Kaluza-Klein Dark Matter and the Positron Excess(4)

The excess of cosmic positrons observed by the HEAT experiment may be the result of Kaluza-Klein dark matter annihilating in the galactic halo. Kaluza-Klein dark matter annihilates dominantly into charged leptons that yield a large number and hard spectrum

B(1)KKDMfallswiththerangemeasuredbyWMAPformB(1)=550to800GeV.ThelowervalueoftheB(1)masscorrespondstoalevel-oneKKspectrumwithright-handedKKleptonsonly1%heavierthanB(1),lead-ingtosigni cantcoannihilations.Tonaturally ttheHEATobservationsweideallyneedmB(1)<～400GeV,

(1)

andtherefore,theBrelicdensityisnaivelytoolowbyafactorof2-3.However,variationsintheKKspectrum,suchasloweringthemassesoftheKKquarks,leadstoadditionalcoannihilationchannels(whichwerenotcalcu-latedinRef.[15])thatcanenhancetherelicdensityandthereforelowerthemassofB(1)neededtogettherelicdensityupintotheWMAPrange.TherecouldalsobenonthermalsourcesofKKDMthatboosttherelicden-sity.Inanycase,weareencouragedthatthethermalrelicdensityisatleastroughlyintherightrangethatisconsistentwiththeB(1)massrangeneededtoexplaintheHEATobservations.

Futureexperiments,suchasPAMELAandAMS-02,willbecapableofmeasuringthecosmicpositronspec-trumtomuchhigherenergiesandwithgreaterpreci-sion.Thecon rmationinsuchexperimentsofariseinthepositronspectrum,asshowninFigs.2and3,wouldfurtherfavorKKDMasthesourceofthepositronex-cess.Inaddition,adistinctivespikeinthespectrumatEe+=mB(1)isexpected[16]thatwouldprovideagoodmeasurementoftheB(1)mass.IndirectdetectionisalsoexpectedatnextgenerationneutrinotelescopessuchasIceCubethatshould ndbetweentensandathousandeventsperyearfromKKDMannihilationsintheSun[17].Existingneutrinotelescopes,suchasAMANDA-II,maypotentiallybesensitivetothisscenarioaswell.DirectdarkmatterdetectionexperimentsarealsoapproachingthesensitivityneededtoobservesuchaWIMP[16,18].Finally,futurecolliderexperimentsincludingtheTeva-tronandparticularlytheLargeHadronCollider(LHC)areexpectedtoproducemostofthelevel-oneandpossi-blyhigherlevelKKmodesthatwillleadtofascinating(andconfusing)signals[14]inthisscenario.

Inconclusion,wehaveshownthatKaluza-KleindarkmatterannihilatinginthegalactichalocanaccountfortheexcessinthecosmicpositronspectrumobservedbytheHEATexperiment.We ndanexcellent ttotheobservedpositronspectrumforawiderangeofpositronpropagationparametersandB(1)darkmattermasses.Additionally,withreasonablevaluesofthelocaldarkmatterdensityandclumpiness,theannihilationrateof300-400GeVKaluza-Kleindarkmatterandthecorre-spondingpositron uxcanbesu cienttoaccountfortheHEATobservations.Thisisincontrasttosupersym-metricdarkmatter,inwhichanunnaturalamountofdarkmattersubstructureisinvariablyrequiredtopro-ducethenecessarypositron ux.IftheHEATobser-vationsarecon rmedbyfuturemeasurementsofaris-ingcosmicpositronspectrumfromPAMELAandAMS-02,theninterpretingtheHEATexcessasarisingfrom

4

KKDMannihilationinthegalactichaloimpliesotherdarkmatterdetectionandcolliderexperimentsshouldsoonseecon rmingsignalsofaworldwithextraspatialdimensionsonlyslightlyabovetheelectroweakscale.Acknowledgments:WethankAndrewStrongandJoakimEdsj¨oforusefulcommunications.DHissup-portedbytheLeverhulmeTrust.GDKissupportedbyaFrankandPeggyTaplinMembershipandbytheDepart-mentofEnergyundercontractDE-FG02-90ER40542.

