CERN–TH/2001-061
Non-Baryonic Dark Matter1
Leszek Roszkowski1,2
1Department of Physics, Lancaster University, Lancaster LA1 4YB, England
2TH Division, CERN, CH-1211 Geneva 23, Switzerland
Abstract
There exist several well-motivated candidates for non-baryonic cold dark matter, including neutralinos, axions, axinos, gravitinos, Wimpzillas. I review the dark matter properties of the neutralino and the current status of its detection. I also discuss the axino as a new interesting alternative.
CERN–TH/2001-061 February 2001
1Invited plenary review talk given at 6th International Workshop on Topics in Astroparticle and Underground Physics (TAUP 99), 6-10 September, 1999, Paris, France. 2
Non-Baryonic Dark Matter † Leszek Roszkowski
Department of Physics, Lancaster University, Lancaster LA1 4YB, England
There exist several well-motivated candidates for non-baryonic cold dark matter, including neutralinos, axions, axinos, gravitinos, Wimpzillas. I review the dark matter properties of the neutralino and the current status of its detection. I also discuss the axino as a new interesting alternative.
1. Introduction will also comment about recent results regarding axinos. The puzzle of the hypothetical dark matter Neutrinos, the only WIMPs that are actually (DM) in the Universe still remains unresolved [1]. known to exist, do not seem particularly attrac- While there may well be more than one type of tive as DM candidates. It has long been believed DM, arguments from large structures suggest that that their mass is probably very tiny, as suggested a large, and presumably dominant, fraction of by favored solutions to the solar and atmospheric DM in the Universe is made of massive particles neutrino problems, which would make them hot, which at the time of entering the epoch of mat- rather than cold DM. This picture has recently ter dominance would be already non-relativistic, been given strong support by first direct evidence or cold. From the particle physics point of view, from Superkamiokande for neutrinos’ mass [4]. cold DM (CDM) could most plausibly be made While the new data only gives the µ τ neu- of so-called weakly interacting massive particles −3 2 trino mass-square difference of 2.2 10− eV ,it (WIMPs). it very unlikely that there would exist× two mas- There exist several interesting WIMP candi- sive neutrinos with cosmologically relevant mass dates for CDM that are well-motivated by the of 5 to 40 eV and such a tiny mass difference. underlying particle physics. The neutralino is The strength with which the WIMP candidates considered by many a “front-runner” by be- listed above interact with ordinary matter spans ing perhaps the most natural WIMP: it comes many orders of magnitude. For the neutrali- as an unavoidable prediction of supersymmetry 2 5 nos it is a fraction ( 10− 10− )ofthe (SUSY). The axion is another well-motivated can- weak strength. Interactions∼ −∼ of axions and axi- didate [2]. But by no means should one forget 2 16 nos are suppressed by (mW /fa) 10− ,where about some other contenders. While some old 10 ∼ fa 10 GeV is the scale at which the global picks (sneutrinos and neutrinos with mass in the Peccei-Quinn∼ U(1) symmetry is broken. Interac- GeV range) have now been ruled out as cosmolog- tions of gravitinos and other relics with only grav- ically relevant CDM by LEP, axinos (SUSY part- itational interactions are typically suppressed by ners of axions) have recently been revamped [3]. 2 33 (mW /MPlanck) 10− . One may wonder how Another well-known candidate is the fermionic such vastly different∼ strengths can all give the partner of the graviton, the gravitino. Most of relic abundance of the expected order of the crit- the review will be devoted to the neutralino but I ical density. The answer lies in the different ways they could be produced in the early Universe: the
