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29th International Cosmic Ray Conference Pune (2005) 9, 179182Search for neutralino dark matter with the AMANDA neutrino detectorD. Hubert , A. Davourand C. de los Herosfor the IceCube Collaboration(a) Vrije Universiteit Brussel, Dienst ELEM, B-1050 Brussels, Belgium(b) Division of High Energy Physics, Uppsala University, S-75121 Uppsala, SwedenPresenter: D. Hubert (hubert@hep.iihe.ac.be), bel-hubert-D-abs1-he23-oralData taken with the AMANDA high energy neutrino telescope can be used in a search for an indirect darkmatter signal. This paper presents current results from searches for neutralinos accumulated in the Earth andthe Sun, using data from 1997-1999 and 2001 respectively. We also discuss future improvements for higherstatistics data samples collected during recent years.1. IntroductionCosmological observations have long suggested the presence of non-baryonic dark matter on all distance scales.The WMAP results [1] confirmed our current understanding of the Universe, summarised in the concordancemodel. In this model the Universe contains about 23% non-baryonic cold dark matter, but nothing is predictedabout the nature of this dark matter.A massive, weakly interacting and stable particle appears in Minimally Supersymmetric extensions to theStandard Model that assume R-parity conservation. Indeed, the supersymmetric partners of the electroweakneutral Standard Model bosons mix into an interesting dark matter candidate, the neutralino, whose mass isexpected in the GeV-TeV range [2].On their trajectory through the Universe these particles will scatter weakly on normal matter and lose energy.Eventually, the dark matter particles will be trapped in the gravitational field of heavy celestial objects, like theEarth and the Sun [3]. The particles accumulated in the center of these bodies can annihilate pairwise. Theneutrinos produced in the decays of the Standard Model annihilation products can then be detected with a highenergy neutrino detector as an excess over the expected atmospheric neutrino flux.In this paper we present the results of searches with the AMANDA detector for neutralino dark matter accu-mulated in the Earth (1997-1999 data set) and the Sun (2001 data set). We also summarize current techniquesthat continue these efforts on higher statistics data samples accumulated during recent years.The Antarctic Muon And Neutrino Detector Array [4] at the South Pole uses the polar ice cap as a Cherenkovmedium for the detection of relativistic charged leptons produced in high energy neutrino interactions withnuclei. The detector was constructed between 1996 and 2000. Now totaling 677 light sensitive devices dis-tributed on 19 strings, the detector is triggered when at least 24 detector modules are hit within a sliding 2.5  swindow. Before 2000, the detector configuration consisted of between 10 and 13 strings and consequentlya lower multiplicity trigger condition was able to cope with the high rate of events produced by cosmic rayinteractions with the atmosphere.Reconstruction of muons, with their long range, offers the angular resolution required to reject the atmosphericbackground and search for a neutralino-induced signal, which, due to the geographic location of AMANDA,yields vertical upward-going (Earth) or horizontal (Sun) tracks in the instrumented volume. Indeed, it is pos-sible to eliminate the dominant background, downward-going atmospheric muons. However, upward-goingatmospheric neutrinos will always contaminate the final, selected data sample.180 D. Hubert, A. Davour and C. de los Heros Filter LevelRelative efficiencyWIMP 250GeV(hard channel)DataAtm. µAtm. ν1997-9910-1010-910-810-710-610-510-410-310-210-110 1+2 3 4 5 6 σSI > σSIlim   CDMSEthµ = 1 GeV0.05 < Ωχh2  < 0.2Neutralino Mass (GeV)Muon flux from the Earth (km-2 yr-1 )BAKSAN 1997BAIKAL 1998-99MACRO 2002AMANDA 1997-99AMANDA 1997SUPER-K 200411010 210 310 410 510 102103104Figure 1. (a) Detection efciencies relative to trigger level for the different lter levels in the terrestrial neutralino analysis(  =250 GeV, hard spectrum) for 1997-1999 data, neutralino signal, atmospheric muons and neutrinos. (b) As a functionof neutralino mass, the 90% condence level upper limit on the muon ux coming from hard neutralino annihilations inthe center of the Earth compared to our results from 1997 data [9] and other indirect experiments [10]. Markers showpredictions for cosmologically relevant MSSM models, the dots represent parameter space excluded by CDMS [11].2. Signal and background simulationWe have used the DARKSUSY program [5] to generate dark matter induced events for seven neutralino massesbetween 