Frascati Physics Series Vol. XXXVII (2004) pp. 187–1921877192 FrontierFRONTIER ScienceSCIENCE 2004, PhysicsPHYSICS andAND AstrophysicsASTROPHYSICS inIN SpaceSPACE Frascati, 14-19 June, 2004

ANTIMATTER SEARCHES AS PROBES OF SUSY DARK MATTER

StefanoSTEFANO ProfumoPROFUMo a,b“vb,, PieroPIERo UllioULLIO a,bW

a SISSA/ISAS, via Beirut 2-4,2—4, 34013 Trieste,Trieste, Italy

b INFN, Sez. didi Trieste,Trieste, 34014 Trieste,Trieste, Italy

Abstract

We investigate the discrimination perspectives for neutraneutralinolino dark matter at future antimatter search experiments. Theoretically wewellll motivated benchmark scenarios are introduced, with the purpose of comcomparingparing projected experimental search strategies, and in order to sshowhow in which cases antimatter searches could be of particular relevancerelevance.. We resort to visibility ratios ((i.e.i.e. signal-to-sensitivitysignal—to—sensitivity ratios) which allow a comparison among differentdifierent neutralino dark matter detection techniquetechniques.s. We also introduce a novel quantity, II¢,φ, defineddefined as an integral of the squared expected exotic signal over the background, which allows to estimate the future reach of antimatter search experiments independindependentlyently of the experimental apparatus. We point out that antiprotons, positrons and antideuterons searches constitute a primary neutralinneutralinoo dark matter search strategy for a class of large annihilation rates modemodels,ls, which will be largely probed at upcoming experiments, like PAMELA and AAMS—OZ.MS-02.

187 188 Frontier Science 20042004,, Physics and Astrophysics in Space

1 Introduction

The Dark Matter (DM) problem, i.e.i.e. the observation that the majority of the matter content of the Universe is made of some kind of non-non—luminousluminous and non-baryonicnon—baryonic gravitationally interacting substance, stands as one of the greatest challenge in astroparticle physics. As now, many pparticlearticle candidates have been proposed to solve the DM puzzle, and an extensive exexperimentalperimental program has been deployed with the purpose of detecting and uunderstandingnderstanding this elusive not less than seemingly inevasible component ooff the Universe. One of the best theoretically motivated candidates is the lightestlightest supersymmetric particleparticle,, LSP, of the minimal supersymmetric extension of the StandaStandardrd Model (MSSM),(MSSM), which turns out to be an ideal DM claimant, provided iitt is the lightest neutralino (as it is the case in large parameter space regionregionss of many SUSY breaking scenarios). Complementarity between direct detection and indirect detdetectionection methods has been repeatedly stressed [1].[1]. However, the question regregardingarding how competitive a given detection technique is compared to the oothers,thers, can be addressed only resorting to particular particle physics frframeworks,ameworks, and making definitedefinite assumptions about the features of the Milky Way DM HaHalo.lo. In the present paper we address the issue of confronting a set of leading DM search strategies, focusing in particular on antimatter searches [2].[2]. Pair annihilation is the mechanism which sets the thermal relic aabundancebundance of neutralinos; although the density of neutralinos in DM halohaloss today is much smaller than in the early Universe environment, there is stistillll a finitefinite probability for WIMPsWIMPS in the Galactic halo to annihilate in pairs. In thesthesee annihilations the same amount of matter and antimatter is produced; while tthehe matter component is likely to be very subdominant compared to standstandardard astrophysical sources, there seems to be no standard primary source of antiantimatter,matter, with the bulk of the (scarce) antimatter component in cosmic rays whiwhichch is likely to be of secondary origin, i.e. generated in the interaction of prprimaryimary cosmic rays (mainly(mainly protons) with the interstellar medium (mainly(mainly hydrhydrogenogen and helium). The goal is then to identify, through their peculiar spectraspectrall features, the WIMP-WIMP— induced antimatter fluxes,fluxes, or at least to exclude those DM cancandidatesdidates which would overproduce antimatter compared to the relatively loloww background term. We will firstfirst outline our particle physics setup, in sec. 2, definingfining a set of benchmark neutralino models which we claim to be of particulparticularar relevance at antimatter searches; then, in sec. 3, we provide the details of our calculation of the antimatter fluxes.fluxes. Finally, sec. 4 is devoted to the questquestionion of the future discrimination sensitivity at antimatter search facilitifacilities,es, and summarizes the comparison of the latter with differentdifferent direct and indirect ddetectionetection techniques. We draw our conclusions in sec. 5. S. Profumo and P. UllioUllio Antimatter searches as probes of SUSY dark matter 189