†Invited plenary review talk given at 6th International neutralinos are mainly produced “thermally”: at Workshop on Topics in Astroparticle and Underground some temperature they decouple from the ther- Physics (TAUP 99), 6-10 September, 1999, Paris, France. 3 mal bath. But in addition there exist also several due to assumed R-parity. A perfect candidate for mechanisms of “non-thermal” production. This aWIMP! is how CDM axinos, gravitinos can be produced The neutralino is a well-motivated candidate. (in addition to “thermal” production), as well as It is an inherent element of any phenomenologi- Wimpzillas [5]. cally relevant SUSY model. Being neutral, it is It is obvious that WIMPs do not necessarily a natural candidate for the LSP (although one have to interact only via weak interactions per should remember that this is often only an as- se. WIMPs are generally required to be electri- sumption); it couples to ordinary matter with a cally neutral because of stringent observational weak-interaction strength (reduced by mixing an- constraints on the abundance of stable charged gles) which is within the range of sensitivity of relics. On the other hand, they could in princi- present-day high energy, as well as dark matter, ple carry color charges. (For example, a stable detectors. Finally, as a bonus, it naturally gives 2 gluino above 130 150 GeV [6], or in an experi- Ωχh 1 for broad ranges of masses of SUSY mentally allowed− window 25 35 GeV [7], could particles∼ below a few TeV and for natural ranges still be the lightest SUSY particle− (LSP) although of other SUSY parameters. its relic abundance would be very small [6].) In a Much literature has been devoted to the neu- halo they would exist as neutral states by form- tralino as DM, including a number of comprehen- ing composites with gluon or quarks. We can see sive and topical reviews (see, e.g., Refs. [9,10]). that generally WIMPs are expected to have sup- Here I will only summarize the main results and pressed effective couplings with ordinary matter. comment on some recent developments and up- Otherwise, they would dissipate their kinetic en- dates. ergy. Neutralino properties as DM and ensuing im- 2 What is the WIMP relic abundance Ωχh = plications for SUSY spectra are quite model de- ρχ/ρcrit [8]? Current estimates of the lower pendent but certain general conclusions can be bound on the age of the Universe lead to drawn. First, its cosmological properties are very 2 ΩTOTALh < 0.25. Recent results from rich clus- different depending on a neutralino type. The ters of galaxies and high-redshift supernovae type relic abundance of gaugino-like (mostly bino-like) Ia imply Ωmatter 0.3. The Hubble parameter is neutralinos is primarily determined by the (light- now constrained' to 0.65 0.1. Big Bang nucle- est) sfermion exchange in the LSP annihilation ± 2 ¯ 2 4 2 osynthesis requires Ωbaryonh < 0.015. Assuming into ff:Ωh m /m . In order to have Ωχ 1, ∝ f χ ∼ that most matter in the Universe∼ is made of CDM the lightest sfermion cannot be too light (below WIMPs, one therefore obtains 100 GeV) nor tooe heavy (heavier than a few ∼ 2 hundred GeV) [11] which is a perfectly natural 0.1 < Ωχh < 0.15 (1) ∼ ∼ range of values. or so. At the very least, requiring that a dominant On the other hand, higgsino-like χ’s are fraction of CDM is located only in galactic halos strongly disfavored. Firstly, due to GUT relations 2 gives Ωχh > 0.025. among gaugino masses, higgsinos correspond to ∼ rather large gluino mass values, above 1 TeV, 2. Neutralinos which may be considered as unnatural [11]. Fur- thermore, higgsino-like neutralinos have been The DM candidate that has attracted perhaps shown to provide very little relic abundance. For the most wide-spread attention from both the mχ >mZ ,mW ,mt the χ pair-annihilation into theoretical and experimental communities is the those respective final states (ZZ, WW, tt¯)is neutralino. It is a neutral Majorana particle, the very strong. But both below and above those lightest of the mass eigenstates of the fermionic thresholds, there are additional co-annihilation partners of the gauge and Higgs bosons: the bino, 0 [12] processes of the LSP with χ1± and χ2, which wino and the two neutral higgsinos. If it is the are in this case almost mass-degenerate with the lightest SUSY particle, it is massive and stable, 4
