The large–scale U.S. halo axion search experiment has been taking production data since February 9th 1996. After a brief introduction to the theory and phenomenology of axions, I present the current status of the experiment and our latest results. I conclude by outlining some future research goals of the project.

We have searched for axions which could have been produced in the solar core using an axion helioscope which is equipped with a 2.3 m-long 4 T superconducting magnet, PIN-photodiode x-ray detectors, and a telescope mount mechanism to track the sun. A gas container to hold dispersion-matching gas has been developed and a mass region up to m_a = 0.26 eV was newly explored. Preliminary analysis sets a limit on axion-photon coupling constant to be g_aγγ < 6.4 ~ 9.6 × 10^-10GeV^-1 for the axion mass of 0.05 < m_a < 0.26 eV at 95% confidence level from the absence of the axion signal. This is more stringent than the limit inferred from the solar age consideration and also more stringent than the recent helioseismological bound.

We report on the search for nearly vertical up-going muon neutrinos from WIMP annihilations in the center of the Earth with the AMANDA-Bl0 detector. The whole data sample collected in 1997, 10⁹ events, has been analyzed and a final sample of 15 up-going events is found in a restricted zenith angular region where a signal from WIMP annihilations is expected. A preliminary upper limit at 90% confidence level on the annihilation rate of WIMPs in the center of the Earth is presented.

The ANTARES Collaboration is currently constructing a large area neutrino telescope for deployment in the Mediterranean Sea, approximately 30 km off the southern French coast. Results from site evaluation studies demonstrating the suitability of the proposed site are reported, along with results from a demonstrator string deployed in late 1999 and retrieved in July 2000. The collaboration plans to deploy a 0.1 km² detector array by the end of 2003. The design and expected performance of this array are described, and the prospects for its use in indirect searches for neutralino dark matter are briefly reviewed.

A qualitative explanation of all present evidence for neutrino mass (the solar and atmospheric neutrino deficits, LSND, and significant dark matter) is provided by a neutrino mass-mixing scheme which also successfully avoids the "alpha effect," allowing r-process nucleosynthesis in the neutrino-heated ejecta of supernovae. The neutrino properties required to ensure production of heavy nuclei provides independent evidence for (1) at least one light sterile neutrino, ν_s; (2) a near maximally-mixed ν_µ-ν_τ doublet (which also explains the atmospheric anomaly and provides hot dark matter) split from a lower mass ν_e-ν_s doublet (needed also for the solar ν_e deficit); (3) ν_µ-ν_e mixing ≳ 10^-4; and (4) a splitting between the doublets (measured by the ν_µ-ν_e mass difference) ≳ 1 eV², favoring the upper part of the LSND range. There is a quantitative problem with the solar observations, which do not in detail fit this or any other model. If, however, the ν_s is a bulk neutrino in extra dimensions the solar data can be fit remarkably well with a combination of vacuum and MSW oscillations. Models with large extra dimensions lead to naturally light sterile neutrinos in the process of giving small mass to the known neutrinos. In this particular model radiative effects split a Dirac neutrino, giving a small mass difference between its Majorana constituents which provides vacuum oscillations of the ν_e to the zero mode of the ν_s, while the ν_e transitions to the Kaluza-Klein states of the ν_s give MSW oscillations.

Trimaximal mixing is the mixing hypothesis with maximal symmetry. In trimaximal mixing there remains a still worrying conflict between the large values of Δm² preferred by the atmospheric fits (possibly supported by the early K2K data) and reactor limits on ν_e-mixing. However, the latest solar results do seem to point to energy-independent (ie. 'no-scale') solar solutions, like the trimaximal solution.

The MiniBooNE experiment has been designed to unequivocally confirm or refute the neutrino oscillation signal reported by the LSND collaboration. The status of MiniBooNE's preparations is presented along with a summary of the experiment's prospects.

We built a low background detector based on a 1 m³ time projection chamber surrounded by an active anti-Compton shielding to measure the elastic cross section at low energy. The detector has been installed close to a nuclear reactor in Bugey and it is running since almost one year. After having reduced the electron background by more than 2 orders of magnitude we are now taking data to be sensitive to a neutrino magnetic moment in the region below 10^-10 Bohr magnetons.

We discuss two experiments – OPERA and ICARUS – that have been proposed for the direct observation of the ν_τ appearance from the ν_µ ↔ ν_τ oscillations in the CNGS neutrino beam. Neutrinos are produced at the CERN SPS with an average energy of 17 GeV and are directed towards the Gran Sasso Laboratory 730 km away.