I Model I MAll1/22,, mm33/22,, MA433 I tan βfi I sgn(sgn(M)µ) I DefiningDefining Condition I Funnel 700 ÷+ 1450 55 > 0 2 mmXχ ≃2 mmAA AMSB 23 ÷+ 231 50 > 0 mm00 = 1500 GeV e NUGM 879 ÷+ 1096 50 > 0 MN11/1W31/M3 = 10, M[Hg/1W32/M3 = 2, H = 9999.8%.8%

Table 1: The definingdefining parameters for three SUSY models under cconsiderationonsideration (see(see [2][2] for details).

2 Large Annihilation Rates Benchmark Models

From the point of view of indirect detection experiments,experiments7 ththee most favorable situation in the particle physics setup occurs when neutralneutralinosinos feature large pair annihilation rate (canny).σannv . Since the thermal relic abundance of neutralinos h i − − in a standard cosmological setup forces (canny)σ v r: 3 - 1010’27cm3s’1/QCDMh2,27cm3s 1/Ω h2, h ann i ≃ · CDM resorting to models with a large annihilation rate translattranslateses into assuming that either non-thermalnon—thermal mechanisms of neutralino productiproduction,on, or cosmological enhancement processes are operative [3].[3]. We focus here on ththreeree benchmark neutralino scenarios, respectively featuring bino, wino aandnd higgsino like LSP’sLSP7s (see(see Tab. 1).1). The models present remarkable purity in their ddominantominant gauge-gauge— eigenstate component, in all cases larger than 98%, and the vvaluesalues for (ammo)σannv − − h − i lie, at a sample neutralino mass of 300 GeV, between 1010’2424 and 1010’2525 cmcm3s’1.3s 1.

3 Computing Antimatter Yields

We discuss here the case of neutralino-inducedneutralino—induced antiproton,antiproton7 positron and antideuteron cosmic ray fluxes.fluxes. The stable antimatter specispecieses generated by neutralino annihilations are simulated using the Monte CarCarlolo code Pythia [4][4] 6.154, and we take the conservative cored Burkert profileprofile as tthehe reference DM halo model [5].[5I. Concerning the propagation of charged cosmicosmicc rays through the Galactic magnetic fields,fields, we consider an effectiveeffective two-ditwo—dimensionalmensional diffusiondiffusion model in the steady state approximation where the diffusiondiffusion ccoefficientoefficient D takes the form of a broken power law in rigidity,rigidity7 RR,,

0.6 D =2 DD0 ((R/R0)0'6R/R ) if R 2 RR0 0 0 ≥ 0 DD=D0= D0 if RR

We take eq. (1) with DD0 =2= 2.5.5 X 10102828 cmcm22 ss’1−1 and RR0 = 4 GV,GV7 in a cylindrical 0 × 0 diffusiondiffusion region of radius equal to 30 kpc and half height equaequall to 4 kpc, plus a galactic wind term (see [2][2] for details). Finally,Finally, to sketcsketchh solar modulation effectseffects we implement the one parameter model based on the analanalyticalytical force-force— fieldfield approximation [7][7] taking for simplicity the parameter smod to be charge-charge— independent. 190 Frontier Science 20042004,, Physics and Astrophysics in Space

