DARK MATTER SEARCHES WITH AMS-02 EXPERIMENT

A.Malinin a, For AMS Collaboration

IPST, University of Maryland, MD-20742, College Park, USA

Abstract. The Alpha Magnetic Spectrometer (AMS), to be installed on the In-ternational Space Station, will provide data on cosmic radiations in a large rangeof rigidity from 0.5 GV up to 2 TV. The main physics goals in the astroparti-cle domain are the anti-matter and the dark matter searches. Observations andcosmology indicate that the Universe may include a large amount of unknownDark Matter. It should be composed of non baryonic Weakly Interacting MassiveParticles (WIMP). A good WIMP candidate being the lightest SUSY particle inR-parity conserving models. AMS offers a unique opportunity to study simulta-neously SUSY dark matter in three decay channels from the neutralino annihila-tion: e

+, anti-proton and gamma. The supersymmetric theory frame is consideredtogether with alternative scenarios (extra dimensions). The expected flux sen-sitivities in 3 year exposure for the e

+/e− ratio, anti-proton and gamma yields

as a function of energy are presented and compared to other direct and indirectsearches.

Introduction

The first evidence for the existence of Dark Matter comes from the observationof rotation velocities across the spiral galaxies, derived from the variation inthe red-shift. The rotation velocities rise rapidly from the galactic center, thenremain almost constant to the outermost regions of a galaxy. The observationsare consistent with the gravitation motion only if the matter in the Universeis mostly non luminous ”dark matter”. The recent WMAP results [1] confirmthat about 83% of the matter in the Universe exists in the form of cold DarkMatter (DM). The mystery of the Dark Matter remains unsolved. Many can-didates such as massive neutrino, Universal Extra Dimensions Kaluza-Kleinstates and Super Symmetry theory (SUSY) heavy neutralinos were proposed.If Dark Matter, or a fraction of it, is non-baryonic and consists of almost non-interacting particles like neutralinos, it can be detected in cosmic rays throughits annihilation into positrons or anti-protons, resulting in deviations (in caseof anti-protons) or structures (in case of positrons) to be seen in the otherwisepredictable cosmic ray spectra [2]. Considering the hypothesis of a possibleclumpy DM, the expected fluxes of such primary positrons, γ-s or anti-protonsmay be enhanced [3] since the annihilation rate is proportional to the squareDM density contrary to the direct DM searches which will suffer from a de-creased probability for the Earth to be contained into an eventual DM clump.

ae-mail: malinin@mail.cern.ch

1 AMS-02 Instrument

The Alpha Magnetic Spectrometer (AMS) is a particle physics experiment inspace. Its initial space mission on board of the Space Shuttle Discovery (STS-91) in June, 1998 confirmed the basic concept of the experiment [4]. Duringthis short flight AMS measured of the GeV cosmic-ray fluxes over most of theEarth’s surface [5–8], and provided the impetus to upgrade the instrumentfor the ISS 3 year mission (hereafter called AMS-02). These upgrades includeamong others a stronger, BL2 = 0.9T superconducting magnet to achieve themaximal detectable rigidity of 1 TV (the rigidity resolution better than 2%up to 20 GV) in the Silicon Tracker, as well as the addition of a TransitionRadiation Detector (TRD), a Ring Imaging Cherenkov (RICH) and an Elec-tromagnetic Calorimeter (ECAL). The upgraded instrument will provide dataon cosmic radiation in a large range of energy from a fraction of GeV to 3 TeVwith very high accuracy and free from the atmospheric corrections needed forballoon-born measurements. Its main physics goals in the astroparticle domainare the Antimatter and the Dark Matter searches as well as the cosmic raycomposition and propagation study.

2 AMS-02 Sensitivity for DM search

The Monte Carlo study, based on the AMS-02 mathematical model, was per-formed to estimate the instrument sensitivity for the indirect DM search chan-nels [9–13]. More than 109 events containing p+−, He, e+− and γ at differentenergies have been fully simulated [14–17] passing through the detector andthen reconstructed. The results of the study in the anti-proton, e+ and γ can-nels are presented in the figures 1-9. The background rejection factors up to10−6 necessary to extract the tiny anti-proton and e+ signals were achievedby combining the redundant information from the TRD, RICH and ECAL de-tectors. The selection criteria were tuned and the resulting efficiencies wereused to simulate the measured spectra. Comparison with the existing datademonstrates that the AMS-02 will have an adequate sensitivity to address theenhancement in the positron fraction measurement reported by HEAT [9, 10],simultaneously constraining the DM signal parameter space by combining theanti-proton, e+ and γ channels [18]. The Galactic Center γ signal measure-ment by AMS-02 would provide 95% CL exclusion limits for several mSUGRAmodels in 3 years.

Figure 1: The acceptance for the anti-proton signal including the selection efficiency.

Figure 2: The background rejection factors for the anti-proton signal.