2 LSP. Co-annihilation typically reduces Ωχh be- the theoretical criterion of naturalness: expecting low any interesting level although it has been re- that no SUSY masses should significantly exceed cently argued that the effect of co-annihilation is that value. Again, GUT relations among gaugino not always as strong as previously claimed [13]. masses cause the above constraint to provide a Higgsino-like LSPs are thus rather unattractive, much more stringent (although still only indica- although still possible DM candidates, especially tive) bound of about 150 200 GeV. Remarkably, − in the large mass (mχ > 500 GeV) regime. One in the scenario with additional GUT unification also arrives at the same∼ conclusions in the case of spin-zero mass parameters, just such a typi- of ‘mixed’ neutralinos composed of comparable cal upper bound derives from the cosmological 2 fractions of gauginos and higgsinos. This is be- constraint Ωχh < 1. Only in a relatively nar- cause, even without co-annihilation, in this case row range of parameters where the neutralino be- the neutralino pair-annihilation is less suppressed comes nearly mass degenerate with a SUSY part- and one invariably finds very small Ωχ there [11]. ner of the tau, τR, the effect of the neutralino’s Remarkably, just such a cosmologically pre- co-annihilation opens up the allowed range of mχ ferred gaugino type of neutralino typically up to about 600e GeV [17]. emerges in a grand-unified scenario with addi- Summarizing, we can see that, despite the com- tional assumptions that the mass parameters of plexity of the neutralino parameter space and a all spin-zero particles are equal at the unification large number of neutralino annihilation channels, scale 1016 GeV. What one finds there is that one can, remarkably, select the gaugino-like neu- the lightest∼ bino-like neutralino emerges as essen- tralino in the mass range between roughly 30 and tially the only choice for a neutral LSP [14,15]. (It 150 GeV as a natural and attractive DM candi- is still possible to find cases with a higgsino-like date. Furthermore, one is able to derive relatively LSPs but they are relatively rare.) Furthermore, stringent conditions on the mass range of some 2 in order for Ωχh not to exceed one or so, other sfermions, which are consistent with our basic ex- (most notably sfermion) SUSY partner masses pectations for where SUSY might be realized. have to be typically less than 1 TeV [14,15]. Thus our phenomenological expectations for low- 3. Neutralino WIMP Detection energy SUSY to be realized roughly below 1 TeV are nicely consistent with a bino-like neutralino 3.1. Predictions as a dominant component of DM in the Universe. The local halo density of our Milky Way is es- 3 What about the neutralino mass? In princi- timated at 0.3GeV/cm with a factor of two or ple, it could be considered as a nearly free pa- so uncertainty [9]. For neutralino WIMPs as a rameter. It would be constrained from below dominant component of the halo this translates 2 to about 3000 LSPs with mass mχ = 100 GeV by the requirement Ωχh < 1 to lie above some 2 3 GeV [16] which is a neutralino version of the per cubic meter. With typical velocities in the Lee-Weinberg− bound [1]. This is because, in this range of a few hundred km/s, the resulting flux mass range, the neutralino may be viewed as a of WIMPs is actually quite large massive neutrino with somewhat suppressed cou- 9 2 Φ=vρ /m 10 (100 GeV/m ) χ0s/m /s. (2) plings. Much stronger bounds are in many cases χ χ ≈ χ provided by LEP. Unfortunately, collider bounds A massive experimental WIMP search pro- are rather model dependent and are no longer gramme has been developed during the last few provided for the general SUSY model normally years. Although a variety of techniques have been considered in DM studies. Nevertheless, one can explored, most of them follow one of the two ba- reasonably expect that mχ is now ruled out below sic strategies. One can look for DM neutralinos roughly 30 GeV, except in cases when the sneu- directly, through the halo WIMP elastic scatter- trino is nearly degenerate with the LSP in which ing off nuclei, χN χN, in a detector. Indi- case the lower bound may disappear altogether. rect searches look for→ traces of decays of WIMP A rough upper bound of 1 TeV follows from pair-annihilation products. One promising way 5 is to look for multi-GeV energy neutrinos com- the nucleus. ing from the Sun and/or the core of the Earth. The differential cross section for a WIMP scat- A One can also look for monochromatic photons, tering off a nucleus XZ with mass mA is therefore positrons or antiprotons produced in WIMP pair- given by annihilation in a Galactic halo. I will not discuss dσ dσscalar dσaxial these indirect methods here. Perhaps I should = + , (3) d ~q 2 d ~q 2 d ~q 2 only mention about an interesting new way of | | | | | | looking for WIMPs at the