- Email: [email protected]
Direct search for relic particles
PROCEEIMNGS SUPPLElVtENTS
Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
Direct search for relic particles David 0. CaldwelP’ %stitute for Nuclear and Particle Astrophysics and Cosmology and Physics Department, University of California, Santa Barbara, CA 93106-9530, U.S.A. One of the most important issues in fundamental physics is identifying the major component of the dark matter of the universe. Thus there has been a rapid expansion in techniques to pursue this goal, with about 27 experiments in various stages to look for dark matter particles by detecting the cold dark matter particle directly. While axion experiments, now producing useful limits, seek a unique signature, experiments to find WIMPS seek nuclear recoils via semiconductors, scintillators, a variety of cryogenic techniques, superheated droplets, and some visual means. Some of these experiments will be utilized here to illustrate features relevant to the search and give some idea of the progress and prospects of this vital task.
1. INTRODUCTION Just a few years ago reviewing the search for a cold dark matter particle was easy, since a very few groups were engaged in this pursuit and all used the same technique. Now the importance to astrophysics, particle physics, and cosmology of this endeavor has been recognized widely, and a remarkably large number of groups are involved, using a full panoply of approaches. The various techniques will be presented here, but rather than an encyclopedic covering of all experiments, representative ones will be used to illustrate the relative advantages of the different approaches, as well as to show the relevant issues in the search. In this process some idea of the present state of the searches will be given, although direct comparisons among experiments is often made difficult by a lack of uniformity in analysis of data and the presentation of results. Some indication will be given of the improvement in results to be expected. The search for the major component of the missing mass of the universe is focused on two types of particles, one quite specific and the other rather general. For the specific one, the axion, a particular signature is available so that its detection would be certain. The problem is that limits on its mass are quite wide, so that an extended search is required. The second type is a *Supported
in part by the U.S. Department
of Energy.
0920-5632/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PI1 SO920-5632(98)00386-7
Weakly Interacting Massive Particle, or WIMP, which could be anything that produces recoil energy when it hits a nucleus. Thus this search has the advantage that it is a broad one, accommodating many potential candidates, but the disadvantage that it is difficult to identify a positive signal and indeed to separate such from background effects. With some candidate particles, such as very massive neutrinos, eliminated by earlier experiments [l], the specific WIMP now used to set goals is the supersymmetric neutralino. The axion and neutralino share the advantage of being undiscovered particles whose conjectured existence has nothing to do with being dark matter but whose mass and abundance could allow them to be the major dark matter component. Of these two theoretically best motivated candidates, the neutralino has less well defined detection parameters. Its interactions could be mainly spin-dependent or spin-independent, with possible interaction cross sections varying over an uncomfortably wide range. 2. AXION
SEARCHES
The axion, which arises in models having the strong-CP problem solved by the Peccei-Quinn mechanism, would have a mass between 10m6 and 10e3 eV, as constrained by experimental and astrophysical limits. The lower limit would provide universe closure density, and despite this small
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
Axion couplings and masses exFigure 1. cluded at the 90% C.L. “This Work” is the LLNL/MIT/Florida/FNAL/LBNL/INR experiment, and earlier results are from Rochester/LBNL/FNAL (“RBF”) and University of Florida (“UF”). The KSVZ and DFSZ model predictions are also shown.
mass such a particle would be cold dark matter because axions would never have been in thermal equilibrium in the early universe. Since axions would couple to two photons, they could be detected by being converted to a single photon in the presence of a magnetic field. The Lawrence Livermore/M.I.T./Florida/ Fermilab/Lawrence Berkeley/I.N.R. Moscow experiment therefore uses a high-Q (- 2 x 105) cavity solenoid cooled to N 1.3”K in a superconducting of 7.6T. To cover a range of axion masses, the radial position in the cavity of two tuning rods is varied, and if an axion were detected there would be a peak of fractional width - 10m6 in the cavity power spectrum at a frequency corresponding to the axion mass. The signal is initially amplified by GaAs HEMT devices at the cavity temperature, since the signal power is only - 5 x 10-22W. This experiment is taking data, and at the time of writing it has excluded the mass range between 2.9 and 3.3 PeV for KSVZ axions at the 90% C.L., as shown in Fig. 1 [2]. Also shown there are results from two earlier experiments [3] and the added sensitivity needed to reach the other extreme axion model, designated “DFSZ”. In the near future, the experiment should search a substantial portion of the lower decade of the allowed
axion range at a sensitivity equal to or greater than the present results. Longer term plans call for achieving sensitivity to even the DFSZ axion by using DC SQUID amplifiers operating at 0.3”K. The experiment being done by the Kyoto group [4] also uses a tuned cavity in a 7T magnetic field, but it achieves a greater sensitivity by using a second tuned cavity into which the photon from the axion would go. An atomic beam puts rubidium atoms into the detection cavity, and these are excited by lasers so that they are in a very high state near ionization. These so-called Rydberg atoms can then be ionized by the photon from the axion. Both the conversion and detection cavities are cooled by a dilution refrigerator to O.Ol”K so that all background photons are reduced to less than the expected number of axion-converted photons. The initial search in Kyoto is centered around a 10peV axion mass at the DFSZ axion level. They hope to cover 15% of this mass region by the end of 1997. Funding has been obtained to install a bigger superconducting magnet and larger cavities by September 1998 so that they can begin a search for 2 to 30 PeV with a combination of the present and the new system over a period of 4 years. A quite different type of experiment is the search for solar axions by the Primakoff conversion of axions to photons in the Coulomb field of nuclei. Coherent conversion could occur when the incident angle from the sun fulfills the Bragg condition for a given crystalline plane of the germanium detector. A pilot experiment [5] has been carried out by the SOLAX collaboration (PNL/USC/Zaragoza/TANDAR) using a lkg Ge crystal in the Sierra Grande mine in Argentina. Another Ge detector is currently taking data for the same purpose in the Canfranc tunnel in Spain. Results will be found elsewhere in the Proceedings. 3. SEARCH
FOR WIMPS WITH CONDUCTOR DETECTORS
SEMI-
The first searches for WIMPS were carried out as extensions of double beta decay experiments
D.O. CaldweN/Nuclear Physics B (Pmt. Suppl.) 70 (1999) 43-53
using Ge semiconductor detectors [l]. A second generation of such experiments uses enriched 76Ge, which reduces backgrounds caused by cosmogenic activation of other isotopes in normal Ge. It cannot be emphasized too strongly that the primary issue in the direct search for WIMPS is background reduction. This point, and also the effect of threshold, are illustrated in Fig. 2. Figure 2(a) shows the energy spectra in the relevant region for WIMP detection from different detectors of the PNL/USC/Zaragoza group [6]. The Cosme detector has a lower threshold, but the Twin detector has a lower background. Thus in the exclusion plot of Fig. 2(b), the Cosme detector (dashed line) is more sensitive only in the very low mass region, whereas the Twin detector (dot-dashed line) is far more sensitive over most of the mass range. Those results are for spin-independent interactions, since 76Ge is a nucleus with zero spin, and hence comparison is made (solid line) with the weak interaction cross section for Dirac particles. Also shown in Fig. 2(b) are results from the Heidelberg-Moscow group (dotted line), the other experiment which uses enriched 76Ge [7]. This group is now testing a prototype for an experiment in which an n-type enriched Ge crystal will be almost entirely surrounded by a ptype natural Ge detector in anticoincidence [8]. All such experiments use surrounding shields which are usually passive, although NaI scintillators have been used as an active shield, but Ge can be made of such high purity that it should provide an impressive reduction in backgrounds. Far more ambitious is Heidelberg’s proposed GENIUS experiment [9], which would use 300 76Ge detectors (1 ton total) buried in a 7m diameter by 7m high dewar of liquid nitrogen! These detectors always operate at liquid nitrogen temperatures, and the material should be a good passive shield. In addition, such multiple arrays of Ge detectors have provided considerable self-shielding, since the nuclear recoils can occur in only one detector, whereas many background events record in more than one.
