Non-cryogenics dark matter experiments

Non-cryogenics dark matter experimentsAngel Morales∗

Canfranc Underground Laboratory, University of Zaragoza, 50009Zaragoza, Spain

The current status of WIMP direct searches with conventional detectors is overviewed, emphasizing strategies,achievements and prospects.

1. INTRODUCTION

A large body of experimental observations andwell-founded theoretical arguments conclude thatour universe is essentially non-visible. The distri-bution of a flat universe (Ω = ΩM + ΩΛ = 1)attributes to the dark energy about ΩΛ ∼ 73%,whereas the matter density takes the remainingΩM ∼ 27%, consisting of both visible (Ωl ∼ 0.5%)and non-visible (dark) matter. This dark com-ponent is formed by ordinary baryonic matter(ΩB ∼ 3.5%) and a large fraction (up to ΩNB ∼23%) of non-baryonic dark matter, supposedlymade by non-conventional, exotic particles whichwould be filling the galactic halos. It is supposedto be a suitable mixture (cold and hot dark mat-ter) to properly generate the cosmic structures.The minimal requirements to be fulfilled by thenon-baryonic dark particles are to provide theright relic abundance, to have non-zero mass, zeroelectric charge and very weak interaction with or-dinary matter.

There are several candidates to such species ofmatter provided by schemes beyond the StandardModel of Particle Physics. Remarkable examplesare the axions, WIMPs (Weakly Interacting Mas-sive Particles) and neutrinos.

Axions are pseudoscalar Nambu-Goldstonebosons arising from the spontaneous symme-try breaking of the Peccei-Quin U(1)PQ sym-metry invented to solve the strong CP problem.They are very weakly coupled to ordinary matterand the most favorable mass window should be10−5(−6)eV < ma < 10−2(−3)eV .

The weakly interacting, neutral and massive

∗The last version of this paper was finished up by JulioMorales, University of Zaragoza ([email protected]).

particles, WIMPs, are another popular candidate.A particularly attractive kind of WIMPs are pro-vided by the SUSY models, like the neutralinos(the lightest stable particles, LSP) of super sym-metric theories. The mass window of this candi-date is GeV ≤ mχ ≤ TeV (an interesting massregion, for reasons which will become clear lateron, is that of 40GeV ≤ mχ ≤ 200GeV ).

The last candidate is the (non-zero mass) neu-trino of theories beyond the Standard Model.They are the only candidate known to exist, havewell-known weak interaction and only a smallamount of them is needed to explain cosmic data.Their mass window is very wide according to theparticular model (to fit also other phenomeno-logy of ν-physics). It is to be noticed that themasses of the non-baryonic particle candidatesextend along more than 18 orders of magnitude:10−6 eV - 1012 eV.

This talk will be devoted to only one of thesecandidates: the WIMPs. Without entering intoconsiderations about how large the baryonic darkcomponent of the galactic halo could be, we takefor granted that there is enough room for WIMPsin our halo to try to detect them, either directlyor through their by-products.

The indirect detection of WIMPs proceeds cur-rently through two main experimental lines: ei-ther by looking in cosmic rays experiments forpositrons, antiprotons, or other antinuclei pro-duced by the WIMPs annihilation in the halo,or by searching in large underground detectorsor underwater neutrino telescopes for upward-going muons produced by the energetic neutrinosemerging as final products of the WIMPs annihi-lation in celestial bodies (Sun, Earth...).

The direct detection of WIMPs relies on mea-

Nuclear Physics B (Proc. Suppl.) 138 (2005) 135–144

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Table 1History of WIMP direct searches

DETECTOR LABORATORYGERMANIUM 1986 USC-PNNL(Homestake), UCSB-LBL (Oroville), ZAR-USC-PNL (Canfranc),(Ionization) CALTECH-PSI-N (Gothard)

1990 H/M (Gran Sasso), IGEX (Canfranc), COSME (Canfranc),TAN-USC-PNL-ZAR (Sierra G), IGEX (Baksan), HDMS (Gran Sasso)

2003 GENIUS-TF (Gran Sasso)SCINTILLATOR 1990 ZARAGOZA NaI (Canfranc), ROMA lqXe (Gran Sasso),

ROMA/SACLAY NaI (LSM/LNGS), UKDMC NaI (Boulby), DAMA NaI, CaF2 (LNGS),SACLAY NaI (Frejus), ELEGANTS NaI, CaF2 (Oto)