Galactic center [20]. where the transferred momentum ~q = mAmχ ~v mA+mχ If there is a super-massive black hole there (for depends on the velocity ~v of the incident which there is now some evidence), it will ac- WIMP. The effective WIMP-nucleon cross sec- crete WIMPs and thus increase their density in tions σscalar and σaxial are computed by evalu- the core. They will then be annihilating much ating nucleonic matrix elements of corresponding more effectively and the resulting flux of neutri- WIMP-quark and WIMP-gluon interaction oper- nos, photons and other products from the cen- ators. ter of the Milky Way may be strongly enhanced, In the scalar part contributions from individual 5 even up to a factor of 10 in halo models with nucleons in the nucleus add coherently and the fi- central spikes in their profiles. Such spiked halo nite size effects are accounted for by including the models have been obtained in recent N-body sim- scalar nuclear form factor F (q). (The effective in- ulations [18]. teraction in general also includes tensor compo- In the following I would like to make several nents but the relevant nucleonic matrix elements comments about direct detection and in particu- can be expanded in the low momentum-transfer lar about an intriguing claim made by the DAMA limit in terms of the nucleon four-momentum Collaboration regarding a possible evidence for a and the quark (gluon) parton distribution func- WIMP signal in their data. For a more detailed tion. As a result, the non-relativistic WIMP- review, see Ref. [19]. First, I will briefly summa- nucleon Lagrangian contains only scalar interac- rize the theoretical aspects and predictions. tion terms.) The differential cross section for the In direct searches one of the most sig- scalar part then takes the form [9] nificant quantities is the event rate R ∼ dσscalar 1 σ(χN)(ρχ/mχv)(1/mN ) - the product of the 2 2 = [Zfp +(A Z)fn] F (q), (4) elastic cross section σ(χN) of neutralinos from d ~q 2 πv2 − | | nuclei, their flux ρχ/mχv and the density of tar- where fp and fn are the effective WIMP couplings get nuclei with mass mN . to protons and neutrons, respectively, and typi- The elastic cross section σ(χN) of relic WIMPs cally fn fp. Explicit expressions for the case scattering off nuclei in the detector depends on of the supersymmetric≈ neutralino can be found, the individual cross sections of the WIMP scat- e.g., in the Appendix of Ref. [21]. tering off constituent quarks and gluons. For The effective axial WIMP coupling to the nu- non-relativistic Majorana particles, these can be cleus depends on the spin content of the nucleon divided into two separate types. The coherent ∆qp,n and the overall expectation value of the nu- part described by an effective scalar coupling be- cleon group spin in the nucleus
-39 10 (Maxwellian) velocity distribution was fixed at v0 = 220 km/s. Since the Galactic halo has not been directly observed and there still is a consid- -40 10 erable disagreement about a correct halo model, quoted error bars for v0 and the local halo density should, in my opinion, be treated with much cau- -41 10 tion. Varying v0 within a reasonable range leads to a significant enlargement of the region selected by DAMA, [24] as first shown in Ref. [32]. This ] (normalised to nucleon)
2 -42 10 is so because of the significant dependence on v0 of the differential event rate spectrum, as can be seen in Fig. 1 in Ref. [32]. -43 10 As a result, the upper limit of mχ correspond- ing to the claimed signal region increases consid- erably, from about 100 GeV for the initially as- -44 Cross-section [cm 10 sumed value v0 = 220 km/s up to over 180 GeV 1 2 3 10 10 10 at v0 = 170 km/s [33] (at 2σ CL). On the other WIMP Mass [GeV] hand the range of ξ0.3σp is not much affected. Assuming that the effect claimed by DAMA were indeed caused by DM WIMPs, it is interest- ing to ask what ranges of SUSY parameters and LSP relic abundance it would correspond to. This Figure 1. The current and some recent limits on issue was addressed by three groups, each using the WIMP-proton spin-independent (scalar) cross- section. The legend is as follows: dot: Heidelberg- their own code for computing the direct detec- Moscow Ge [30], solid: DAMA NaI [31], dash: CDMS tion rate and the relic abundance [34–36]. While 1999 4 kg day Ge with neutron background sub- there is