45
10
5
deposited
15
energy (keV)
(4
Mass (GeV)
M Figure 2. (a) Energy spectra for two Ge detectors of the PNL/USC/Zaragoza group. (b) Exclusion plot for spin-independent WIMP couplings for the two detectors, showing the effects of energy threshold and background level. The COSME detector (dashed line) has a lower threshold, but the Twin detector (dot-dashed line) has a lower background. Also shown are the limit (dotted line) from the other enriched 76Ge experiment (Heidelberg/Moscow) and the cross section (solid line) for a Dirac neutrino.
D.O. CaldweN/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
46
. backgrounddata
0.08
0.06 0.04 0.02
n ”
0
Figure of the events pulses tion.
100
200
300
time constant(ns)
400
500
3. Pulse shapes from a 6 kg NaI detector UK Collaboration for neutron and gamma compared with the averaged shape of data to provide some background discrimina-
4. WIMP TORS
SEARCH
WITH
SCINTILLA-
Scintillation detectors have recently come into prominence in the search for cold dark matWhile zone-refined Ge is exter candidates. tremely radiopure, scintillating materials have not had that advantage, nor do they have nearly so good intrinsic resolution, making achieving as low thresholds difficult. The advantages of scintillators are the choice of materials (e.g., a nucleus with spin), and cost, hence mass. By strong efforts the experimenters have managed to improve greatly the radiopurity of the materials and even the thresholds. While the use of pulse-shape analysis has been employed in Ge double beta decay experiments, it cannot be utilized as fully in the much lower energy dark matter regime where the statistics of the pulses makes the separation of signal and background far more difficult. Pushing the technique further than distinguishing against noise pulses, it may be used to provide some particle discrimination, not on an event-by-event basis, but statistically over many events. This latter use has been more successfully employed in the caSe of Na.I detectors, as illustrated in Fig. 3. This figure
shows typical pulse shapes produced in a 6 kg NaI detector operated by the United Kingdom Collaboration (Imperial, Rutherford, Sheffield, Birback, and Nottingham) [lo]. The sought nuclear recoils would look like the pulse produced by neutrons, whereas the most common background is that produced by electrons, here labeled by their gamma source. By seeing how much of the averaged shape of the data pulses could represent some fraction of the events being neutron-like as opposed to gamma-like, an order of magnitude improvement in sensitivity is achieved. To do this requires frequent calibrations and considerable effort in maintaining pulse stability. The latter requires, for example, temperature stability, since the rise time changes by 5 nanoseconds/degree K. This means of reducing backgrounds and the relatively large mass of the scintillators has made this technique competitive with Ge for spinindependent interactions. Indeed, the NaI limits from particularly the DAMA group (Rome, Beijing, and Frascati) [ll] surpass Ge limits for most masses. With Na and I both possessing spin, obviously the NaI does much better than earlier Ge experiments which had only 8% of an isotope with spin. Even for Majorana neutralinos, at the present and near future level of detector sensitivity, the spin-independent interactions are the important ones, and there is no known candidate the limits on spin-dependent interactions are testing. Possibly for the future the development of detectors with spin is important. For this reason there is work with the scintillator CaF2, as F has the best matrix element for spin-dependent interactions. Thus the DAMA group [12] has been trying to improve radiopurity and has tested a 0.37 kg prototype, while the Osaka group [13] has employed 3.2 kg of CaF2, getting better spindependent limits than they have with NaI. The Rome/Beijing group has achieved its best spindependent limits above 50 GeV WIMP mass with a 6.5 kg liquid xenon detector enriched to 99.5% in 12gXe [14]. The DAMA limits for both spindependent and spin-independent couplings are shown in Fig. 4 in comparison with some other results discussed above. Xenon is a particularly
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
41
Mass (GeV) Figure 5. Improvement in the spin-independent WIMP exclusion plot (dashed line) obtained by using the non-observance of an annual modulation (solid line). Two Junes minus two Decembers from 1343 kg.y of NaI data were used by the Zaragoza group.
Figure 4. WIMP exclusion plots for A) spindependent and B) spin-independent interactions for some experiments mentioned in the text. Note particularly the Xe, NaI, and CaFs limits, mainly from the DAMA group.
promising scintillator, since tests by the U.K. group and the ICARUS collaboration [15] show the potential for better particle discrimination than is the case for NaI, because states are excited differently by electrons and nuclear recoils, and these can be registered by scintillation vs. ionization, for example. The large mass of scintillators aids directly in setting limits, but also it can be useful in improving those limits by looking for effects of the annual modulation of the expected WIMP signal. Part of the year the earth’s motion around the sun has a component in the direction of the
solar system’s motion in the galaxy, and hence through the WIMP halo, and part of the year this component is opposite to the solar motion. Thus the WIMP flux has a time dependence, the absence of which can improve limits. Since this is a small effect, good statistics and hence large mass is necessary, and the Osaka group [16] has used 547 kg of NaI, the DAMA group 115 kg, and the Zaragoza group 32 kg, although they are now going to 150 kg. Figure 5 illustrates the sort of advantage using annual modulation can provide. This result is from the Zaragoza group, who (with PNL/USC/TANDAR) have also tried using diurnal modulation, which is not so big an effect, even in favored geographical locations [17]. Annual modulation is the source of an indication for a WIMP signal, as seen by the DAMA group [IS]. Using nine 9.7 kg NaI detectors, providing 3364 kgadays exposure in the winter and 1185 kg-days in the summer, they report an allowed region at the 90% C.L. for a spinindependent candidate of mass 59:;: GeV at a cross section on protons of (l.Of$:)lO-” pb, if the WIMP provides a local halo density of 0.3 GeVcmm3. While this result has been criticized
48
D.O. Caldwell/Nuclear Physics B (Pmt. Suppl.) 70 (1999) 43-53
[19], it has also been interpreted [20] in terms of being a neutralino. The DAMA experiment is obtaining more data, and they have certainly provided an added incentive for other experiments. 5. WIMP SEARCH WITH DETECTORS
cq
Woods-Saxon Form Factor
1 10”
5.