2000 ZEPLIN Xe (Boulby), NAIAD NaI (Boulby)2002 ANAIS NaI (Canfranc)2003 LIBRA NaI (LNGS)

THERMAL 1988 MIBETA TeO2 (LNGS),EDELWEISS-0 Al2O3 (Frejus)(Phonons) 90’s CRESST-I Al2O3 (LNGS), ROSEBUD Al2O3/Ge (Canfranc)

2003 CUORICINO TeO2 (LNGS)2005 CUORE TeO2 (LNGS)

CRYO 1988 CDMS-I Si/Ge (SUF)(Phon.+Ioniz.) 90’s EDELWEISS I Ge (Frejus)

2001 EDELWEISS II (Frejus)2002 CDMS-II Ge/Si (Soudan)

CRYO 2000 CRESST-II CaWO4 (LNGS)(Phon.+Light) 2001 ROSEBUD CaWO4 and BGO (Canfranc)SSD R+D of SSD since 80’s, Paris, Munich, Garching, Bern, Zaragoza, Oxford, Lisbon

2001 ORPHEUS Sn (Bern UF)SDD 1997 SIMPLE Freon (Rustrel), PICASSO Freon (SNO)TPC 2002 DRIFT Xe (Boulby)

suring the nuclear recoil produced by their elasticscattering off target nuclei in suitable detectors.The signal rate depends on the type of WIMP andinteraction, whereas simple kinematics says thatthe energy delivered in the WIMP-nucleus inter-action is very small. Even with the small recoilenergy, only a fraction of it is visible in the detec-tor, depending on the type of detector and tar-get, and on the mechanism of energy deposition.The quenching factor, Q, is essentially a unit inthermal detectors, whereas for the nuclei used inconventional detectors it ranges from about 0.1to 0.6. For instance for a Ge nucleus only about1/4 of the recoil energy goes to ionization.

The rare (≤ 10−2c/kg.day) and small (keVrange) WIMP signal falls in the low-energy re-gion of the spectrum, where the radioactive andenvironmental backgrounds accumulate at muchfaster rate and with similar spectral shape. Thatmakes WIMP signal and background practicallyindistinguishable. In conclusion, due to the pro-perties of the expected signals, the direct search

for particle dark matter through their scatte-ring by nuclear targets requires ultralow back-ground detectors of a very low-energy threshold,endowed, when possible, with background dis-crimination mechanisms. All these features to-gether make the WIMP detection a formidableexperimental challenge.

This review will deal with the efforts currentlybeing done in the direct search of WIMPs. Onlyconventional, non-cryogenic detectors will be con-sidered here (the case for cryogenics detectorswill be addressed in the review of W. Seidel, inthese Proceedings). Table 1 gives a rough accountof the “history” of WIMP searches and Table 2gives an overview of the experiments on directdetection of WIMPs currently in operation or inpreparation. General reviews on WIMPs can befound in Ref [1] whereas neutralino dark matterhas been extensively described in Ref [2]. WIMPdirect detection is reviewed in detail, for instance,in Ref [3].

A. Morales / Nuclear Physics B (Proc. Suppl.) 138 (2005) 135–144136

2. DETECTING WIMPs

The method to explore whether there existsor not a WIMP signal contribution in the ex-perimental data is rather simple: one comparesthe predicted event rate with the observed spec-trum; if the former turns out to be larger thanthe measured one, the particle which would pro-duce such event rate can be ruled out as a darkmatter candidate. That is expressed as a con-tour line σ(m) in the plane of the WIMP-nucleonelastic scattering cross section versus the WIMPmass. For each mass m, those particles with across-section above the contour line σ(m) are ex-cluded as dark matter. The level of backgroundsets, consequently, the sensitivity of the experi-ment to eliminate candidates or in constrainingtheir masses and cross-sections.