50 GeV and 5000 GeV, and two annihilation channels for each mass: the 	 channel producesa hard neutrino energy spectrum (  for a 50 GeV neutralino), while  yields a soft spectrum. The cosmicray showers in the atmosphere, in which downward-going muons are created, are generated with CORSIKA[6] with a primary spectral index of  =2.7 and energies between 600 GeV and 10  GeV. The atmospheric neu-trinos are produced with NUSIM [7] with energies between 10 GeV and 10  GeV and zenith angles above 80  .3. Search for neutralino annihilations in the center of the EarthA neutralino-induced signal from the center of the Earth was searched for in AMANDA data collected be-tween 1997 and 1999, with a total effective livetime of 536.3 days. To reduce the risk of experimenter bias, thecomplete data set of 5.0  10  events was divided in a 20% subsample, used for optimisation of the selectionprocedure, and a remaining 80% sample, on which the selection was applied and final results calculated. Simi-larly, the sets of simulated events were divided in two samples: the first for use in the selection optimisation andthe second for the selection efficiency calculations. The simulated atmospheric muon sample contains 4.2  10 triggered events (equivalent to an effective livetime of 649.6 days). The sample of atmospheric neutrinos totals1.2  10  events, which corresponds to 2.2  10  triggers when scaled to the livetime of the analysis.First, we try to suppress the dominant atmospheric muon background which is about 10  times more abundantthan the atmospheric neutrino background. This is partially done by selecting the events that are reconstructedas upward-going and that satisfy a cut correlated with reconstruction quality (“filter level 3”). However, only a10 ﬀ reduction of the atmospheric muons is obtained this way (Fig. 1a) and more elaborate selection criteria areneeded to reject downward-going muon tracks misreconstructed as upward-going. Depending on the detectorconfiguration and the neutralino model under study, the characteristics of the signal differ, which influencesselection efficiencies significantly at this point. Therefore, all further cuts are fine-tuned separately for eachneutralino model and year of data taking. At filter level 4, a neural network is trained using between 8 and 10Search for neutralino dark matter with the AMANDA neutrino detector 18110-810-710-610-510-410-310-210-110 1 2 3 4 5 Filter LevelRelative efficiencyWIMP 500GeV, hardDataAtm. µAtm. νEvents10 210 310 410 510 610 710 810 911010 210 310 410 510 102103104 σSI > σSIlim   CDMSEthµ = 1 GeV0.05 < Ωχh2  < 0.2Neutralino Mass (GeV)Muon flux from the Sun (km-2 yr-1 )BAKSANMACROSUPER-KAMANDA-IIFigure 2. (a) Detection efciencies relative to trigger level for the different lter levels in the solar neutralino analysis(  =500 GeV, hard spectrum) for 2001 data, neutralino signal, atmospheric muons and neutrinos. (b) As a function ofneutralino mass, the 90% condence level upper limit on the muon ux coming from hard neutralino annihilations inthe center of the Sun compared to other indirect experiments [10]. Markers show predictions for cosmologically relevantMSSM models, the dots represent parameter space excluded by CDMS [11].input observables, reaching another 10 ﬁﬀ reduction. Filter level 5 cuts sequentially on observables, with thegoal of removing downward-going muons that resemble signal events.At filter level 5 the data sample is dominated by atmospheric neutrinos (see Fig. 1a). With no significant excessof vertical tracks observed, the final selection on reconstructed zenith angle (filter level 6) was optimised forthe average lowest possible 90% confidence level upper limits on the muon flux. From the number of observedevents and the amount of (simulated) background in the final angular search bin, we infer the 90% confidencelevel upper limit on the number of signal events. Combined with the effective volume at the final cut level andthe livetime of the collected data, this yields an upper limit on the neutrino-to-muon conversion rate, which canthen be related to the muon flux [8] (see Fig. 1b).4. Search for neutralino annihilations in the SunThe AMANDA data used in the search for solar neutralinos consists of 8.7  10  events, corresponding to 143.7days of effective livetime, collected in 2001. In contrast to the search in Section 3, reducing the risk of experi-menter bias in this analysis can be achieved by randomizing the azimuthal angles of the data. The advantage ofthis procedure is that it allows the use of the full data set for cut optimisation. The azimuthal angles are restoredonce the optimisation is finalised and results are calculated. The