CRYOGENIC
One advantage of scintillators which is shared by cryogenic detectors is a wide choice of materials, in the cryogenic case even extending to helium (the HERON experiment at Brown University [21]). Besides the choice of materials with and without spin, and in the latter case particularly choosing the more favorable matrix elements, there are several other material dependent issues. First, the maximum recoil energy is obtained when the mass of the nucleus matches the mass of the WIMP. At this stage one can utilize this only to emphasize a mass region, but it is important to note that these low recoil energies are hard to detect, and that backgrounds generally increase as the energy decreases, so having more energy available to measure can have a big effect. Second, for spin-independent interactions the rate is proportional to the square of the number of neutrons in the nucleus for lighter WIMPS; for heavier masses there is loss of this coherence. This favors heavier targets for some of the WIMP mass range. Third, a lighter target is favored by the nuclear form factor, as illustrated in Fig. 6. The form factor effect increases with increasing recoil energy, another reason why being able to measure lower recoil energies is important. Fourth, there is the quenching factor, how much of the recoil energy is detectable. For scintillators this is defined as the ratio of light from nuclear recoils to light from the same energy electrons. In a cryogenic detector nearly all the recoil energy ends up as phonons, whereas in a semiconductor or a scintillator only a fractionoften a small fraction-of that energy is detected when the recoil energy is small. One has to be very careful in looking at experimental spectra to notice whether detected energy or recoil energy (“equivalent electron energy”) is being displayed. When the quenching factor is small it is very difficult to reach low recoil energies, with the atten-
I
\\
\
\
‘\ ‘\
‘\. Xe
\
or I
\
0
150 100 50 Recoil Energy Q [keV]
Go
Figure 6. Woods-Saxon coherent nuclear form factors for Si, Ge, and Xe (or I) as a function of recoil energy.
dant problems noted above. Quenching factors are not easy to measure, usually involving neutron scattering experiments, and for some materials there is controversy over result. Typical numbers are 0.30 for Na and 0.09 for I. On the other hand, the Milan group, which was the first to use a cryogenic detector, has measured a quenching factor of 1.025 f 0.01 f 0.02 over the 13-170 keV range using a TeOs crystal at O.Ol”K [22]. This group will soon have the largest cryogenic array of 20, 340g TeOs crystals. The main emphasis of this work has been on double beta decay. Getting to low recoil energy is important for WIMP masses below roughly a TeV, as illustrated in Fig. 7, in which for Si and Ge the effect of three threshold energies is shown in an exclusion plot. It will also be noted there that except for very low masses Ge is more effective than Si, so this illustrates three of the issues discussed above, and also emphasizes the importance of the fourth, preferably having unity quenching factor.
49
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
-7
---
GeDnde
-
Nal Rome
w-
ilO-43i
SiO,Oldr>;~Q&30keV b
t
I
-
1 .-.-
Ge 0.01 dru 2. lo’&30
keV
MSSmodels
Cl
12
3 Energy
10‘ WIMP Mass [GeV]
Figure 7. Limits that can be set with Ge and Si detectors as a function of threshold energy, if the background is 0.01 dru (differential rate unit, or counts/keV.kg.day) in both cases. Also shown are best current limits from Ge and NaI, as well as an envelope of possible neutralino model predictions.
The leading experiment for achieving low threshold is CRESST, a Munich/Oxford/LNGS Operating in Gran Sasso, collaboration [23]. they are using four 262g sapphire (AlsOs) crystals with tungsten superconducting phase transition thermometers. The thermometer transition curve has a long linear range in resistance so that the SQUID output voltage varies linearly with temperature rise and hence with the energy deposited. This provides a threshold of a little less than 0.5 keV and corresponding excellent energy resolution of 250 eV FWHM for 1.5 keV x-rays, as shown in Fig. 8. Since only light nuclei are involved, these detectors are sensitive primarily to very light WIMPS. The ROSEBUD experiment of the Orsay/ Paris/Zaragoza group [24] also uses sapphire and has achieved even better resolution of 120 eV FWHM at 1.5 keV, but this is in an order of magnitude smaller crystal. They read out their two, 25g crystals (now going to four crystals) with NTD (neutron transmutation-doped) ther-
4
5
6
7
LkeVl
Figure 8. Energy spectrum from a 262 g cryogenic sapphire detector of the CRESST experiment.
mistors, the resistance of which changes with the rise in temperature caused by an energy deposit in the crystal. At the time of writing this experiment is starting to run in the Canfranc tunnel in Spain. The use of F because of its most favorable matrix element for spin-dependent interactions is not limited to scintillators. The Tokyo group [25] has eight, 21g LiF crystals operating as bolometers. As with the other cryogenic detectors discussed so far, an energy deposit in the crystal, AE, produces a temperature rise AT = AEIC, where the heat capacity at low temperatures (T) is C a T3M. To get a measurable AT in a large mass M therefore requires a very low T. Thus all of these detectors use dilution refrigerators, and it is a basic problem to couple this large, complex refrigerator to the crystal thermally while providing sufficient shielding of the crystal from radioactivity or the effect of cosmic rays. In the CDMS experiment [26] of the U.C. Berkeley (CfPA)/U.C. Santa Barbara/Stanford/ Case Western/San Francisco State/Santa Clara/ Fermilab/Baksan collaboration the refrigerator has an unusual side access so that the cubic foot of cooled region can be located sufficiently far away to allow a complete active and passive shield to surround it. The complex linkage enters the
50
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
0
20
60 40 Recoil Energy PS 1 ReVI
80
f
100
Figure 9. Scatter plot of ionization measurement versus recoil energy measurement (phonons) for a 62 g Ge cryogenic detector exposed to neutrons and photons from a 252Cf source obtained in a calibration run of the CDMS experiment. The ionization measurement is normalized to electron equivalent energy. The shaded line is a fit to the region of nuclear recoil events.
shield in such a way as to avoid exposure of the detectors to the exterior. The CDMS experiment merits description mainly because of the central issue of background rejection. While such rejection has been demonstrated in scintillators on a statistical basis, improving sensitivity by an order of magnitude, the CDMS experiment has shown background rejection on a single pulse basis, providing at least two orders of magnitude improvement of sensitivity against electron backgrounds. This is achieved by measuring not only the temperature rise, but also the ionization produced by a particle. Nuclear recoils give mainly phonons, whereas electron interactions ionize efficiently, as shown in Fig. 9, where a Ge detector read out with NTD thermistors records neutrons and photons from a 252Cf source. Similar particle identification is obtained with Si
10”
_ 10”
-B--g
CDMSRun14SilOOgwith0.52kgd CDMSRun15616Ge165gwith1.74kgd .“‘..‘_ P .‘.“‘.‘_ 10’ 10” WIMP &s [Gev]
“..“m_
. .“,10’
Figure 10. Existing limits set by previous experiments, as well as the first limits from short runs of the CDMS experiment with a 30-keV noise threshold for Si and an 18-keV background threshold for Ge.