The smallness of the predicted rate (which goesfrom ∼ 10 down to 10−5 c/kg.day) implies thatthe sensitivity of the experiment must be drivento the best possible achievable value in this range.The σχN calculations are made within the Mini-mal Supersymmetric extension of the StandardModel, MSSM, as basic frame, implemented invarious schemes (see Ref. [2]). Besides the pecu-liarities of the SUSY model, there is a wide choiceof parameters entering in the calculation of therates: the halo model, the values of the parame-ters in the WIMP velocity distribution, the threelevels of the WIMP-nucleus interaction (quark-nucleon-nucleus) and the constraint of getting theproper relic abundance of the candidates, just tomention a few.

So, the theoretical prediction of the ratesshows considerable spreading and are presentedas “scatter plots” extending along the various or-ders of magnitude quoted above. Some of themost favorable predictions are already testable bythe leading experiments which, in fact, penetrateinto the scatter plot of predictions. The bottomof the plot is still far away from the detector sen-sitivity.

The rarity and smallness of the signals dictatethe obvious strategy: to use ultralow backgrounddetectors of the lowest possible energy thresholdplus one (or various) unambiguous backgroundrejection mechanisms, all these prescriptions car-

ried out in a radioactivity-free environment (in-cluding shielding, experimental devices, ...). Ex-amples of low background recently achieved arethe case of IGEX, with Ge ionization detectors;the cases of the CDMS and EDELWEISS, whichuse Ge thermal detectors which also measure ion-ization to discriminate, and that of ZEPLIN,which uses background discrimination in liquidxenon.

Large masses of targets are also recommended,to increase the probability of detection and thestatistics as, for instance, the case of DAMA andANAIS, CUORICINO and ZEPLIN.

The basic idea behind the background rejec-tion techniques is to discriminate first electron re-coils (tracers of the background) from nuclear re-coils (originated by WIMPs and neutrons). Meth-ods used to discriminate backgrounds from nu-clear recoils are either simply statistical, like apulse shape analysis, PSD (based on the differenttiming behavior of both types of pulses), or onan event by event basis by measuring simultane-ously two different mechanisms of energy depo-sition having different responses for backgroundand signals, like the ionization (or scintillation)and the heat produced by the WIMP-induced nu-clear recoil.

Another discriminating technique is that usedin the two-phase liquid-gas Xenon detector withionization plus scintillation, of the ZEPLIN seriesof detectors. An electric field prevents recombina-tion, the charge being drifted to create a secondpulse in addition to the primary pulse. The am-plitudes of both pulses are different for nuclearrecoils and electrons, and that allows their dis-crimination.

One could use instead threshold detectors – likeneutron dosimeters – which are blind to most ofthe low Linear Energy Transfer (LET) radiation(e, µ, γ) and so are able to discriminate gammabackground from neutrons (and thus WIMPs).Detectors which use superheated droplets whichvaporize into bubbles by the WIMP (or otherhigh LET particles) energy deposition are thoseof the SIMPLE and PICASSO experiments. Anultimate discrimination will be the identificationof the different kinds of particles by the trackingthey left in, say, a TPC, plus the identification of

A. Morales / Nuclear Physics B (Proc. Suppl.) 138 (2005) 135–144 137

Table 2WIMP direct detection in underground facilities experiments currently running (or in preparation)

LABORATORY EXPERIMENT TECHNIQUE

Bern(Switzerland) ORPHEUS (SSD) Tin Superconducting Superheated Detector (0.45 kg)Boulby (UK) NAIAD NaI scintillators (46-65 kg)

ZEPLIN I Liquid Xe scintillator (4 kg)ZEPLIN II Liquid-Gas Xe (scint/ioniz) (30 kg) (R+D)ZEPLIN III Liquid-Gas Xe (scint/ioniz) (6 kg) (R+D)ZEPLIN IV Liquid-Gas Xe (scint/ioniz) (1 T) (R+D)DRIFT Low pressure Xe TPC 1m3 (R+D)

Canfranc (Spain) IGEX Ge ionization detector (2.1 kg)GEDEON Set of Ge ionization detectors (in project) (4 × 7 × 3 × 1 kg)ANAIS NaI scintillators (110 kg)ROSEBUD CaWO4 and BGO scintillating bolometers (50-200 g)

Frejus/Modane (France) EDELWEISS Sets of Ge thermal+ionization detectors (n×320 g)Gran Sasso (Italy) H/M Ge ionization detector (2.7 kg)