[1]S.W.Barwicketal.[HEATCollaboration],Astro-phys.J.482,L191(1997)[arXiv:astro-ph/9703192];S.Coutuetal.,Astropart.Phys.11,429(1999),[arXiv:astro-ph/9902162].

[2]S.Coutuetal.[HEAT-pbarCollaboration],inProceed-ingsof27thICRC(2001).

[3]R.J.Protheroe,Astrophys.J.254,391(1982).

[4]I.V.MoskalenkoandA.W.Strong,Astrophys.J.493,

694(1998)[arXiv:astro-ph/9710124].

[5]M.S.TurnerandF.Wilczek,Phys.Rev.D42,1001

(1990);A.J.Tylka,Phys.Rev.Lett.63,840(1989)[Erratum-ibid.63,1658(1989)].M.KamionkowskiandM.S.Turner,Phys.Rev.D43,1774(1991).

[6]E.Diehl,G.L.Kane,C.F.KoldaandJ.D.Wells,

Phys.Rev.D52,4223(1995)[arXiv:hep-ph/9502399];G.Jungman,M.KamionkowskiandK.Griest,Phys.Rept.267,195(1996)[arXiv:hep-ph/9506380];J.L.Feng,K.T.MatchevandF.Wilczek,Phys.Rev.D63,045024(2001)[arXiv:astro-ph/0008115].G.L.Kane,L.T.WangandJ.D.Wells,Phys.Rev.D65,057701(2002)[arXiv:hep-ph/0108138];E.A.Baltz,J.Edsj¨o,K.FreeseandP.Gondolo,Phys.Rev.D65,063511(2002)[arXiv:astro-ph/0109318];G.L.Kane,L.T.WangandT.T.Wang,Phys.Lett.B536,263(2002)[arXiv:hep-ph/0202156];E.A.Baltz,J.Edsj¨o,K.FreeseandP.Gondolo,arXiv:astro-ph/0211239;H.BaerandJ.O’Farrill,JCAP0404,005(2004)[arXiv:hep-ph/0312350].[7]E.A.BaltzandJ.Edsj¨o,Phys.Rev.D59(1999)023511

[arXiv:astro-ph/9808243].

[8]D.Hooper,J.E.TaylorandJ.Silk,Phys.Rev.D,in

press,arXiv:hep-ph/0312076.

[9]W.deBoer,M.Herold,C.SanderandV.Zhukov,

arXiv:hep-ph/0309029.

[10]T.Appelquist,H.C.ChengandB.A.Dobrescu,

Phys.Rev.D64,035002(2001)[arXiv:hep-ph/0012100];T.AppelquistandH.U.Yee,Phys.Rev.D67,055002(2003)[arXiv:hep-ph/0211023].

[11]E.W.KolbandR.Slansky,Phys.Lett.B135,378

(1984);K.R.Dienes,E.DudasandT.Gherghetta,Nucl.Phys.B537,47(1999)[arXiv:hep-ph/9806292].

[12]I.Antoniadis,Phys.Lett.B246,377(1990);I.Anto-niadis,K.BenakliandM.Quir´os,Phys.Lett.B331,313(1994)[arXiv:hep-ph/9403290].

[13]H.C.Cheng,K.T.MatchevandM.Schmaltz,Phys.

Rev.D66,036005(2002)[arXiv:hep-ph/0204342].

[14]H.C.Cheng,K.T.MatchevandM.Schmaltz,Phys.

Rev.D66,056006(2002)[arXiv:hep-ph/0205314].

[15]G.ServantandT.M.P.Tait,Nucl.Phys.B650,391

(2003)[arXiv:hep-ph/0206071].

[16]H.C.Cheng,J.L.FengandK.T.Matchev,Phys.Rev.