detectors read out by superconducting tungsten transition edge sensors. The latter measure athermal phonons on microsecond time scales versus the millisecond time scale for thermalization required by NTD readout. The athermal phonon timing is fast enough to image the event location in the crystal, allowing definition of a fiducial volume away from surfaces where contamination is more likely. This technique is now being applied to Ge and should be the main type of detector in future CDMS runs. At present, because these new techniques needed to be tried in a convenient location, CDMS I is in a tunnel on the Stanford campus with only a 17 m.w.e. overburden, requiring efficient active and passive shielding. So far problems with inefficiency in ionization collection for electrons in a thin dead layer at the surface of the Ge detectors, some 3H contamination from the NTD’s, and high threshold for the Si detec-
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10"
10'
10'
IO”
10'
WIMP Mass [GeV]
Goals for CDMS-I at the StanFigure 11. ford site (0.01 events/keVqkg.day with 2-keV threshold for Ge), for CDMS-II at Soudan (0.0003 events/keV.kgday with 2-keV threshold for Ge), and for CRESST at Gran Sass0 0.2 events/keV.kg.day with 0.5keV threshold for sapphire). The best current limits are shown for comparison, along with the envelope of some neutralino model predictions, the peak of which corresponds to 3 events/kg-day for Ge.
tors provide poorer than expected exclusion plots for short runs of the experiment. The Si threshold has been improved by an order of magnitude, and it is believed that the Ge problems have been solved also, although no confirming data yet exists. These earlier results are shown in Fig. 10, along with results from many of the experiments described above. If these problems are solved, longer runs will be taken with more 165g Ge and 1OOgSi detectors to approach the CDMS I goal shown in Fig. 11. Instead of a kg-scale experiment at Stanford, CDMS II is to be a 10-20 kg-size experiment at the deep Soudan Mine in Minnesota, for which the goal is also shown in Fig. 11, along with one version of neutralino parameter space
and the goal of the CRESST experiment. The Edelweiss experiment [27] of Saclay/ Orsay/Lyon/Modane/Paris uses a similar Ge-NTD technique and is already at a deep site (Modane). Their 70g Ge detector has a similar surface dead layer problem. A totally different kind of cryogenic experiment is the use of Superheated Superconducting Granules (SSG). An ensemble of spherical superconducting grains of a few pm diameter are placed in a magnetic field. With the temperature adjusted to put the grains in a superheated superconducting state near normal, the addition of a small amount of energy deposited by a particle in a grain will drive the grain normal, producing a change in the flux distribution, since the field is no longer excluded by the grain. This grain flip is read out by a magnetometer. The ORPHEUS experimental group [28] (Bern/PSI/Clausthal) has tested the sensitivity of a small detector to y, p, and nuclear recoils and plans to use a kg of 20pm Sn granules with a 10% filling factor in teflon. The SALOPARD (Paris/Lisbon/Zaragoza) experiment [29] will start as a feasibility study in the Canfranc tunnel in the Spring of 1998 and is planned to have 1OOgof 1Opm Sn spheres with a 20% filling factor. Low thresholds are expected (< 2 keV), and there is evidence for perhaps a factor of 20 in background rejection from the number of flipped grains. 6. OTHER TORS
WIMP
SEARCH
DETEC-
While SSG detectors were one of the earliest types proposed, there is a very new technique which sounds similar but is quite different, the superheated droplet detector. This is based on commercial neutron dosimeters, operating at ambient temperatures and pressures. The detector consists of superheated droplets (lo-100pm) of freon in a gel. Rather like a bubble chamber, these droplets expand to bubbles of N lmm when there is a nuclear recoil, a process which can be picked up by a microphone. The parameters can be set so that the droplets are insensitive to y’s, /3’s, or p’s, and even low-energy neutrons, providing excellent background rejection. Furthermore,
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large detector masses seem feasible, even including such possibly desirable ingredients as fluorine. Whether sufficiently heavy nuclei can be included is yet to be demonstrated. Two groups are testing small detectors, CERN/Lisbon/Paris [30] and Montreal/Chalk River [31]. Another technique insensitive to light particle backgrounds is the use of ancient mica, utilized first for a WIMP search at U.C. Berkeley. Even (Yparticle recoils can be distinguished by the length of the pit formed when tracks are etched. The etched tracks are detected using an atomic-force microscope. While very little mass can be used, mica lo9 years old can be searched, providing a very long WIMP exposure time. The limiting background is from fission neutrons, and it has been proposed [32] to overcome this background by using the directionality of the WIMP tracks. Since WIMPS enter the mica from a preferred direction because of our motion around the galaxy, backgrounds can be reduced statistically. There is other work on directional detectors, the most straightforward of which are visual detectors like a TPC [33]. So far all the techniques for what could be the next generation of detectors suffer from insufficient mass and background problems. With such a blossoming of approaches and so many people entering this field, one hopes that discovery will come soon. Otherwise there will have to be ever increasing cleverness, as this hunt for most of the mass of the universe must be brought to a successful conclusion.
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14. 15.
ACKNOWLEDGMENTS I am grateful to the following experimenters for information and figures: Frank Avignone, Daniel Bauer, R,ita Bernabei, Bias Cabrera, Juan Collar, Susan Cooper, Hiroyasu Ejiri, Eduardo Garcia, Gilles Gerbier, H.V. Klapdor-Kleingrothaus, Robert Lanou, Seishi Matsuki, Makoto Minowa, Angel Morales, Klaus Pretzl, Ezio Previtali, Leslie Rosenberg, Daniel Snowden-Ifft, Neil Spooner, and Karl von Bibber.
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Physics B (Proc. Suppl.) 70 (1999) 43-53
9710295 (1997, to be published). 21. J.S. Adams et al., The Identification of Dark Matter (N.J.C. Spooner ed.) World Scientific, Singapore (1997) 469. 22. A. Alessandrello et al., Phys. Lett. B384 (1996) 316;E. Previtali, these Proceedings. 23. L. Zerle, these Proceedings. 24. N. Coron, these Proceedings. 25. W. Ootani et al., Tokyo University preprint hep-ex/9709019 (1997, to be published); Y. Ito et al., Nucl. Instr. Methods Phys. Res. A386 (1997) 439. 26. D.S. Akerib, these Proceedings. 27. P. DiStefano, these Proceedings. 28. K. Pretzl, these Proceedings; G. Czapek et al., The Identification of Dark Matter (N.J.C. Spooner ed.) World Scientific, Singapore (1997) 434. 29. T.A. Girard et al., The Identification of Dark Matter (N.J.C. Spooner ed.) World Scientific, Singapore (1997) 456. 30. J.I. Collar et al., The Identification of Dark Matter (N.J.C. Spooner ed.) World Scientific, Singapore (1997) 563. 31. L.A. Hamel et al., The Identification of Dark Matter (N.J.C. Spooner ed.) World Scientific, Singapore (1997) 569. 32. D.P. Snowden-Ifft and A.J. Westphal, Phys. Rev. Lett. 78 (1997) 1628. 33. K.N. Buckland, M.J. Lehner, and G.E. Masek, The Identification of Dark Matter (N.J.C. Spooner ed.) World Scientific, Singapore (1997) 475; K.N. Buckland et al., Phys. Rev. Lett. 73 (1994) 1067.