HDMS Ge ionization in Ge wellGENIUS-TF Set of Ge crystals in LN2) (40 kg)DAMA NaI scintillators (∼ 100 kg)LIBRA NaI scintillators 250 kg (starting)Liquid-Xe Liquid Xe scintillator (6 kg)CaF2 ScintillatorCRESST Set of CaWO4 scintillating bolometers (n×300 g)CUORICINO Set of TeO2 thermal detector (41 kg)CUORE 1000×760g TeO2 (in project)

Kamioka (Japan) XMASS Large mass Xe scintillators (R+D)Rustrel (France) SIMPLE (SDD)Superheated Droplets Detectors (Freon)Soudan (USA) CDMS Sets of Ge and Si thermal+ionization detectorsSNO (Canada) PICASSO (SDD) Superheated Droplets Detectors (Freon)Oto (Japan) ELEGANTS V Large set of massive NaI scintillators (670 kg)

ELEGANTS VI CaF2 scintillators

the WIMP through the directional sensitivity ofthe device (DRIFT). Intense R&D programs areunderway to use devices with this kind of sensitiv-ity. Tables 3 to 6 give, synoptically, the main non-cryogenic experiments currently running, summa-rizing some of their features.

Temporal and spatial asymmetries specific tothe WIMP interaction exist, which are not char-acteristics of the background and, in principle,can be used as identification labels of the WIMPs.They are due to the kinematics of the motion ofthe Earth (and of our detectors) in the galac-tic halo. These signatures could be the annualmodulation of the rate [4], the forward/backwardasymmetry of the nuclear recoil [5] or the nucleartarget dependence of the rates [6].

The two kinematical asymmetries character-istic of WIMPs signals are originated by theEarth’s motion through the galactic halo. TheEarth’s orbital motion around the Sun has a sum-

mer/winter variation, which produces a small an-nual modulation of the WIMP interaction rates,of the order O

(vrot,E

vh

)∼ 15

270 ∼ 5% [4]. Theobservation of a tiny modulation of a very smallsignal requires large target mass and exposure,superb stability and extreme control of systema-tics and of other stational effects.

The annual modulation signature is the onlydistinctive signature seriously investigated up tonow. Pioneering searches were carried out inCanfranc (NaI-32), Kamioka (ELEGANTS) andGran Sasso (DAMA-Xe). In July 1997 theDAMA experiment at Gran Sasso, using a set ofNaI scintillators, reported an annual modulationeffect which after seven yearly periods has a 6.3σlevel significance.

A second characteristic signature of the WIMPis provided by the directional asymmetry of therecoiling nucleus. The WIMPs velocity distribu-tion in the Earth’s frame is peaked in the oppo-


site direction of the Earth/Sun motion throughthe halo, and so the distribution of nuclear re-coils direction shows a large asymmetry for-ward/backward (F/B) not easily mimicked by thesupposedly isotropic background. The order ofmagnitude of the effect is large because the solarsystem’ motion around the galactic center vsun,and the typical WIMP velocity in the halo, vh,are of the same order O

(vsunvh

)∼ 230

270 ∼ 1.An increasing interest in developing devices

sensitive to the directionality of nuclear recoilsfrom WIMPs exists and, in general, to the track-ing of particles. Chambers with such purposeare being used or planned for experiments inrare event physics. The DRIFT (Direction RecoilIdentification From Tracks) detector is a TPC ofXe (or other gases), which is sensitive to the di-rectionality of the nuclear recoil.

Another asymmetry is the nuclear target de-pendence of the rate [6], for instance in the nu-clear mass A, or in the nuclear spin J. However,due to the differences in the intrinsic backgroundsof the various targets, it is not easy to get reli-able conclusions. Some experiments are operatingand sets of similarly produced crystals of differ-ent nuclear targets in the same environment asROSEBUD with the objective of exploring suchnuclear target dependence.

3. WIMP SEARCHES

3.1. Germanium detectorsThe high radiopurity and low background

achieved in germanium detectors, their fair low-energy threshold, their reasonable quenching fac-tor (about 25%) and other nuclear merits makegermanium a good option to search for WIMPswith detectors and techniques fully mastered.The first detectors applied to WIMP directsearches (as early as in 1987) were, in fact, Gediodes, as by-products of 2β-decay dedicated ex-periments. Table 3 shows the germanium ioniza-tion detector experiments currently in operationor in preparation.