some difference in approach (somewhat × traction [27]. Marked also are the 2 σ DAMA re- different ranges of input parameters, techniques − gions: solid closed curve: 15, 000 kg day, filled re- and assumptions, etc.) the overall outcome is × gion: 20, 000 kg day [24]. that it is indeed possible to find such SUSY con- × figurations which could reproduce the signal but only for large enough tan β (typically above 10 gion selected by DAMA [27].4 Let us hope that although SUSY points with smaller values can DAMA’s claim will be falsified soon. also sometimes be found). More importantly, the 2 Figure 1 shows the current status of WIMP corresponding values of Ωχh are rather small, searches in the plane of the WIMP-proton scalar typically below 0.06 [34–36] although somewhat 5 larger values can also sometimes be found. These cross-section σp versus the WIMP mass. A claimed DAMA region is also indicated. are clearly small values, well below the favored < 2 < It is worth pointing out that the claimed sig- range 0.1 Ωχh 0.15. This is illustrated in nal region selected by DAMA [24] was too re- Figure 2 [36].∼ ∼ strictive as it did not include astrophysical un- However, one should remember that these re- certainties. The effect is rather sensitive to as- sults have been obtained using commonly used sumptions about a model of the Galactic halo. but often rather uncertain values of several input In the DAMA analysis the peak of the WIMP parameters. In addition to astrophysical ones, the nuclear physics and quark mass inputs in calcu- 4See Note Added. lating the scalar cross-section for the neutralino- 5 The figure has been produced with the help of nucleus elastic scattering are rather poorly de- a particularly useful plotting facility of Gaitskell & Mandic which is now available on the Web: termined, as mentioned above. The effect of the http://cdms.berkeley.edu/limitplots/. latter has been recently re-analyzed in Ref. [37] 9
0.0001 -40 10
-42 10
-44 ] (normalised to nucleon) 10 2
-46 10
-48 Cross-section [cm 10 1 2 3 100 200 10 10 10 WIMP Mass [GeV]
Figure 2. A scan of SUSY points in the plane Figure 3. The current and some future reach lim- 3 (mχ,ξ0.3σp)whereξ0.3 = ρχ/(0.3GeV/cm ). The its on the WIMP-proton spin-independent (scalar) 2 legend: circles: 0.1 < Ωχh < 0.15 (cosmologically fa- cross-section compared with conservative predictions 2 vored range), crosses: Ωχh > 0.25 (excluded), small from minimal SUSY (large shaded region). The leg- 2 points: 0.02 < Ωχh < 0.25 (conservative range). end is as follows: upper dot: Heidelberg-Moscow The 2 σ region (upper left) of DAMA is marked Ge [30], solid: DAMA NaI [31], upper dash: CDMS with full− thick dots. 1999 4 kg day Ge with neutron background subtrac- × tion [27]. Marked also are the 2 σ DAMA regions: − solid: 15, 000 kg day, filled: 20, 000 kg day [24]. Future reach of× some experiments (as claimed× by and found to affect the overall scalar cross-section the respective groups): lower dash: CDMS at Soudan, lower dot: CRESST. Some other experi- byafactoroftenorso. ments (e.g., Edelweiss, UKDMC-Xe, GENIUS) ex- It is worth presenting Figure 3 again with an pect to reach roughly similar sensitivities. (Source: outlook for some expected limits (as claimed by http://cdms.berkeley.edu/limitplots/). the respective groups) and compare them with predictions of minimal SUSY obtained for very broad ranges of SUSY parameters and neglect- ing nuclear input uncertainties mentioned above. testing predictions coming from SUSY. What I The SUSY region is bounded from above by a find promising is that several experiments us- 2 somewhat arbitrary requirement Ωχh > 0.025. ing different detector materials and often differ- From below it is limited by a generous bound ent methods of [attempts at] distinguishing signal 2 Ωχh < 1. The SUSY points falling into the cur- from background will explore a large fraction of 2 rently expected range 0.1 < Ωχh < 0.15 (not the SUSY parameter space within the next few indicated in the plane) form∼ a sub-region∼ reach- years. Especially reassuring would be an observa- 6 ing up to roughly a few times 10− pb. tion of a positive signal in more than type of DM It is clear that today’s experiments are now detector, although many experimentalists would only reaching the sensitivity required to begin probably remark that I am asking for too much. 10