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Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
Direct search for relic particles David 0. CaldwelP’ %stitute for Nuclear and Particle Astrophysics and Cosmology and Physics Department, University of California, Santa Barbara, CA 93106-9530, U.S.A. One of the most important issues in fundamental physics is identifying the major component of the dark matter of the universe. Thus there has been a rapid expansion in techniques to pursue this goal, with about 27 experiments in various stages to look for dark matter particles by detecting the cold dark matter particle directly. While axion experiments, now producing useful limits, seek a unique signature, experiments to find WIMPS seek nuclear recoils via semiconductors, scintillators, a variety of cryogenic techniques, superheated droplets, and some visual means. Some of these experiments will be utilized here to illustrate features relevant to the search and give some idea of the progress and prospects of this vital task.
1. INTRODUCTION Just a few years ago reviewing the search for a cold dark matter particle was easy, since a very few groups were engaged in this pursuit and all used the same technique. Now the importance to astrophysics, particle physics, and cosmology of this endeavor has been recognized widely, and a remarkably large number of groups are involved, using a full panoply of approaches. The various techniques will be presented here, but rather than an encyclopedic covering of all experiments, representative ones will be used to illustrate the relative advantages of the different approaches, as well as to show the relevant issues in the search. In this process some idea of the present state of the searches will be given, although direct comparisons among experiments is often made difficult by a lack of uniformity in analysis of data and the presentation of results. Some indication will be given of the improvement in results to be expected. The search for the major component of the missing mass of the universe is focused on two types of particles, one quite specific and the other rather general. For the specific one, the axion, a particular signature is available so that its detection would be certain. The problem is that limits on its mass are quite wide, so that an extended search is required. The second type is a *Supported
in part by the U.S. Department
of Energy.
0920-5632/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PI1 SO920-5632(98)00386-7
Weakly Interacting Massive Particle, or WIMP, which could be anything that produces recoil energy when it hits a nucleus. Thus this search has the advantage that it is a broad one, accommodating many potential candidates, but the disadvantage that it is difficult to identify a positive signal and indeed to separate such from background effects. With some candidate particles, such as very massive neutrinos, eliminated by earlier experiments [l], the specific WIMP now used to set goals is the supersymmetric neutralino. The axion and neutralino share the advantage of being undiscovered particles whose conjectured existence has nothing to do with being dark matter but whose mass and abundance could allow them to be the major dark matter component. Of these two theoretically best motivated candidates, the neutralino has less well defined detection parameters. Its interactions could be mainly spin-dependent or spin-independent, with possible interaction cross sections varying over an uncomfortably wide range. 2. AXION
SEARCHES
The axion, which arises in models having the strong-CP problem solved by the Peccei-Quinn mechanism, would have a mass between 10m6 and 10e3 eV, as constrained by experimental and astrophysical limits. The lower limit would provide universe closure density, and despite this small
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
Axion couplings and masses exFigure 1. cluded at the 90% C.L. “This Work” is the LLNL/MIT/Florida/FNAL/LBNL/INR experiment, and earlier results are from Rochester/LBNL/FNAL (“RBF”) and University of Florida (“UF”). The KSVZ and DFSZ model predictions are also shown.
mass such a particle would be cold dark matter because axions would never have been in thermal equilibrium in the early universe. Since axions would couple to two photons, they could be detected by being converted to a single photon in the presence of a magnetic field. The Lawrence Livermore/M.I.T./Florida/ Fermilab/Lawrence Berkeley/I.N.R. Moscow experiment therefore uses a high-Q (- 2 x 105) cavity solenoid cooled to N 1.3”K in a superconducting of 7.6T. To cover a range of axion masses, the radial position in the cavity of two tuning rods is varied, and if an axion were detected there would be a peak of fractional width - 10m6 in the cavity power spectrum at a frequency corresponding to the axion mass. The signal is initially amplified by GaAs HEMT devices at the cavity temperature, since the signal power is only - 5 x 10-22W. This experiment is taking data, and at the time of writing it has excluded the mass range between 2.9 and 3.3 PeV for KSVZ axions at the 90% C.L., as shown in Fig. 1 [2]. Also shown there are results from two earlier experiments [3] and the added sensitivity needed to reach the other extreme axion model, designated “DFSZ”. In the near future, the experiment should search a substantial portion of the lower decade of the allowed
axion range at a sensitivity equal to or greater than the present results. Longer term plans call for achieving sensitivity to even the DFSZ axion by using DC SQUID amplifiers operating at 0.3”K. The experiment being done by the Kyoto group [4] also uses a tuned cavity in a 7T magnetic field, but it achieves a greater sensitivity by using a second tuned cavity into which the photon from the axion would go. An atomic beam puts rubidium atoms into the detection cavity, and these are excited by lasers so that they are in a very high state near ionization. These so-called Rydberg atoms can then be ionized by the photon from the axion. Both the conversion and detection cavities are cooled by a dilution refrigerator to O.Ol”K so that all background photons are reduced to less than the expected number of axion-converted photons. The initial search in Kyoto is centered around a 10peV axion mass at the DFSZ axion level. They hope to cover 15% of this mass region by the end of 1997. Funding has been obtained to install a bigger superconducting magnet and larger cavities by September 1998 so that they can begin a search for 2 to 30 PeV with a combination of the present and the new system over a period of 4 years. A quite different type of experiment is the search for solar axions by the Primakoff conversion of axions to photons in the Coulomb field of nuclei. Coherent conversion could occur when the incident angle from the sun fulfills the Bragg condition for a given crystalline plane of the germanium detector. A pilot experiment [5] has been carried out by the SOLAX collaboration (PNL/USC/Zaragoza/TANDAR) using a lkg Ge crystal in the Sierra Grande mine in Argentina. Another Ge detector is currently taking data for the same purpose in the Canfranc tunnel in Spain. Results will be found elsewhere in the Proceedings. 3. SEARCH
FOR WIMPS WITH CONDUCTOR DETECTORS
SEMI-
The first searches for WIMPS were carried out as extensions of double beta decay experiments
D.O. CaldweN/Nuclear Physics B (Pmt. Suppl.) 70 (1999) 43-53
using Ge semiconductor detectors [l]. A second generation of such experiments uses enriched 76Ge, which reduces backgrounds caused by cosmogenic activation of other isotopes in normal Ge. It cannot be emphasized too strongly that the primary issue in the direct search for WIMPS is background reduction. This point, and also the effect of threshold, are illustrated in Fig. 2. Figure 2(a) shows the energy spectra in the relevant region for WIMP detection from different detectors of the PNL/USC/Zaragoza group [6]. The Cosme detector has a lower threshold, but the Twin detector has a lower background. Thus in the exclusion plot of Fig. 2(b), the Cosme detector (dashed line) is more sensitive only in the very low mass region, whereas the Twin detector (dot-dashed line) is far more sensitive over most of the mass range. Those results are for spin-independent interactions, since 76Ge is a nucleus with zero spin, and hence comparison is made (solid line) with the weak interaction cross section for Dirac particles. Also shown in Fig. 2(b) are results from the Heidelberg-Moscow group (dotted line), the other experiment which uses enriched 76Ge [7]. This group is now testing a prototype for an experiment in which an n-type enriched Ge crystal will be almost entirely surrounded by a ptype natural Ge detector in anticoincidence [8]. All such experiments use surrounding shields which are usually passive, although NaI scintillators have been used as an active shield, but Ge can be made of such high purity that it should provide an impressive reduction in backgrounds. Far more ambitious is Heidelberg’s proposed GENIUS experiment [9], which would use 300 76Ge detectors (1 ton total) buried in a 7m diameter by 7m high dewar of liquid nitrogen! These detectors always operate at liquid nitrogen temperatures, and the material should be a good passive shield. In addition, such multiple arrays of Ge detectors have provided considerable self-shielding, since the nuclear recoils can occur in only one detector, whereas many background events record in more than one.