The International Germanium Experiment(IGEX) which was optimized to search for thedouble beta decay of germanium [7] is using oneenriched detector of 76Ge of ∼2.1 kg to look for

Figure 1. Exclusion plots obtained by the mostadvanced experiments taking as reference theircrossing of the region defined by the DAMA can-didate.

WIMPs in the Canfranc Underground Labora-tory. It has an energy threshold of 4 keV andan energy resolution of 0.8 keV at the 75 keVPb x-ray line [8]. The H/M experiment [9] isanother enriched-Ge experiment (enriched 76Gecrystal of 2.7 kg and energy threshold of 9 KeV),already completed, which has been running atGran Sasso.

The best exclusion plot derived for these Geexperiments, that of IGEX-2002, is depicted inFig. 2. It improves the exclusion of the otherGe-ionization experiments for a mass range from20 GeV up to 200 GeV, which encompass theDAMA mass region [10]. In particular, IGEXexcludes WIMP-nucleon cross-sections above 7×10−6 pb for masses of 40-60 GeV and entersthe DAMA region excluding the upper left partof this region. That is the first time that a directsearch experiment with a Ge-diode without back-ground discrimination, but with very low (raw)background, enters such region. A further 50%background reduction between 4 keV and 10 keVwould allow IGEX to explore practically all theDAMA region in 1 kg.y of exposure.

New experimental projects to look for WIMPswith Ge detectors exist. GEDEON (GErmaniumDEtectors in ONe cryostat), is planned to use 56


Table 3Ge ionization experiments (Q∼ 0.25)

Experiment & Site Mass EThr Low Energy Observations(kg) (keV) B(c/keVkgd)

IGEX. Canfranc 2.1 4 0.21 (4-10 keV) 1 detector from IGEX enriched 76Ge set0.10 (10-20 keV) R+D improving BKG0.04 (20-40 keV) New bound after stripping of tritium

H/M. LNGS 2.76 9 0.16 (9-15 keV) 1 detector from H/M enriched 76Ge set (2β)0.042 (15-40 keV)

HDMS. LNGS inn. 0.2 2.5 0.2 (11-40 keV) Small detector inside a well-type outer crystalout. 2.1 7.5 0.07 (40-100 keV)

GENIUS-TF. LNGS 40 0.5 nom. goal 10−2 14 Ge crystals embedded in LN2(2.7x14) 12 eff. (12-100 keV) housed in zone-refined Ge bricks

GENIUS Project 100→10T > 12 goal 10(−4),(−5) Large set of naked p-type Ge detectors in LN2(cosm.) (12-100 keV)

GEDEON Project 28→84 1-2 nom. goal 10(−2),(−3) Phase I: 28 Ge diodes in one single cryostatCanfranc 3x4x7 12 eff. (2-50 keV) Archaeological lead shielding melt underground

(cosm.) Detectors and components made underground

kg og Ge of natural isotopic abundance [12]. Itwill use the technology developed for the IGEXexperiment, and it would consist of a set of ∼2 kggermanium crystals, of a total mass of about 56kg, placed together in a compact structure insideone only cryostat in the Canfranc UndergroundLaboratory. This approach could benefit from an-ticoincidences between crystals and a lower com-ponents/detector mass ratio to further reduce thebackground with respect to IGEX.

The background final goal of GEDEON, be-low 100 keV, would be in the region of 10−3

c/keV.kg.day and this value has been used tocalculate anticipated σ(m) exclusion plots in themost favourable case. The expected threshold as-sumed has been Ethr = 2 keV and the energy res-olution in the low-energy region has been takenΓ ∼ 1 keV. The exclusion plot which could be ex-pected with such proviso in a first step (28 kg.y ofexposure) is shown in Fig. 2. Moreover, followingthe annual modulation sensitivity plots presentedin [13], GEDEON would be massive enough tosearch for the WIMP annual modulation effect[4] and explore positively an important part ofthe WIMP parameter space.

Two more Ge experiments running or in prepa-ration, both in Gran Sasso, are that of the Hei-delberg Dark Matter Search (HDMS)[14] and theGENIUS-Test Facility. The small detector ofHDMS has achieved a background still higher

than that of H/M, and so the results will not beincluded here. GENIUS-TF, now in operation,is intended to test the GENIUS project [15] andat the same time to search for WIMP. It consistsof a set of 14 natural abundance HP Ge detec-tors (a total of 40 kg) suspended in a holder andsubmerged directly in liquid nitrogen, in a steelvessel inside a polystyrene box (∼ 1m3).