Figure 3: The simulated AMS-02 three year combined measurement of the cosmic p− spec-

trum and the residual background (left plot). The comparison of the AMS-02 expectedthree year p

− spectrum measurement with existing data (right plot). Lines show differentsecondary anti-proton flux models.

Figure 4: MC prediction for the AMS-02 positron energy reconstruction (left plot). Thesimulated AMS-02 three year combined measurement of the cosmic e

+ spectrum (right plot).

Figure 5: The HEAT positron fraction data with an example of estimated 1 year AMS-02measurement. Solid lines show one of the most favorable SUSY neutralino scenario and thestandard LBM prediction. SUSY signal enhancement 100 (clumpy DM) is necessary to fit

data.

Figure 6: The integrated γ flux from the Galactic Center as a function of mχ for the NFWhalo profile (left plot) and cuspy NFW halo (right plot) parameterizations with the standardset of parameters. The considered models are the mSUGRA scheme, AMSB scenario andKaluza-Klein Universal Extra Dimensions. The various selections were done by varying Ωh2

cuts.

Figure 7: Combined example. The anti-proton flux as a function of kinetic energy, assuming150 GeV SUSY neutralino mass (left plot) and 50 GeV Kaluza-Klein boson mass (right plot)

for 3 years of AMS-02 data taking.

Figure 8: Combined example. The positron flux as a function of energy, assuming 150 GeVSUSY neutralino mass (left plot) and 50 GeV Kaluza-Klein boson mass (right plot) for 3

years of AMS-02 data taking.

Figure 9: Combined example. The γ flux as a function of energy, assuming 150 GeV SUSYneutralino mass (left plot) and 50 GeV Kaluza-Klein boson mass (right plot) for 3 years ofAMS-02 data taking. IMBH associated DM clumps at different distance: 20 kpc (case 1)

and 2 kpc (case 2) are shown.

Conclusions

During the 3 year mission in space, AMS-02 will perform precise, high statisticscosmic ray measurements in the 1 GeV to few TeV energy range. It will allowto combine all indirect Dark Matter search channels, constraining the existingmodels and will have a high discovery potential of the Dark Matter signal.

References

[1] D.N.Spergel et al., ApJS, 148, 175 (2003).[2] I.V.Moskalenko and A.W.Strong, Adv.Space Res. 27, 717 (2001).[3] E.A.Baltz et al. ”Cosmic-ray positron excess and neutralino dark mat-

ter”,Phys.Rev. D 65 63511 (2002).[4] G.M.Viertel, M.Capell, ”The Alpha Magnetic Spectrometer”,

Nucl.Inst.Meth. A 419, 295 (1998).[5] J.Alcaraz et al., ”Search for Antihelium in Cosmic Rays”, Phys. Lett. B

461, 387 (2000).[6] J.Alcaraz et al., ”Protons in Near Earth Orbit”, Phys.Lett. B 472, 215

(2000).[7] J.Alcaraz et al., ”Leptons in Near Earth Orbit”, Phys.Lett. B 484, 10

(2000).[8] J.Alcaraz et al., ”Cosmic Protons”, Phys.Lett. B 490, 27 (2000).[9] S.W.Barwick et al. (HEAT Collaboration), ”Measurements of the

Cosmic-Ray Positron Fraction from 1 to 50 GeV”, Astro-phys.J. 482,L191 (1997).

[10] S.Coutu et al. (HEAT-pbar Collaboration), ”Positron Measurements

with the HEAT-pbar Instrument”, in (Proceedings of 27th ICRC), 2001.[11] R.L.Golden et al., ”Observation of cosmic ray positrons in the region

from 5 to 50 GeV”, A&A 188, 145 (1987).[12] L.Bergstrom, J.Edsjo, P.Ullio, ”Cosmic Antiprotons as a Probe for Su-

persymmetric Dark Matter”, Astro-phys.J. 526, 215 (1999).[13] D.Hooper, G.Kribs, ”Kaluza-Klein Dark Matter and the Positron Ex-

cess”, Phys.Rev. D 70 115004 (2004)[14] R.Brun et al., ”GEANT 3”, CERN/DD/EE/84-1, 1987.[15] S.Giani et al., ”GEANT 4 An Object-Oriented Toolkit for Simulation in

HEP”, CERN/LHCC/98-44, 1998.[16] V.Choutko, G.Lamanna, A.Malinin, (Trento II International Workshop

on Matter Antimatter and Dark Matter Proceedings), Int.J.Mod.Phys.

A 17, N12-13, 1817 (2002).[17] A.Jacholkowska, et al., ”An indirect dark matter search with diffuse

gamma rays from the Galactic Centre with the Alpha Magnetic Spec-

trometer”, arXiv:astro-ph/0508349, 23 May 2006.[18] S.Rosier-Lees in (Proceedings of 30th ICRC), ID0618, Merida, 2007.