45
10
5
deposited
15
energy (keV)
(4
Mass (GeV)
M Figure 2. (a) Energy spectra for two Ge detectors of the PNL/USC/Zaragoza group. (b) Exclusion plot for spin-independent WIMP couplings for the two detectors, showing the effects of energy threshold and background level. The COSME detector (dashed line) has a lower threshold, but the Twin detector (dot-dashed line) has a lower background. Also shown are the limit (dotted line) from the other enriched 76Ge experiment (Heidelberg/Moscow) and the cross section (solid line) for a Dirac neutrino.
D.O. CaldweN/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
46
. backgrounddata
0.08
0.06 0.04 0.02
n ”
0
Figure of the events pulses tion.
100
200
300
time constant(ns)
400
500
3. Pulse shapes from a 6 kg NaI detector UK Collaboration for neutron and gamma compared with the averaged shape of data to provide some background discrimina-
4. WIMP TORS
SEARCH
WITH
SCINTILLA-
Scintillation detectors have recently come into prominence in the search for cold dark matWhile zone-refined Ge is exter candidates. tremely radiopure, scintillating materials have not had that advantage, nor do they have nearly so good intrinsic resolution, making achieving as low thresholds difficult. The advantages of scintillators are the choice of materials (e.g., a nucleus with spin), and cost, hence mass. By strong efforts the experimenters have managed to improve greatly the radiopurity of the materials and even the thresholds. While the use of pulse-shape analysis has been employed in Ge double beta decay experiments, it cannot be utilized as fully in the much lower energy dark matter regime where the statistics of the pulses makes the separation of signal and background far more difficult. Pushing the technique further than distinguishing against noise pulses, it may be used to provide some particle discrimination, not on an event-by-event basis, but statistically over many events. This latter use has been more successfully employed in the caSe of Na.I detectors, as illustrated in Fig. 3. This figure
shows typical pulse shapes produced in a 6 kg NaI detector operated by the United Kingdom Collaboration (Imperial, Rutherford, Sheffield, Birback, and Nottingham) [lo]. The sought nuclear recoils would look like the pulse produced by neutrons, whereas the most common background is that produced by electrons, here labeled by their gamma source. By seeing how much of the averaged shape of the data pulses could represent some fraction of the events being neutron-like as opposed to gamma-like, an order of magnitude improvement in sensitivity is achieved. To do this requires frequent calibrations and considerable effort in maintaining pulse stability. The latter requires, for example, temperature stability, since the rise time changes by 5 nanoseconds/degree K. This means of reducing backgrounds and the relatively large mass of the scintillators has made this technique competitive with Ge for spinindependent interactions. Indeed, the NaI limits from particularly the DAMA group (Rome, Beijing, and Frascati) [ll] surpass Ge limits for most masses. With Na and I both possessing spin, obviously the NaI does much better than earlier Ge experiments which had only 8% of an isotope with spin. Even for Majorana neutralinos, at the present and near future level of detector sensitivity, the spin-independent interactions are the important ones, and there is no known candidate the limits on spin-dependent interactions are testing. Possibly for the future the development of detectors with spin is important. For this reason there is work with the scintillator CaF2, as F has the best matrix element for spin-dependent interactions. Thus the DAMA group [12] has been trying to improve radiopurity and has tested a 0.37 kg prototype, while the Osaka group [13] has employed 3.2 kg of CaF2, getting better spindependent limits than they have with NaI. The Rome/Beijing group has achieved its best spindependent limits above 50 GeV WIMP mass with a 6.5 kg liquid xenon detector enriched to 99.5% in 12gXe [14]. The DAMA limits for both spindependent and spin-independent couplings are shown in Fig. 4 in comparison with some other results discussed above. Xenon is a particularly
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
41
Mass (GeV) Figure 5. Improvement in the spin-independent WIMP exclusion plot (dashed line) obtained by using the non-observance of an annual modulation (solid line). Two Junes minus two Decembers from 1343 kg.y of NaI data were used by the Zaragoza group.
Figure 4. WIMP exclusion plots for A) spindependent and B) spin-independent interactions for some experiments mentioned in the text. Note particularly the Xe, NaI, and CaFs limits, mainly from the DAMA group.
promising scintillator, since tests by the U.K. group and the ICARUS collaboration [15] show the potential for better particle discrimination than is the case for NaI, because states are excited differently by electrons and nuclear recoils, and these can be registered by scintillation vs. ionization, for example. The large mass of scintillators aids directly in setting limits, but also it can be useful in improving those limits by looking for effects of the annual modulation of the expected WIMP signal. Part of the year the earth’s motion around the sun has a component in the direction of the
solar system’s motion in the galaxy, and hence through the WIMP halo, and part of the year this component is opposite to the solar motion. Thus the WIMP flux has a time dependence, the absence of which can improve limits. Since this is a small effect, good statistics and hence large mass is necessary, and the Osaka group [16] has used 547 kg of NaI, the DAMA group 115 kg, and the Zaragoza group 32 kg, although they are now going to 150 kg. Figure 5 illustrates the sort of advantage using annual modulation can provide. This result is from the Zaragoza group, who (with PNL/USC/TANDAR) have also tried using diurnal modulation, which is not so big an effect, even in favored geographical locations [17]. Annual modulation is the source of an indication for a WIMP signal, as seen by the DAMA group [IS]. Using nine 9.7 kg NaI detectors, providing 3364 kgadays exposure in the winter and 1185 kg-days in the summer, they report an allowed region at the 90% C.L. for a spinindependent candidate of mass 59:;: GeV at a cross section on protons of (l.Of$:)lO-” pb, if the WIMP provides a local halo density of 0.3 GeVcmm3. While this result has been criticized
48
D.O. Caldwell/Nuclear Physics B (Pmt. Suppl.) 70 (1999) 43-53
[19], it has also been interpreted [20] in terms of being a neutralino. The DAMA experiment is obtaining more data, and they have certainly provided an added incentive for other experiments. 5. WIMP SEARCH WITH DETECTORS
cq
Woods-Saxon Form Factor
1 10”
5.
