Kaluza-Klein Dark Matter and the Positron Excess

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

hep-ph/0406026

Kaluza-KleinDarkMatterandthePositronExcess

DanHooper1andGrahamD.Kribs2

2

1

Astrophysics,UniversityofOxford,Oxford,UK

SchoolofNaturalSciences,InstituteforAdvancedStudy,Princeton,NJ08540,USA

arXiv:hep-ph/0406026v1 2 Jun 2004

TheexcessofcosmicpositronsobservedbytheHEATexperimentmaybetheresultofKaluza-Kleindarkmatterannihilatinginthegalactichalo.Kaluza-Kleindarkmatterannihilatesdomi-nantlyintochargedleptonsthatyieldalargenumberandhardspectrumofpositronsperannihi-lation.GivenaKaluza-Kleindarkmatterparticlewithamassintherangeof300-400GeV,noexceptionalsubstructureorclumpingisneededinthelocaldistributionofdarkmattertogener-ateapositron uxthatexplainstheHEATobservations.Thisisincontrasttosupersymmetricdarkmatterthatrequiresunnaturallylargeamountsofdarksubstructuretoproducetheobservedpositronexcess.Futureastrophysicalandcollidertestsareoutlinedthatwillcon rmorruleoutthisexplanationoftheHEATdata.

In1994and1995,theHigh-EnergyAntimatterTele-scope(HEAT)reportedanexcessofcosmicpositrons,peakingintherangeof7-10GeV,andcontinuingtohigherenergies[1].In2000,anadditionalHEAT ightcon rmedthisobservation[2].Manypreviousexperi-ments,althoughlessprecise,alsorecordedalargerthanexpectedpositron uxaboveabout10GeV(seeRef.[1]andreferencestherein).Thestudyoftheastrophysicalproductionofpositrons[3]hasbeenthoroughlyinves-tigated[4],withtheconclusionthattheratioofcosmicpositronstoelectronsaboveabout10GeVishigherthanissuggestedbysecondaryproductioninamodelofadif-fusivehalo.

Galacticpositronspotentiallyprovideaninterestingprobeofparticledarkmatterannihilationinthegalactichalo[5].Theprospectsforsupersymmetricdarkmat-terannihilationproducingpositrons,includingwithinthecontextoftheHEATobservations,havebeenex-tensivelydiscussed,forexample,inRefs.Insupersymmetricmodels,thelightestsupersymmetricparticleisstable(withexactR-parity)andisusuallyaneutralino.Neutralinoannihilationdirectlyto + is,however,helicity-suppressed,andthuspositronsariseonlythroughcascadedecayssuchasfromdecaysofgaugebosons.Thistypicallyresultsinarathersoftspectrumofpositronsandis,therefore,hardtoreconcilewiththepositron uxandspectrumobservedbyHEATwithoutanunnaturallylargedegreeofclumpinessinourgalacticneighborhood.

Afascinatingalternativetosupersymmetricdarkmat-terarisesinmodelswith“universal”extradimensionsThepremiseisthatallstandardmodel eldsprop-agateinahigherdimensionalbulkthatiscompacti edonaspacewhosesizeisaboutTeV 1(forearlierwork,see[11,12]).Higherdimensionalmomentumconserva-tioninthebulktranslatesintoKaluza-Klein(KK)modenumberconservationinfourdimensionsthatisbrokenbyorbifoldboundaryconditionstoadiscretesubgroup,calledKKparity.Allodd-levelKKmodesareoddunderKKparity,andthereforethelightestlevel-oneKKpar-ticle(LKP)doesnotdecay.ThemostnaturalcandidatefortheLKPisthe rstKKexcitationofthehyperchargegaugeboson,B(1)13,14,15].Inadditiontobeingstable,neutralandcolorless,thethermalrelicdensityofB(1)sisconsistentwiththemeasurementsfromWMAPwhenthemassofB(1)isintherangeofhundredsofGeVuptoaboutaTeVThepreciseLKPrelicdensitydependsonthemassspectrumofthelevel-oneKKex-citations,however.WewillconsiderLKPswithmassesaslightasallowedbyprecisionelectroweakconstraints,mB(1)>～300GeVDirectandindirectdetectionstrategiesforB(1)Kaluza-KleinDarkMatter(KKDM)havebeenexplored17,18,19].Indirectdetectionisparticularlypromisinggiventhelargedarkmattermass,annihila-tioncrosssectionandannihilationfractionintoleptons[16,17].Thepositron uxandspectrumfromannihi-latingKKDMwas rstconsideredinRef.[16].Inthisletter,werevisitthepositron uxandspectrumspeci -callytoexplorethepossibilitythatKKDMannihilatinginthegalactichaloisresponsibleforthepositronexcessobservedbyHEAT.

B(1)darkmatterdominantlyannihilatesthrought-channelexchangeofotherlevel-oneKKparticles.Theannihilationcrosssectionintofermionsisproportionaltothe nalstatefermion’shyperchargetothefourthpower.Thusright-handedleptonsdominatewithanannihilationfractionof20-23%pergenerationforanapproximatelydegeneratelevel-oneKKspectrum.Thismeansenergetic(“hard”)positronsarecopiouslyproducedinKKDMan-nihilationbothdirectlyandthroughcascadesofmuonandtaudecay.Theremainingannihilationfractionisprimarilyintoright-handedup-typequarksthatcanalsoproducepositronsviacascading,butmainlyatloweren-ergies.

InFig.1,weshowthepositronspectrumthatresultsfromgenericparticledarkmatterannihilationtoτ+τ ,µ+µ ,b¯bandgaugebosonpairs.Clearly,annihilation

+

intoee,µ+µ andτ+τ producesamuchharderspectrumthanthemodesthattypicallydominateforsu-persymmetricdarkmatter(gaugebosonsorb¯b).Theseinitialpositronspectra,includingcascadedecays,were