3.2. NaI scintillator detectorsThe sodium iodide detectors are very attrac-

tive devices to look for WIMPs. Both nuclei havenon-zero spin (23Na J = 3

2 , 127I J = 52 ) and

then are sensitive also to spin-dependent interac-tion. Iodine is a heavy nucleus favorable for spin-independent interactions. Q is small (< 10%) forI, and medium for Na (∼ 30−40%). Backgroundslesser than or of the order of ∼ 1 c/keV.kg.day inthe few keV region have been achieved. Table 4shows the main features of the current NaI expe-riments.

The United Kingdom Dark Matter Collabora-tion (UKDMC) uses a set of 6-8 encapsulatedand unencapsulated NaI crystals (46 to 65 kg)in Boulby [16]. Typical resolution of ∼0.5 keV at4 keV and energy threshold of 2 keV have beenobtained.

ANAIS (Annual modulation with NaIs) is alarge mass (∼107 kg) NaI scintillator experimentplanned to investigate the seasonal modulationeffects of galactic WIMPs. A prototype has been


Table 4NaI scintillation experiments

Experiment & Site Mass EThr B(c/keVkgd) Observations(kg) (keV) aver. before PSD

DAMA. LNGS 9 x 9.70 2 ∼ 1.5 (at 2-3 keV) Annual modulation effect reported along sevenCompleted ∼ 2 (at 3-6 keV) annual cycles (6.3σ).

Phys. Lett. B450 (99)448;Riv.Nuovo Cim 26(2003)1-73

ELEGANT. Oto Cosmo 670 4-5 8-10 (at thr.) Old set-up upgradedLarge BKG from 210Pb (10 mBq/kg)

ANAIS. Canfranc Prot: 10.7 2 ∼1.5 (at 4 keV) 107 kg intended for ann. mod. search.Total 107 Old set upgraded plus new radiopure crystals

Preliminary 1200 kg day. In operationNAIAD. Boulby 48 → 56 2 ∼6-10 (4-20 keV) Set of NaI unencapsulated and unencapsulated

8 crystals In operationLIBRA-DAMA. LNGS 250 Set of NaI crystals. R+D on detector radiopurity

crystals from ultrapure powders. In operation

Figure 2. IGEX-DM projections are shown for aflat background rate of 0.1 c/keVkgday and 0.04c/keVkgday down to the threshold at 4 keV, for1 kg.year of exposure. The exclusion contour ex-pected for GEDEON is also shown as explainedin the text.

developed in Canfranc obtaining a background of1.2 events/keV.kg.day from the threshold ( 4 keVwith only one PMT) up to 10 keV. The completeexperiment is underway.

The DAMA experiment [10], now completed,uses 9 radiopure NaI crystals of 9.7 kg each,viewed by two PMT in coincidence. The software

energy threshold is at EThr = 2 keV and the en-ergy resolution at 2-5 keV is Γ ∼ 2.5 keV. Noiseis removed by discriminating timing behaviour ofnoise and true NaI pulses. The main objectiveof DAMA was to search for the annual modula-tion of the WIMP signal. Such modulation hasbeen found and attributed by the collaborationto a WIMP signal. 107731 kg.day of statisticslead to a WIMP of mass and cross-section givenby MW = (52+10

−8 )GeV ξσp = (7.2+0.4−0.9)× 10−6pb.

A maximum likelihood favours the hypothesis ofpresence of modulation with the above MW , ξσp

values at 6.3σ C.L. The (σ, m) region for spin in-dependent coupled WIMP is the “triangle” zonedepicted in Fig. 2. An extension of DAMA up to250 kg of NaI (LIBRA) is being prepared.

Other experiments (IGEX, CDMS, EDEL-WEISS, ZEPLIN) not sensitive to annual mod-ulation but simply comparing the expected sig-nal with observed spectrum have excluded to agreater or lesser extent the DAMA region by us-ing a particular model framework. There is alarge controversy in the confrontation. New, inde-pendent experiments, sensitive to seasonal modu-lation, should look for such effect. Some of themare ready: CUORICINO (41 kg), GENIUS-TF(40 kg), LIBRA (250 kg), ANAIS (110 kg), NA-IAD (56 kg). We will have a look at the experi-mental perspectives of such exploration, using theannual modulation signature of the WIMP.