−1 10107 l A BESS−98BESSi98 CAPRICE−98CAPRICEi98 — Background 1010'2−2 ------Funnel , — — - AMSB ]

−1 NUGM srgl] sr 41 −1 s s

42 −3 −2 10 cm −1 [GeVglcm −4 105 Flux

−5 101075 , mml:300χ=300 GeV Antiproton Antiproton Flux [GeV smod=0.61smod:0.6l

Background + SUSY Contribution \ −6 1010’ 7 l l l l 1010−1' 1010°0 101011 101022 1010"3

Tpbar [GeV][GeV]

Figure 1: The solar modulated antiproton flux,flux, as a function ooff the antiprotons kinetic energy Tparpbar [2].[2].

Fig. 1 shows the spectral features, after solar modulation fforor a given step along the solar activity cycle, of primary antiprotons and ooff the background, comparing the total expected signals to the data taken durinduringg the corresponding modulation phase. The figurefigure refers to a common neutralino mamassss of 300 GeV. As regards the secondary antimatter fluxes,fluxes, which play hhereere the role of backgrounds, our estimates are produced running the Galprop [6][6] code in the configurationconfiguration for propagation parameters we have adopted foforr the signals. We remark that we find,find, for both antiprotons and positrons, that the computed backgrounds provide by themselves excellent fitsfits of the datadata:: we obtain, for background only, a reduced χX22 equal to 0.82 for antiprotons and to 0.95 for positrons. Excessive exotic signals may be therefore statistatisticallystically ruled out [2].[2].

4 Discrimination of SUSY DM Through Antimatter

New generation space-basedspace—based experiments for antimatter seasearchesrches PAMELA and AMS [8][8] will tremendously enhance the resolution and accuraaccuracycy of positron and antiproton spectra measurements, as compared to existiexistingng balloon borne results. With the purpose of assessing discrimination capacapabilitiesbilities of future experimental facilities, we will sketch here the possibilipossibilityty of disentangling an exotic component out of a standard secondary background. To this extent, we introduce a novel quantity, II¢,φ, to compare the case of a pure background S. Profumo and P. UllioUllio Antimatter searches as probes of SUSY dark matter 191

100100 200 300 400 500 600 −3 l l l l 1010

\ \ CurrentCurrant excl. limits AMSB −4 * \ 10l 0 \ JL\ 7 7 AntiprotonsAntipmtons −575

10 ] ’ \ Positrons ] 41 −1 w s s / l

−6 4‘7 −1

10 sr sr / 42w −2 7 Pamela 1y \ \c −7,7

10 [cm [cm φ I,” I 7 Pamela 3y ‘ \ \_ __r 10−8*3

104 10−9

102

0 VisibilityVisibility LincLine , 10 Ratios −2 105

10−4 Visibility σ Visibility Ratios Direct Detection ((6}),p, SI) −670 , , , , Neutrino Tlsc. ((1.1µ f.f, Sun) 10 Antideuterons (Satellite Around the Earth)

10−8 l l l l 100100 200 300 400 500 600 Neutralino Mass (GeV)

Figure 2: The comparison of future sensitivities for variouvariouss detection techniques in the AMSB model. measurement to that of the occurrence of a signal. It can be shshownown [2][2] that a signal with fluxflux φgbss can be discriminated from a background fluxflux φgbbb at a payload with geometrical factor A and time of data acquisition TT,, at 95% C.L.,CL, if

