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Indirect detection of WIMPs
J. Edsjoa∗
aDepartment of Physics, Stockholm Universuty, AlbaNova University CenterSE-106 91 Stockholm, Sweden
There is compelling evidence for the existence of dark matter in the Universe. One of the favourite candidatesis a Weakly Interacting Massive Particle (WIMP). We will here focus on indirect ways to search for WIMPsand compare their advantages and disadvantages, the emphasis though will be on searches via neutrinos fromthe Earth/Sun. As a concrete WIMP example, we will focus on the neutralino that arises in supersymmetricextensions of the standard model.
1. Introduction
Weakly Interacting Massive Particles (WIMPs)are very good dark matter candidates as they nat-urally give a relic density of the right order ofmagnitude. There are many different WIMP can-didates (see e.g. [1]), and as a concrete example,we will here use the neutralino, that naturally ap-pears as the lightest supersymmetric particle inmany supersymmetric extensions of the standardmodel (see e.g. [2]).
We will work in the Minimal SupersymmetricStandard Model (MSSM), with the usual low-energy parameters: µ, M2, tanβ, mA, m0, Ab
and At. See [3,4] for more details.We will use DarkSUSY [5] to calculate the var-
ious rates in indirect and direct searches. Therelic density of neutralinos is calculated includ-ing coannihilations [4,6,16]. We will here onlyinclude cosmologically interesting models, wherethe neutralinos can make up a major part of thedark matter in the Universe without overclosingit.
2. Indirect searches and comparison with
direct searches
There are many different ways to search forneutralino dark matter (for a review see e.g. [1,2]).We will here focus on the neutrinos coming fromannihilation in the Earth and the Sun.
∗E-mail: [email protected]
2.1. Direct searches
The most stringent direct detection limitsto date comes from the CDMS experiment atSoudan [7], that has already started to excludesome of our MSSM models. In the coming plots,the models excluded by CDMS will be indicatedby green filled circles.
2.2. Neutrino-induced muons in neutrino
telescopes
WIMPs can be gravitationally trapped by e.g.the Sun and the Earth, where they can annihilateand produce e.g. muon neutrinos. These can pro-duce muons that neutrino telescopes like Super-Kamiokande, Macro, Baksan, Amanda, Antaresor IceCube could search for. The Sun capturesWIMPs directly from the halo, while this is notvery efficient for the Earth, being deep inside thepotential well of the Sun. Instead the Earth cap-tures from a population of WIMPs that have beengravitationally scattered from the halo and dif-fuse around in the solar system [8]. In [8], it wasshown that the velocity distribution at the Eartheffectively is the same as if the Earth was in freespace due to these diffusion processes. However,in recent years [9], a concern about the effectsof solar capture has been raised. In [10], the ef-fects of solar capture were studied in detail viaextensive numerical simulations. The main resultof these simulations is that if it were not for so-lar capture, the results of [8] would hold, i.e. thevelocity distribution at the Earth would be as if
Nuclear Physics B (Proc. Suppl.) 143 (2005) 435–438
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doi:10.1016/j.nuclphysbps.2005.01.244
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Figure 1. Expected neutrino-induced muon fluxes from neutralino annihilation in a) the Earth and b)the Sun. The current experimental limits are also shown together with the expected future limits fromAntares and IceCube. Models that are excluded by the current CDMS limit are shown with green filledcircles (see text for details).
the Earth was in free space. However, solar cap-ture reduces the density of low-velocity WIMPsby almost an order of magnitude. Since these low-velocity WIMPs are the ones that could be cap-tured by the Earth, especially for heavier WIMPs,the corresponding capture rates will be reducedby up to an order of magnitude. As capture anannihilation is typically not in equilibrium in theEarth, the annihilation rates will be suppressedfurther (by up to two orders of magnitude). Inthe following we will use this new estimate of thecapture and annihilation rates in the Earth.