3.3. Xenon scintillator detectorsThe search for WIMPs with xenon scintilla-

tors benefits from a well-known technique. More-over, background discrimination can be done bet-ter than in NaI. They are also targets of heavynuclear mass (A ∼ 130) for enhancement of thespin-independent coherent interaction.

They have achieved a fairly good level of ra-diopurity, have a good quenching factor (∼ 50%)and a high density (∼ 3gr/cm3). Summing upthese properties, one concludes that xenon scin-tillator based detectors are a good option to lookfor WIMPs.

One of the pioneer searches using xenon is theDAMA liquid-xenon experiment. The spectra oflimits on recoils in WIMP-129 Xe elastic scatte-ring using PSD and exclusion plots were pub-lished in Ref. [17]. Recent results of the DAMAliquid xenon experiment refer to limits on WIMP-129 Xe inelastic scattering [18].

The ZEPLIN program [19] uses a series ofxenon-based scintillator devices able to discrim-inate the background from the nuclear recoilsin liquid or liquid-gas detectors in various ways.Either using the Scintillation Pulse Shape ormeasuring the scintillation and the ionization(an electric field prevents recombination, thecharge being drifted to create a second scintil-lation pulse), and capitalizing on the fact thatthe primary (direct) scintillation pulse and thesecondary scintillation pulse amplitudes differfor electron recoils and nuclear recoils, the sec-ondary scintillation being smaller for nuclear re-coils. That feature provides a powerful back-ground rejection. The secondary scintillationphotons are produced by a proportional scintilla-tion process in liquid-xenon, as in the ZEPLIN-Idetector, (where a discrimination factor of 98% isachieved) or by electro luminescence photons ingas-xenon (as in the case of the ZEPLIN-II de-tector prototype) in which the electrons (ioniza-tion) drift to the gas phase where electrolumines-cence takes place (the discrimination factor being> 99%) [20]. Some prototypes leave been testedand various different projects of the ZEPLIN se-ries are underway [19,21] in Boulby. A recentrunning has provided a remarkable exclusion plot(see [11]) which traverses entirely the DAMA re-

gion. Table 5 is a sketch of the various xenonexperiments or projects.

3.4. Time projection chambersThere have been several efforts to develop de-

tectors sensitive to nuclear recoil direction: thelow-pressure TPC gas detectors (suggested inthe 80s by Spiro and Rich, now materialized inprojects like DRIFT, ITEP, etc.), the use of or-ganic crystals (anthracene), the ejection of atomsfrom surfaces, the detection of the recoiling sil-icon atoms in the surface layers of silicon chipsby means of thin film thermal detectors, rotonsin superfluid helium, imprints left by WIMPs inancient mica, and others.

DRIFT is a detector project sensitive to direc-tionality [22]. It uses a low-pressure (40 Torr)TPC filled with xenon to measure the nuclearrecoil track in WIMP-nucleus interactions. Thedirection and orientation of the nuclear recoilprovide a characteristic signature of the WIMP.The diffusion constrains the track length obser-vable but DRIFT reduces the diffusion (transver-sal and longitudinal) using negative ions to driftthe ionization instead of drift electrons: gasCS2 is added to capture electrons and so CS(−)

2

ions are drifted to the avalanche regions (wherethe electrons are released) for multiwire read-out(no magnetic field needed). The negative ionTPC has a millimetric diffusion an a millimet-ric track resolution. The proof-of-principle hasbeen performed in mini-DRIFTs, where the di-rection and orientation of nuclear recoils havebeen seen. The event reconstruction, the mea-surement of the track length and orientation, thedetermination of dE/dx and the ionization mea-surement permit a powerful background discrim-ination (99.9% gamma rejection and 95% alpharejection) leading to a rate sensitivity of R <10−2c/kg. day. DRIFT will permit recognitionof the forward/backward asymmetry and the nu-clear recoils angular distribution, which, as previ-ously noted, are the clearest distinctive signaturesof WIMPs. A DRIFT prototype of 1 m3 is run-ning at Boulby for test. A project of 10 m3 (Xe)scaling up the TPC of 1 m3 (Xe) is under way.