This european long baseline programme aims at further understanding of the deficit seen in the atmospheric neutrinos experiments both in the ν_µ to ν_e flavor ratio and in the ν_µ zenith angle distribution. It aims at very high sensitivity in the parameter region suggested by the atmospheric neutrinos experiments and in particular by Super-Kamiokande. It also foresees a sensitive ν_µ ↔ ν_e appearance programme to perform an analysis of neutrino oscillation with three-flavour mixing. The direct observations of the oscillation products provide an unique tool to firmly assess the neutrino oscillation scenario.

The enthusiasm for building a muon storage ring as an intense source of neutrinos is growing rapidly world-wide. This paper will outline the physics that could be done with such a neutrino facility, called a neutrino factory, and discuss how it might be built. The status of related R&D projects will also be briefly described.

We describe an approach to the study of neutrino masses that combines quantum optics techniques with radiation detectors to obtain unprecedented sensitivity. With it the search for Majorana neutrino masses down to ~10 meV will become accessible. The experimental technique uses the possibility of individually detecting Ba⁺-ions in the final state of ¹³⁶Xe double-beta decay via resonant excitation with a set of lasers aimed at a specific location in a large Time Projection Chamber. The specificity of the atomic levels provides tagging and, together with more traditional event recognition parameters, greatly suppresses radioactive backgrounds.

ORLaND, the Oak Ridge Laboratory for Neutrino Detectors, is a proposed underground laboratory adjacent to the target station of the Spallation Neutron Source (SNS) under construction at the Oak Ridge National Laboratory (ORNL). The neutrons, copious π-mesons, and hence neutrinos, will be generated by a 1.3 GeV proton beam (2 MW) impinging on a mercury target with 600 ns wide pulses at 60 Hz.

The present status of the Boulby Underground Laboratory is reviewed as well as a discussion of the future plans for developing the site. Future experiments are also briefly discussed.

The GENIUS proposal is described and some of it's physics potential is outlined. Also in the light of the contradictive results from the CDMS and DAMA experiments the Genius TF, a new experimental setup is proposed. The Genius TF could probe the DAMA evidence region using the WIMP nucleus recoil signal and WIMP annual modulation signature simultaneously. Besides that it can prove the long term feasibility of the detector technique to be implemented into the GENIUS setup and will in this sense be a first step towards the realization of the GENIUS experiment.

Several dark matter detectors are know set to achieve sensitivities approaching the 10^-8 level for spindependent neuhalino scattering cross-sections. This is within a factor of 10 or so of the lowest cosmologically interesting cross-sections within standard minimal supersymmetry. The next logical step would be to try to reach this significant milestone and try to close this hvoured region. This will require much larger detectors and here I explore the possibilities offered by advanced two-phase xenon detectors.

Experimentally have we got what it takes to pursue the direct observation of WIMP interactions down to a sensitivities of a few events /(100 kg)/year? For a Ge target with a low energy threshold (< 20 keVr) this corresponds to a WIMP-nucleon σ ~ 10^-46cm². A number of recent theoretical papers, making calculations in SUSY based frameworks, show many (> 5) orders of magnitude spread in the possible interaction rates for models consistent with existing Cosmology and Accelerator bounds. Some theorists, but certainly not all, are able to generate models, that lead to interaction rates at the few /kg/day that would be implied by the current DAMA annual modulation signal. All theorists demonstrate models that generate much lower interaction rates. This paper takes an unashamed experimentalist's view of the issues that arise when looking forward to constructing 1 tonne WIMP detectors.

Based on the recently identified ν_e-capture reaction¹ on ¹⁶⁰Gd this programme is designed to investigate the construction of a real-time, low-threshold, spectroscopic solar neutrino detector SIREN (Solar neutrino Interactions by Real time Excitation of Nuclei). To date, solar neutrino experiments capable of achievinglow energy thresholds have relied on radio-chemical methods, while real-time experiments, using Cerenkov techniques, have suffered from high thresholds (> 5MeV). The SIREN concept allows, for the first time, the possibility of measuring the solar neutrino flux in real-time and with sensitivity to the lowest neutrino energies, giving spectral data at E_ν > 250keV. The unique spectroscopic information provides the prospect of resolving the difference between competing scenarios postulated to explain the solar neutrino deficit. In particular, it would be possible at last to distinguish between the just-so vacuum oscillations, MSW effect and flat-energy dependence of the neutrino suppression factor².

New types of element-loaded (B and Gd) organic scintillators for neutron detection and neutrino experiments have been synthesised recently in the JINR. Their optical, spectral, scintillation and radiopurity characteristics are presented and discussed. It is shown that 5% B-loaded plastic scintillator has a light output as much as 70% relative to the unloaded one. The same characteristic for the 3% Gd-loaded plastic sample is equal to 51%. Transparency and other properties of the produced scintillators did not changed at normal conditions for at least one year.

The following sections are included:

• LIST OF PARTICIPANTS