Time will tell. e.g., the process
4. Axinos χ aγ (11) → Axinos are a natural prediction of the Peccei- as showne in Fig. 1 in Ref. [3]. This process was Quinn solution to the strong CP-problem and already considered early on in Ref. [39] (see also SUSY. The axino is the fermionic partner of the Ref. [38]) in the limit of a photino NLSP and only axion. Similarly to the axion, the axino couples for both the photino and axino masses assumed to ordinary matter with a very tiny coupling pro- to be very low, m 1GeVand m 300 eV, portional to 1/f where 109 1010 GeV < f < γ ≤ a ≤ a a the former case now excluded by experiment. In 12 − ∼ ∼ 10 GeV. that case, the photinoe lifetime was typicallye much It is plausible to consider the axino as the LSP larger than 1 second thus normally causing de- since its mass is basically a free parameter which struction of primordial deuterium from Big Bang can only be determined in specific models. As we nucleosynthesis (BBN) by the energetic photon. have seen above, the neutralino has been accepted Avoiding this led to lower bounds on the mass in the literature as a “canonical” candidate for of the photino, as a function of fa, in the MeV the LSP and dark matter. But with current LEP range [39]. bounds between about 30 and 60 GeV (depend- Because both the NLSP neutralino and the ing on a SUSY model), it becomes increasingly CKR axino are both massive (GeV mass range), plausible that there may well be another SUSY the decay (11) is now typically very fast. In the particle which will be lighter than the neutralino, theoretically most favored case of a nearly pure and therefore a candidate for the LSP and dark bino, [11,15] the neutralino lifetime can be writ- matter. ten as Primordial axinos decouple from the thermal soup very early, around T fa, similarly to the 2 3 ' 2 fa/(NCaY Y ) 100 GeV axions. The early study of Ragagopal, et al. [38] τ 3.3 10− s (12) ' × 1011 GeV m concluded that, in order to satisfy Ωh2 < 1, the χ primordial axinos had to be light (< 2 keV), cor- where the factor NC is of order one. One responding to warm dark matter, unless∼ inflation aY Y can see that it is not difficult to ensure that would be invoked to dilute their abundance. In the decay takes place well before 1 second in either case, one did not end up with axino as cold order for avoid problems with destroying suc- DM. cessful predictions of Big Bang nucleosynthesis. However, it has recently been shown [3] that The axino number density is equal to that of the the axino can be a plausible cold dark matter can- NLSP neutralino. Therefore its relic abundance didate, and that its relic density can naturally be is Ω h2 = m /m Ω h2. The axinos are ini- of order the critical density. The axino can be a a χ χ tially relativistic but, by the time of matter dom- produced as a non-thermal relic in the decays of inance they become red-shifted by the expansion heavier SUSY particles. Because its coupling is e e and become cold DM. so much weaker, superparticles first cascade de- There are other possible production mecha- cay to the next lightest SUSY partner (NLSP) nisms and cosmological scenarios for massive ax- for which the most natural candidate would be inos.6 Even if the primordial population of axi- the neutralino. The neutralino then freezes out nos is inflated away (which would happen if the from thermal equilibrium at T m /20. If it f χ reheating temperature T f ), they can be were the LSP, its co-moving number' density af- reh a regenerated from thermal background processes ter freeze-out would remain nearly constant. In at high enough T . the scenario of Covi et al. (CKR) [3], the neu- reh tralino, after decoupling from the thermal equi- librium, subsequently decays into the axino via, 6For a recent comprehensive study, see [40]. 11