Table 5Xenon scintillation experiments

Experiment & Site Detector Mass EThr Meth. of Discr. eff./ BKG at Thr(keV) discrim Reject. factor before PSD

DAMA-Xe. LNGS Liquid Xe 6.5 kg 13 PSD 50%(13-15 keV) 0.895%(16-20 keV) c/keVkgd

ZEPLIN I. Boulby Liquid Xe 4 kg PSD NR from PSD10-20% of BKG

ZEPLIN II. Boulby Two-Phase Xe 1 kg → 30 kg 10 Scint/ioniz > 99% GoalIn preparation Liquid-Gas Low field (luminisc) 10−2 c/kgd

ZEPLIN III. Boulby Two-Phase Xe 6 kg Scint/ionizProject 2004 low threshold High field improved

ZEPLIN IV. Boulby Two-Phase 1 Ton Scint/ioniz GoalProject 2006 Liquid-Gas Extens. of Z-II 10−4c/kgd

X-MASS. Kamioka Liquid-Gas Xe 100 kg → 10 t natural Xe 99% (10-100 keV)XENON Proj. Liquid Xe 1 Ton 10 Scint/ioniz 99.5% 4 × 10−4

Columbia, Brown, sets of 100 kg c/keVkgdPrinceton, Rice,LLNL

Table 6Other techniques

Experiment Detector Goal Observationsand site and Mass

DRIFT. Boulby Xenon TPC R< 10−2,−3c/kg.d Low pressure (10-80 Torr) xenon negative ions TPC1m3 → 10m3 to detect nuclear recoil track. sensitive to directionality

SIMPLE. Rustrel Freon C2ClF5 Superheated droplet detector. Blind to low LET particles7×15g R115 σSD

p ∼ 10−1,−2pb Res. 1999 (0.19 kgd): σSDp ∼ 5 − 10pb; m∼(10-100 GeV)

2000: 25 kg.d

PICASSO. Sudbury 1.34 g Freon Superheated droplet detector(CCl2F2, C3F8,C4F10,C2ClF5)Res. (117 days): σSD

p ∼ 10pb; m∼(10-100 GeV)

3.5. Superheated droplet detectorsWIMP detectors, using the metastability of the

medium where the nuclear targets are embedded,are the superheated drop detectors (SDD) likeSIMPLE and PICASSO [23,24]. They consist ofa dispersion of droplets ( ∼ 10µm) of super-heated liquid (freon) in a gel matrix. The energydeposition of a WIMP in the droplets produces aphase transition from the superheated to normalstate causing vaporization of droplets into bub-bles ( ∼ 1mm), detected acoustically (the soundaccompanying vaporization is picked up by smallpiezoelectric transducer and amplified).

SSD are essentially insensitive to low LET par-ticles (e, γ, µ), and, thus, good for detectingWIMPs and neutrons. Bubbles can be produced

only for particles having large stopping power≥ 200keV/µm.

4. CONCLUSIONS AND OUTLOOK

The direct search for WIMP dark matter pro-ceeds at full strength. There are many expe-riments and projects on direct detection goingon. The new experiments are focusing in theidentification of WIMPs, discriminating the nu-clear recoils from the background (rather thanin constraining or excluding their parametersspace) and looking for distinctive WIMP signals.Large target mass experiments are now startingto be operative, looking for kinematical featuresof WIMPs not shared by the backgrounds.


Raw background rate levels achieved stand ataround a few tenths of counts per kg and day,still far from the main regions of the scatter ofpredictions ( 1-10−5 c/kg.day) from SUSY modelsimplemented in various alternative schemes. Asmall fraction of this window is testable by someof the leading experiments.

An unequivocal annual modulation effect wasreported by DAMA (seven yearly periods), whichhas been shown to be compatible (DAMA) witha neutralino-WIMP, in an extended, variable re-gion of m∼ 50 − 60GeV and σSi

n ∼ 7 × 10−6pb.Some experiments, using specific model frame-works, have gone below the DAMA-signal region,and the controversy in the confrontation with theDAMA-WIMP is open. Experiments sensitive toannual modulation (now in operation) will hope-fully contribute to clarify the situation.

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