CRYOGENIC
One advantage of scintillators which is shared by cryogenic detectors is a wide choice of materials, in the cryogenic case even extending to helium (the HERON experiment at Brown University [21]). Besides the choice of materials with and without spin, and in the latter case particularly choosing the more favorable matrix elements, there are several other material dependent issues. First, the maximum recoil energy is obtained when the mass of the nucleus matches the mass of the WIMP. At this stage one can utilize this only to emphasize a mass region, but it is important to note that these low recoil energies are hard to detect, and that backgrounds generally increase as the energy decreases, so having more energy available to measure can have a big effect. Second, for spin-independent interactions the rate is proportional to the square of the number of neutrons in the nucleus for lighter WIMPS; for heavier masses there is loss of this coherence. This favors heavier targets for some of the WIMP mass range. Third, a lighter target is favored by the nuclear form factor, as illustrated in Fig. 6. The form factor effect increases with increasing recoil energy, another reason why being able to measure lower recoil energies is important. Fourth, there is the quenching factor, how much of the recoil energy is detectable. For scintillators this is defined as the ratio of light from nuclear recoils to light from the same energy electrons. In a cryogenic detector nearly all the recoil energy ends up as phonons, whereas in a semiconductor or a scintillator only a fractionoften a small fraction-of that energy is detected when the recoil energy is small. One has to be very careful in looking at experimental spectra to notice whether detected energy or recoil energy (“equivalent electron energy”) is being displayed. When the quenching factor is small it is very difficult to reach low recoil energies, with the atten-
I
\\
\
\
‘\ ‘\
‘\. Xe
\
or I
\
0
150 100 50 Recoil Energy Q [keV]
Go
Figure 6. Woods-Saxon coherent nuclear form factors for Si, Ge, and Xe (or I) as a function of recoil energy.
dant problems noted above. Quenching factors are not easy to measure, usually involving neutron scattering experiments, and for some materials there is controversy over result. Typical numbers are 0.30 for Na and 0.09 for I. On the other hand, the Milan group, which was the first to use a cryogenic detector, has measured a quenching factor of 1.025 f 0.01 f 0.02 over the 13-170 keV range using a TeOs crystal at O.Ol”K [22]. This group will soon have the largest cryogenic array of 20, 340g TeOs crystals. The main emphasis of this work has been on double beta decay. Getting to low recoil energy is important for WIMP masses below roughly a TeV, as illustrated in Fig. 7, in which for Si and Ge the effect of three threshold energies is shown in an exclusion plot. It will also be noted there that except for very low masses Ge is more effective than Si, so this illustrates three of the issues discussed above, and also emphasizes the importance of the fourth, preferably having unity quenching factor.
49
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
-7
---
GeDnde
-
Nal Rome
w-
ilO-43i
SiO,Oldr>;~Q&30keV b
t
I
-
1 .-.-
Ge 0.01 dru 2. lo’&30
keV
MSSmodels
Cl
12
3 Energy
10‘ WIMP Mass [GeV]
Figure 7. Limits that can be set with Ge and Si detectors as a function of threshold energy, if the background is 0.01 dru (differential rate unit, or counts/keV.kg.day) in both cases. Also shown are best current limits from Ge and NaI, as well as an envelope of possible neutralino model predictions.
The leading experiment for achieving low threshold is CRESST, a Munich/Oxford/LNGS Operating in Gran Sasso, collaboration [23]. they are using four 262g sapphire (AlsOs) crystals with tungsten superconducting phase transition thermometers. The thermometer transition curve has a long linear range in resistance so that the SQUID output voltage varies linearly with temperature rise and hence with the energy deposited. This provides a threshold of a little less than 0.5 keV and corresponding excellent energy resolution of 250 eV FWHM for 1.5 keV x-rays, as shown in Fig. 8. Since only light nuclei are involved, these detectors are sensitive primarily to very light WIMPS. The ROSEBUD experiment of the Orsay/ Paris/Zaragoza group [24] also uses sapphire and has achieved even better resolution of 120 eV FWHM at 1.5 keV, but this is in an order of magnitude smaller crystal. They read out their two, 25g crystals (now going to four crystals) with NTD (neutron transmutation-doped) ther-
4
5
6
7
LkeVl
Figure 8. Energy spectrum from a 262 g cryogenic sapphire detector of the CRESST experiment.
mistors, the resistance of which changes with the rise in temperature caused by an energy deposit in the crystal. At the time of writing this experiment is starting to run in the Canfranc tunnel in Spain. The use of F because of its most favorable matrix element for spin-dependent interactions is not limited to scintillators. The Tokyo group [25] has eight, 21g LiF crystals operating as bolometers. As with the other cryogenic detectors discussed so far, an energy deposit in the crystal, AE, produces a temperature rise AT = AEIC, where the heat capacity at low temperatures (T) is C a T3M. To get a measurable AT in a large mass M therefore requires a very low T. Thus all of these detectors use dilution refrigerators, and it is a basic problem to couple this large, complex refrigerator to the crystal thermally while providing sufficient shielding of the crystal from radioactivity or the effect of cosmic rays. In the CDMS experiment [26] of the U.C. Berkeley (CfPA)/U.C. Santa Barbara/Stanford/ Case Western/San Francisco State/Santa Clara/ Fermilab/Baksan collaboration the refrigerator has an unusual side access so that the cubic foot of cooled region can be located sufficiently far away to allow a complete active and passive shield to surround it. The complex linkage enters the
50
D.O. Caldwell/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
0
20
60 40 Recoil Energy PS 1 ReVI
80
f
100
Figure 9. Scatter plot of ionization measurement versus recoil energy measurement (phonons) for a 62 g Ge cryogenic detector exposed to neutrons and photons from a 252Cf source obtained in a calibration run of the CDMS experiment. The ionization measurement is normalized to electron equivalent energy. The shaded line is a fit to the region of nuclear recoil events.
