Dark matter from late decays and the small-scale s

arXiv:hep-ph/0701007v214Mar2008Darkmatterfromlatedecaysandthesmall-scalestructureproblemsFrancescaBorzumati,1,2,∗TorstenBringmann,3,†andPieroUllio3,‡1ICTP,StradaCostiera11,34014Trieste,Italy2DepartmentofPhysics,NationalCentralUniversity,Chung-Li32054,Taiwan3SISSA/ISASandINFN,Trieste,ViaBeirut2-4,I-34014Trieste,ItalyThegenerationofdarkmatterinlatedecaysofquasi-stablemassiveparticleshasbeenproposedasaviableframeworktoaddresstheexcessofpowerfoundinnumericalN-bodysimulationsforcolddarkmattercosmologies.Weidentifyaconvenientsetofvariablestoillustratewhichrequirementsneedtobesatisfiedinanygenericparticlephysicsmodeltoaddressthesmallscaleproblemsandtofulfillotherastrophysicalconstraints.Asaresultofthismodel-independentanalysis,wepointoutthatmeetingtheserequirementsinacompletelynaturalwayisinherentlydifficult.Inparticular,were-examinetheroleofgravitinosandKaluza-Kleingravitonsinthiscontextandfindthemdisfavouredasasolutiontothesmall-scaleproblemsincasetheyareDMcandidatesgeneratedinthedecayofthermallyproducedWIMPs.Weproposeright-handedsneutrinosandright-handedKaluza-Kleinneutrinosasalternatives.Wefindthattheyareviabledarkmattercandidates,butthattheycancontributetoasolutionofthesmallscaleproblemsonlyincasetheassociatedDiracneutrinomasstermappearsasasubdominantcontributionintheneutrinomassmatrix.PACSnumbers:98.80.Cq,98.80.Es,95.35+d,12.60.Jv,14.80.LyI.INTRODUCTIONCosmologicalobservationshavegivenoverwhelmingevidencethatnonbaryonicdarkmatter(DM)isthebuildingblockofstructuresintheUniverse[1].Atthesametime,fromaparticlephysicsperspective,littleisknownaboutthedetailedpropertiesofDMparticles.Inthestandardcosmologicalscenario,acolddarkmatter(CDM)termisintroducedasagenericcomponentthatisi)“dark”ordissipationless,asitcouplestophotonsandbaryonsonlythroughgravity,ii)“cold”,i.e.withnegligiblefree-streamingeffects,andiii)collisionless.Thisscenariocanbeaccommodatedinextensionstothestandardmodel(SM)ofparticlephysics.ThemostattractiveschemeisprobablytheoneinwhichCDMisintroducedasathermalreliccomponent:stablemassiveparticlesthatacquirearelicdensityoftheorderoftheDMdensityintheUniverse,providedtheircouplingtoSMparticlesisofweaktype.Thelistofweaklyinteract-ingmassiveparticles(WIMPs)thathavebeenproposedasDMcandidatesincludes,amongothers,thelightestneutralinoinsupersymmetricmodelsinwhichthisisthelightestsupersymmetricparticle(LSP),andthefirstKaluza-Klein(KK)hyperchargegaugebosoninmodelswithuniversalextradimensions(forreviews,see[2]).InCDMcosmologies,structuresformhierarchically,withsmallstructurescollapsingfirstandthenmergingintolargerandlargerbodies.Thispicturehasproventoberemarkablysuccessfultodescribethedistributionandcorrelationofstructuresonlargescales.Thereis,however,sometensionwithobservationsonsmallscales∗Electronicaddress:[e-mail]†Electronicaddress:[e-mail]‡Electronicaddress:[e-mail]-linearstructureformationregimewhichneedstobestudiedwithnumericalN-bodysimulations.Thefocushasbeen,inparticular,ontwoissues.ThefirstistheoverabundanceofsubstructuresinsimulatedhalosofMilkyWaysize,withrespecttotheobservednumberofgalacticsatellites[3,4].Thesecondregardstheriseintherotationcurvesofsmall,DM-dominatedgalaxieswhichseems,onaverage,topointtoprofilesofDMden-sitywithaflatinnercore(see,e.g.,[5,6,7]),asopposedtothelargeconcentrationsandcuspyprofilesfoundinsimulations[8,9,10].Thedebateonwhetherthesedis-crepanciesarecallingforadeeperunderstandingoftheastrophysicalandcosmologicalprocessesinconnectiontostructureformation,orwhethertheyareactuallypoint-ingtoadrasticchangeoftheCDMframework,isstillopen.Inbothcases,thediscrepanciesbetweenobservationsandsimulationsarealleviatedinschemeswithasuppres-sionofthepowerspectrumatsmallscales.Variantstothestandardcosmologicalmodelembeddingthisfeature,whileleavingthepictureatlargescalesunchanged,mostofteninvolvea(mild)violationofatleastoneofthethreepropertieslistedaboveforCDM.Proposalsinclude:self-interactingDM[11,12],warmdarkmatter(WDM)[13],DMwithaverylargepairannihilationrate[14],andfuzzyDM[15].Ingeneral,itismuchmorecontrivedtoconstructSMextensionswithparticlesofthiskind.Forexample,asterileneutrinowithamassinthekeVrange,oneofthemostpromisingcandidateforWDMhasre-centlybeenexcludedasathermalDMcandidate[16,17].Analternativeapproachinvolvesintroducingtwodis-tinctphasesinstructureformation.ThisisachievedbyassumingthatatleastpartoftheDMobservedintheUniversetodayhasbeenproduced,atlatetimes,inthedecayofalong-livedspecies.Insuchasetuptwodif-ferentmechanismscanprovidetheneededsuppressionofthepowerspectrum,dependingonthenatureofthe2decayingspecies.IfallDMisgeneratedinthedecayofachargedspecies,thematterpowerspectrumisessentiallycutoffonscalesthatenteredthehorizonbeforethedecay[18].Thisisbecausethechargedspeciesistightlycoupledtothephoton-baryonfluid.InRef.[19],anexplicitmodelwasconstructedintheminimalsupersymmetricstan-dardmodel(MSSM)context.Inthismodel,afractionoftoday’sDMneutralinosisproducedinthelatede-cayofstaus,implyingascale-dependentmatterpowerspectrum,andinturn,areducedpoweronsmallscales.Asrecentlynoti