EEmaxmax φ2 ((X2)95%χ2)95% I E —3dEs dE > —bnb , (2)2 φ¢ ≡ /Emm φQ51) AA-TT ( ) ZEmin b · where (X2)?L‘:%2 95% corresponds to nb degrees of freedom, namely the number of where (χ )nb corresponds to nb degrees of freedom, namely the number of energy bins of the data. We focus, for definiteness,definiteness, on the cascasee of the PAMELA detector, with an effectiveeffective area of 24.5 cmcm2sr,2sr, an exposure time of 3 years, and resorting to a trial energy binning as sketched in ref. [8].[8]. TThishis corresponds to a critical ILb r: 33.2-.2 1010’8cm’2sr’1s’1.−8cm−2sr−1s−1. φ ≃ · As regards antideuterons, the background is expected to be ttotallyotally negligible in the low energy regime, and we refer here to the pproposedroposed gaseous antiparticle spectrometer (GAPS) [9],[9], looking for antideuterons in the energy interval 0.1-0.40.1—0.4 GeV per nucleon, with an estimated ssensitivityensitivity level of − 22.6.6 X 1010’9m’2sr’1GeV’1s’1.−9m−2sr−1GeV 1s−1. × Fig. 2 compares future detection perspectives in the AMSB benchmark model [2].[2]. Direct and indirect detection techniques rates aarere given in terms of signal to sensitivity ratios ((VisibilityVisibility RatiosRatios).). Remarkably, the most promising 192 Frontier Science 20042004,, Physics and Astrophysics in Space detection strategies, as emerging from the upper panel of ththee figure,figure, reside in antiproton searches, but both positron and antideuteron sesearchesarches look more promising than direct detection. We point out that our conclconclusionsusions could have been even stronger had we resorted to a cuspy halo profileprofile [2].[2].

5 Conclusions

We considered antimatter yields in three benchmark scenariscenariosos with large annihilation rates, respectively featuring a bino, winoWino anandd a higgsino-likehiggsino—like lightest neutralino. We introduced a new parameter ILbφ which allows, given a SUSY model, to reliably assess its visibility at given futufuturere experiments. The comparison of future experimental DM search strategies shoshowsws that antimatter searches may be highly competitive with respect to both diredirectct detection and neutrino telescopes. In some cases, such as for antiprotons in a wino DM scenario, antimatter searches may be thethe onlyonly viable DM detectiondetection techniquetechnique.. In the context of SUSY models with large annihilation cross ssections,ections, and in view of the imminent launch of space-basedspace—based dedicated experiexperiments,ments, antimatter searches are therefore to be considered as a highly promisinpromisingg path towards the detection of DM.

References

[1][1] G. Jungman, M. Kamionkowski and K. Griest, Phys. Rep. 267 (1996)(1996) 195;195; L. Bergstr¨om,Bergstrom, Rept. Prog. Phys. 63 (2000)(2000) 793. [2][2] S. Profumo and P. Ullio, JCAP 0407 (2004)(2004) 006 [arXiv:hep-ph/0406018].[arXiv:hep—ph/0406018]. [3][3] S. Profumo and P. Ullio, JCAP 0311 (2003)(2003) 006 [arXiv:hep-ph/0309220];[arXiv:hep—ph/0309220]; S. Profumo and P. Ullio, arXiv:astro-ph/0404390.arXivzastro—ph/0404390.

[4] T. Sj¨ostrand,Sjostrand, arXiv:hep-ph/9508391.arXivshep—ph/9508391. [5] A. Burkert, Astrophys. J. 447 (1995)(1995) L25. [6] Galprop numerical package, http://www.mpe.mpg.de/http://www.mpe.mpg.de/Naws/propagatehtmlaws/propagate.html ∼ [7] L.J. Gleeson and W.I. Axford, Astrophys. J. 149 (1967)(1967) L115; L.A. Fisk, J. Geophys. Res. 76 (1971) 221. [8][8] O. Adriani et al. ((PAMELAPamela Collaboration), Proc. of the 26th ICRC, Salt Lake City, 1999, OG.4.2.04; P. Picozza and A. Morselli, J. PhPhys.ys. G29 (2003)(2003) 903; S. Ahlen et al. ((AMSAms Collaboration), Nucl. Instrum. Methods A350 (1994)(1994) 351;351, [9][9] K. Mori et al., Astrophys. J. 566 (2002)(2002) 604.