In Fig. 1 we show the expected fluxes from neu-tralino annihilation in the Earth and the Sun [10].We also show the current limits from Baksan [11],Macro [12], Super-Kamiokande [13] and Amanda[14]. All limits have been converted to limits onthe muon flux above 1 GeV (without an angularcut-off). We see that these neutrino telescopeshave already started to explore the MSSM pa-rameter space. We also indicate which models
are excluded by the direct search by CDMS [7]:green filled circles are excluded by CDMS, bluecrosses would be excluded with a factor of ten in-creased sensitivity and red crosses would requirean even higher sensitivity. We clearly see thatfor the Earth, CDMS has already excluded thosemodels that would produce the highest fluxes inneutrino telescopes, whereas for the Sun, the cor-relation is not as high. The reason for the strongcorrelation for the Earth is that both the signalin CDMS and the signal from the Earth dependsstrongly on the spin-independent scattering crosssection. For the Sun, on the other hand, the sig-nal depends strongly also on the spin-dependentscattering cross section, on which the limits arenot as good from direct searches. One shouldkeep in mind, however, that the correlation seenin Fig. 1 depends strongly on the assumed halovelocity profile (and for the Earth on the diffusionof neutralinos in the solar system [8]). The cap-ture by the Earth and the Sun is most efficient for
J. Edsjö / Nuclear Physics B (Proc. Suppl.) 143 (2005) 435–438436
low-velocity neutralinos whereas the direct detec-tion rates are higher for high-velocity neutralinos.Hence, it could be possible to break this strongcorrelation in signal strengths with a different ve-locity profile. Fig. 1 is produced with the assump-tion of a standard gaussian halo profile (and theeffects of solar capture on the Earth rates). Alsoindicated in the figure are the expected futurelimits from Antares[15] and IceCube.
3. Future searches
If we look 5–10 years into the future wecan compare the expected sensitivities of futuresearches by indicating which parts of the MSSMparameter space they will probe. We will hereuse the gaugino fraction of the neutralino, Zg,and project the MSSM parameter space onto theZg/(1 − Zg) versus neutralino mass plane. InFig. 2 we show the regions of the parameterspace where future direct detection experiments,neutrino telescopes and gamma ray searches aresensitive. For the direct detection sensitivity,a generic experiment like e.g. GENIUS sensitivedown to σp = 10−9 pb is assumed. For neutrinotelescopes, the expected (optimistic) sensitivity ofIceCube is used. Finally, for the gamma rays, weshow the sensitivity of upcoming Air CherenkowTelescopes (ACTs) and GLAST looking towardsthe galactic center. That signal depends stronglyon the halo profile, and in this figure an NFWprofile is assumed. As can be seen, these differ-ent searches complement each other fairly well,except for the signal from the Earth, that at leastfor standard assumptions of the halo profile, willbe better probed by direct detection experiments.
4. Conclusions
WIMPs are natural dark matter candidatesand a concrete example of a WIMP would be theneutralino, which in many cases is the lighest su-persymmetric particle in supersymmetric exten-sions of the standard model. The current directdetection experiments have already started to ex-plore the MSSM parameter space. The same isalso true for neutrino telescopes searching for neu-trinos from the Earth and the Sun, even if the ex-
pected rates from the Earth have decreased dueto a new calculation of the effects of solar captureon WIMPs diffusing in the solar system. Futuresearches are going to probe substantial parts ofthe MSSM parameter space, with a good comple-mentarity between different searches.
Acknowledgments
J.E. thanks the Swedish Research Council forsupport.
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Figure 2. The Zg/(1−Zg) versus neutralino mass plane and the expected sensitivity of future experiments.In the top-left panel, the sensitivity of future direct detection experiments is shown. In the top-right, thesensitivity of future neutrino telescopes looking towards the Earth is shown. In the bottom-left, the same,but for the Sun, is shown. In the bottom-right panel, the sensitivity of future gamma ray experiments isshown. Regions where all models can be probed are shown with green dots, regions where some modelscan be probed are shown with blue triangles, and, finally, regions where no models can be probed, areshown with red crosses. See text for more details.
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