shield in such a way as to avoid exposure of the detectors to the exterior. The CDMS experiment merits description mainly because of the central issue of background rejection. While such rejection has been demonstrated in scintillators on a statistical basis, improving sensitivity by an order of magnitude, the CDMS experiment has shown background rejection on a single pulse basis, providing at least two orders of magnitude improvement of sensitivity against electron backgrounds. This is achieved by measuring not only the temperature rise, but also the ionization produced by a particle. Nuclear recoils give mainly phonons, whereas electron interactions ionize efficiently, as shown in Fig. 9, where a Ge detector read out with NTD thermistors records neutrons and photons from a 252Cf source. Similar particle identification is obtained with Si
10”
_ 10”
-B--g
CDMSRun14SilOOgwith0.52kgd CDMSRun15616Ge165gwith1.74kgd .“‘..‘_ P .‘.“‘.‘_ 10’ 10” WIMP &s [Gev]
“..“m_
. .“,10’
Figure 10. Existing limits set by previous experiments, as well as the first limits from short runs of the CDMS experiment with a 30-keV noise threshold for Si and an 18-keV background threshold for Ge.
detectors read out by superconducting tungsten transition edge sensors. The latter measure athermal phonons on microsecond time scales versus the millisecond time scale for thermalization required by NTD readout. The athermal phonon timing is fast enough to image the event location in the crystal, allowing definition of a fiducial volume away from surfaces where contamination is more likely. This technique is now being applied to Ge and should be the main type of detector in future CDMS runs. At present, because these new techniques needed to be tried in a convenient location, CDMS I is in a tunnel on the Stanford campus with only a 17 m.w.e. overburden, requiring efficient active and passive shielding. So far problems with inefficiency in ionization collection for electrons in a thin dead layer at the surface of the Ge detectors, some 3H contamination from the NTD’s, and high threshold for the Si detec-
51
D.O. CaldweN/Nuclear Physics B (Proc. Suppl.) 70 (1999) 43-53
10"
10'
10'
IO”
10'
WIMP Mass [GeV]
Goals for CDMS-I at the StanFigure 11. ford site (0.01 events/keVqkg.day with 2-keV threshold for Ge), for CDMS-II at Soudan (0.0003 events/keV.kgday with 2-keV threshold for Ge), and for CRESST at Gran Sass0 0.2 events/keV.kg.day with 0.5keV threshold for sapphire). The best current limits are shown for comparison, along with the envelope of some neutralino model predictions, the peak of which corresponds to 3 events/kg-day for Ge.
tors provide poorer than expected exclusion plots for short runs of the experiment. The Si threshold has been improved by an order of magnitude, and it is believed that the Ge problems have been solved also, although no confirming data yet exists. These earlier results are shown in Fig. 10, along with results from many of the experiments described above. If these problems are solved, longer runs will be taken with more 165g Ge and 1OOgSi detectors to approach the CDMS I goal shown in Fig. 11. Instead of a kg-scale experiment at Stanford, CDMS II is to be a 10-20 kg-size experiment at the deep Soudan Mine in Minnesota, for which the goal is also shown in Fig. 11, along with one version of neutralino parameter space
and the goal of the CRESST experiment. The Edelweiss experiment [27] of Saclay/ Orsay/Lyon/Modane/Paris uses a similar Ge-NTD technique and is already at a deep site (Modane). Their 70g Ge detector has a similar surface dead layer problem. A totally different kind of cryogenic experiment is the use of Superheated Superconducting Granules (SSG). An ensemble of spherical superconducting grains of a few pm diameter are placed in a magnetic field. With the temperature adjusted to put the grains in a superheated superconducting state near normal, the addition of a small amount of energy deposited by a particle in a grain will drive the grain normal, producing a change in the flux distribution, since the field is no longer excluded by the grain. This grain flip is read out by a magnetometer. The ORPHEUS experimental group [28] (Bern/PSI/Clausthal) has tested the sensitivity of a small detector to y, p, and nuclear recoils and plans to use a kg of 20pm Sn granules with a 10% filling factor in teflon. The SALOPARD (Paris/Lisbon/Zaragoza) experiment [29] will start as a feasibility study in the Canfranc tunnel in the Spring of 1998 and is planned to have 1OOgof 1Opm Sn spheres with a 20% filling factor. Low thresholds are expected (< 2 keV), and there is evidence for perhaps a factor of 20 in background rejection from the number of flipped grains. 6. OTHER TORS
WIMP
SEARCH
DETEC-
While SSG detectors were one of the earliest types proposed, there is a very new technique which sounds similar but is quite different, the superheated droplet detector. This is based on commercial neutron dosimeters, operating at ambient temperatures and pressures. The detector consists of superheated droplets (lo-100pm) of freon in a gel. Rather like a bubble chamber, these droplets expand to bubbles of N lmm when there is a nuclear recoil, a process which can be picked up by a microphone. The parameters can be set so that the droplets are insensitive to y’s, /3’s, or p’s, and even low-energy neutrons, providing excellent background rejection. Furthermore,
52
D.O. Caldwell/Nuclear Physics B (Pmt. Suppl.) 70 (1999) 43-53
large detector masses seem feasible, even including such possibly desirable ingredients as fluorine. Whether sufficiently heavy nuclei can be included is yet to be demonstrated. Two groups are testing small detectors, CERN/Lisbon/Paris [30] and Montreal/Chalk River [31]. Another technique insensitive to light particle backgrounds is the use of ancient mica, utilized first for a WIMP search at U.C. Berkeley. Even (Yparticle recoils can be distinguished by the length of the pit formed when tracks are etched. The etched tracks are detected using an atomic-force microscope. While very little mass can be used, mica lo9 years old can be searched, providing a very long WIMP exposure time. The limiting background is from fission neutrons, and it has been proposed [32] to overcome this background by using the directionality of the WIMP tracks. Since WIMPS enter the mica from a preferred direction because of our motion around the galaxy, backgrounds can be reduced statistically. There is other work on directional detectors, the most straightforward of which are visual detectors like a TPC [33]. So far all the techniques for what could be the next generation of detectors suffer from insufficient mass and background problems. With such a blossoming of approaches and so many people entering this field, one hopes that discovery will come soon. Otherwise there will have to be ever increasing cleverness, as this hunt for most of the mass of the universe must be brought to a successful conclusion.
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14. 15.
ACKNOWLEDGMENTS I am grateful to the following experimenters for information and figures: Frank Avignone, Daniel Bauer, R,ita Bernabei, Bias Cabrera, Juan Collar, Susan Cooper, Hiroyasu Ejiri, Eduardo Garcia, Gilles Gerbier, H.V. Klapdor-Kleingrothaus, Robert Lanou, Seishi Matsuki, Makoto Minowa, Angel Morales, Klaus Pretzl, Ezio Previtali, Leslie Rosenberg, Daniel Snowden-Ifft, Neil Spooner, and Karl von Bibber.
16.
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