simulated atmospheric background sample attrigger level totals 1.6  10  muons (equivalent to 32.5 days of effective livetime) and 1.9  10  neutrinos.The solar neutralino analysis suffers the same backgrounds as the terrestrial neutralinos, but the signal isexpected from a direction near the horizon, due to the trajectory of the Sun at the South Pole. This analysiswas only possible after completion of the AMANDA-II detector, whose 200 m diameter size provides enoughlever arm for robust reconstruction of horizontal tracks.We adopted a similar analysis strategy as in Section 3. First, we select events with well-reconstructed horizontaltracks (filter level 1-3). The remaining events are passed through a neural network that was trained separatelyfor the neutralino models under study and used data as background (filter level 4). Although a data reduction ofﬂ 10 ﬃ compared to trigger level is achieved, the data sample is still dominated by misreconstructed downward-182 D. Hubert, A. Davour and C. de los Herosgoing muons. As shown in Fig. 2a, they are removed with extra cuts on observables related to reconstructionquality (filter level 5).There was no sign of a significant excess of tracks from the direction of the Sun in the final data sample. Theexpected background in the final search bin around the Sun was estimated from off-source data in the samedeclination band, which eliminates the effects of uncertainties in background simulation. Combining this withthe number of observed events, the effective volume and the detector livetime, we obtain 90% confidence levellimits on the muon flux coming from annihilations in the Sun for each considered neutralino mass [12], asshown in Fig. 2b.5. Discussion and outlookFigures 1b and 2b present the AMANDA limits on the muon flux from neutralino annihilations into 	(hard channel) in the Earth and the Sun respectively, together with the results from other indirect searches.Limits have been rescaled to a common muon threshold of 1 GeV using the known energy spectrum of theneutralinos. Also shown are the cosmologically relevant MSSM models allowed (crosses) and disfavoured(dots) by the direct search from CDMS [11]. Compared to our search for a terrestrial neutralino signal in 1997AMANDA data [9], the limit has been improved by a factor which is more than that expected from additionalstatistics alone. This is due mainly to the separate cut optimization for each neutralino mass, which exploitsthe characteristic muon energy spectrum of each model.In 2001 an extra trigger was installed that lowered the energy threshold of the detector. This trigger takesinto account spatio-temporal correlations in the event hit pattern. A preliminary analysis with data taken in2001 and 2002 shows an improvement of a factor of about 5 in the effective volume in the search for 50 GeVneutralinos (soft annihilation channel) from the Earth with respect to the analysis presented in this note. We arecurrently performing searches for a dark matter signal both from the Earth and the Sun with data taken from2000 and later. The increased detector exposure combined with improved reconstruction techniques and thenew trigger setting will result in improved limits from these analyses (note that a 4-year exposure alone wouldalready give an improvement of a factor of two).References[1] D. N. Spergel et al., Astrophys. J. Suppl. Ser. 148, 175 (2003).[2] G. Jungman, M. Kamionkowski and K. Griest, Phys. Rept. 267, 195 (1996).[3] A. Gould, Astrophys. J. 328, 919 (1988); J. Lundberg and J. Edsjo¨, Phys. Rev. D69, 123505 (2004); W.H.Press and D.N. Spergel, Astrophys. J. 296, 679 (1985).[4] J. Ahrens et al., Nucl. Instr. Meth. A524, 169 (2004).[5] P. Gondolo et al., Journ. of Cosm. & Astrop. Phys. 0407, 008 (2004).[6] D. Heck et al., FZKA report 6019 (1998).[7] G.C. Hill, Astropart. Phys. 6, 215 (1997).[8] P. Ekstro¨m, PhD thesis, Stockholm University (2004).[9] J. Ahrens et al., Phys. Rev D66, 032006 (2002).[10] M. Boliev et al., in Proc. of Dark Matter in Astro- and Particle Physics, edited by H.V. Klapdor-Kleingrothaus and Y. Ramachers (World Scientific, 1997); M. Ambrosio et al., Phys. Rev. D60, 082002(1999); S. Desai et al., Phys. Rev. D70, 083523 (2004), erratum ibid D70, 109901 (2004); Zh.-A. Dzilk-ibaev et al., Nucl Phys B (Proc. Suppl.) 143, 335 (2005).[11] D.S. Akerib et al., Phys. Rev. Lett. 93, 211301 (2004).[12] Y. Minaeva, PhD thesis, Stockholm University (2004); M. Ackermann et al., submitted to Astrop. Phys.

Search for neutralino dark matter with the AMANDA neutrino detector

Search for neutralino dark matter with the AMANDA neutrino detector

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