5. Conclusions 3. L. Covi, L. Roszkowski and J.E. Kim, Phys. Rev. Lett. 82, 4180 (1999) (hep-ph/9905212). Looking for the invisible is not easy but is cer- 4. Super-Kamiokande Collaboration, tainly worthwhile. The discovery of dark matter Y. Fukuda, et al., Phys. Lett. B433,9 will not only resolve the mystery of its nature but (1998); Phys. Lett. B436, 33 (1998); Phys. is also likely to provide us with much information Rev. Lett. 81, 1562 (1998). about the particle physics beyond the Standard 5. D.J.H. Chung, E.W. Kolb, and A. Riotto, Model. What I find most encouraging is that we Phys. Rev. Lett. 81, 4048 (1998). have a very good chance of fully testing the neu- 6. H. Baer, K. Cheung and J.F. Gunion, tralino as a WIMP by the end of the decade. De- Phys.Rev. D59 (1999) 075002 (hep- tecting other candidates, like the axino, will be ph/9806361). the task for the more distant future. 7. A. Mafi and S. Raby, hep-ph/9912436. 8. For a detailed discussion and references, see J. Primack, in the Proceedings. Note Added: 9. G. Jungman, M. Kamionkowski, K. Griest, After the Conference both the DAMA Phys. Rep. 267, 195 (1996). and CDMS Collaborations published new re- 10. L. Roszkowski, in the Proceedings of the 2nd sults. Based on the combined statistics of Rencontres du Vietnam, Eds. Nguyen van 57, 986 kg day of data collected in a NaI detec- HiˆeuandJ.Trˆan Thanh Vˆan, Editions Fron- tor since November× ’96, the DAMA Collaboration tiers 1996. reported [41] a statistically significant (4 σ CL) 11. L. Roszkowski, Phys. Lett. B 262,59 effect which it interprets as being caused by a (1991); see also J. Ellis, D.V. Nanopoulos, WIMP annual modulation signal. L. Roszkowski, and D.N. Schramm, Phys. The CDMS experiment using germanium and Lett. B245, 251 (1990). silicon crystals at Stanford published [42] a new 12. K. Griest and D. Seckel, Phys. Rev. D 43 limit on scalar WIMP-proton cross section. The (1991) 3191; M. Drees and M. Nojiri, Phys. new result is based on the total of 10.6kg day of Rev. D 47 (1993) 376; S. Mizuta and M. Ya- data collected a current shallow site (17 mwe)× at maguchi, Phys. Lett. B 298 (1993) 120. Stanford during 1999. A powerful event-by-event 13. M. Drees, M. Nojiri, D.P. Roy, Y. Yamada, discrimination method allows CDMS to reach a Phys.Rev.D56 (1997) 276. sensitivity matching that of DAMA with only less 14. R.G. Roberts and L. Roszkowski, Phys. Lett. than 0.2% of DAMA’s statistics. B 309 (1993) 329. The new 90% CL CDMS limit excludes most 15. G.L. Kane, C. Kolda, L. Roszkowski, and of the signal region claimed by DAMA. In par- J. Wells, Phys. Rev. D49, 6173 (1994). ticular, it fully rules out the previous 2 σ region 16. K. Griest and L. Roszkowski, Phys. Rev. D based on the combined data of 19, 511 kg day 46 (1992) 3309. from runs I and II [24] and the new 3 σ region× at 17. J. Ellis, T. Falk, and K. Olive, Phys.Lett. more than 84% CL. B444 367-372 (1998); J. Ellis, T. Falk, An updated compilation of these and K. Olive, and M. Srednicki, hep-ph/9905481. other data can be found on the Web: 18. See, e.g., B. Moore, et al., Ap. J. Lett. http://cdms.berkeley.edu/limitplots/.) 499, 5 (1998); Y.P. Jing and Y. Suto, astro- ph/9909478. 19. A. Morales, in the Proceedings. 20. P. Gondolo and J. Silk, Phys. Rev. Lett. 83, REFERENCES 1719 (1999) (astro-ph/9906391). 1. E. Kolb and M. Turner, The Early Universe, 21. H. Baer and M. Brhlik, Phys. Rev. D 57 (Addison-Wesley, New York, 1989). (1998) 567. 2. P. Sikivie, in the Proceedings. 22. A. Drukier, K. Freese, and D. Spergel, Phys. 12
Rev. D 33 (1986) 3495. 23. K. Freese, J. Frieman, and A. Gould, Phys. Rev. D 37 (1988) 3388. 24. R. Bernabei, et al. (The DAMA Collabora- tion), Phys. Lett. B450, 448 (1999). 25. R. Bernabei, et al. (The DAMA Collabora- tion), Phys. Lett. B424, 195 (1998). 26. G. Gerbier, et al., Astropart. Phys. 11, (1999) 287. 27. R. Gaitskell, in the Proceedings. 28. P. F. Smith, et al., Physics Reports 307 (1998) 275. 29. D. Abriola, et al., Astropart. Phys.10, (1999) 133 (astro-ph/9809018). 30. L. Baudis, et al.,Phys.Rev.D59, 022001 (1999). 31. R. Bernabei, et al., Phys. Lett. B 389 (1996) 757. 32. M. Brhlik and L. Roszkowski, Phys. Lett. B464, 303 (1999) (hep-ph/9903468). 33. P. Belli, et al., Phys.Rev. D61, 023512 (2000) (hep-ph/9903501). 34. A. Bottino, F. Donato, N. Fornengo, S. Scopel, Phys. Rev. D59, 095004 (1999). 35. P. Nath and R. Arnowitt, Phys. Rev. D60, 044002 (1999) (hep-ph/9902237) 36. Work in progress. 37. A. Bottino, F. Donato, N. Fornengo and S. Scopel, hep-ph/9909228. 38. K. Ragagopal, M.S. Turner, and F. Wilczek, Nucl. Phys. B358, 447 (1991). 39. J.E. Kim, A. Masiero, and D.V. Nanopoulos, Phys. Lett. B139, 346 (1984). 40. L. Covi, L. Roszkowski, H.-B. Kim and J.E. Kim, hep-ph/0101009. 41. R. Bernabei, et al. (The DAMA Collab- oration), INFN/AE-00/01 (February 2000), Phys. Lett. B480, 23 (2000). 42. The CDMS Collaboration, Phys. Rev. Lett. 84, 5699 (2000) (astro-ph/0002471).