Dark Matter in the Cosmos - Its Direct Detection a

arXiv:nucl-th/0411021v14Nov2004DARKMATTERINTHECOSMOS-ItsDirectDetectionandtheroleofNuclearPhysics.J.D.VERGADOSA,BATheoreticalPhysicsDivision,UniversityofIoannina,Gr45110,Ioannina,Greece.BT-DO,TheoreticalPhysicsDivision,LANL,LosAlamos,N.M.87545.ExoticdarkmattertogetherwithdarkenergyorcosmologicalconstantseemtodominateintheUniverse.Anevenhigherdensityofsuchmat-terseemstobegravitationallytrappedinourGalaxy.Thenatureofdarkmattercanbeunveiledonly,ifitisdetectedinthelaboratory.Thustheaccomplishmentofthistaskiscentraltophysicsandcosmology.Currentfashionablesupersymmetricmodelsprovideanaturaldarkmattercandi-date,whichisthelightestsupersymmetricparticle(LSP).Suchmodelscombinedwithfairlywellunderstoodphysicslikethequarksubstructureofthenucleoncanyieldtheeffectivetransitionoperatoratthenucleonlevel.Weknow,however,thattheLSPismuchheavierthantheproton,whileitsaverageenergyisinthekeVregion.Thusthemostlikelypossibil-ityforitsdirectdetectionisviaitselasticscatteringwithanucleartargetbyobservingtherecoilingnucleus.Inordertoevaluatetheeventrateoneneedsthenuclearstructure(formfactorand/orspinresponsefunction)forthespecialnucleartargetsofexperimentalinterest.Sincetheexpectedratesforneutralino-nucleusscatteringareexpectedtobesmall,oneshouldexploitallthecharacteristicsignaturesofthisreaction.Suchare:(i)InthestandardrecoilmeasurementsthemodulationoftheeventrateduetotheEarth’smotion.(ii)Indirectionalrecoilexperimentsthecorrelationoftheeventratewiththesun’smotion.Onenowhasbothmodulation,whichismuchlargeranddependsnotonlyontime,butonthedirectionofobser-vationaswell,andalargeforward-backwardasymmetry.(iii)InnonrecoilexperimentsgammaraysfollowingthedecayofexcitedstatespopulatedduringtheNucleus-LSPcollision.Branchingratiosofabout6percentarepossible.(iv)novelexperimentsinwhichoneobservestheelectronsprodusedduringthecollisionoftheLSPwiththenucleus.Branchingratiosofabout10percentarepossible.21.IntroductionOneofthegreatestdiscoveriesofthelastcoupleofdecaysistherealiza-tionthatthematter,whichcanshine,constitutesaverysmallfractionoftheuniverse.Theuniverseiscomposedprimarilyofdarkmatteranddarkenergy,whichareincapableofemittinglight.IfonedefinesΩi=ρiρcforthespeciesi,whereρcthecriticaldensity(seesec.2),therecentcosmologicalobservations[1],whencombined,leadto:Ωb=0.05,ΩCDM=0.30,ΩΛ=0.65forordinarymatter,darkmatteranddarkenergyrespectively.Theirsumis[1]Ω=1.11±0.07,i.e.unitywithintheexperimentalerrors,whichmeansthattheuniverseisflat.Additionalindirectinformationcomesfromtherotationalcurves[2].Therotationalvelocityofanobjectincreasessolongissurroundedbutmatter.Onceoutsidematterthevelocityofrotationdropsasthesquarerootofthedistance.Suchobservationsarenotpossibleinourowngalaxy.Theobservationsofothergalaxies,similartoourown,indicatethattherotationalvelocitiesofobjectsoutsidetheluminousmatterdonotdrop(seeFig.1).Sotheremustbeahaloofdarkmatteroutthere.Aswehavealreadymentioned,however,itisonlythedirectdetec-tionofdarkmatterinthelaboratory,whichwillunravelthenatureoftheconstituentsofdarkmatter.Infactonesuchexperiment,theDAMA,hasclaimedtheobservationofsuchsignals,whichwithbetterstatisticshassubsequentlybeeninterpretedasmodulationsignals[3].Thesedata,how-ever,iftheyareduetothecoherentprocess,arenotconsistentwithotherrecentexperiments,seee.g.EDELWEISSandCDMS[4].Supersymmetrynaturallyprovidescandidatesforthedarkmattercon-stituents.InthemostfavoredscenarioofsupersymmetrytheLSPcanbesimplydescribedasaMajoranafermion(LSPorneutralino),alinearcom-binationoftheneutralcomponentsofthegauginosandhiggsinos[6]-[9].WearenotgoingtoaddressissuesrelatedtoSUSYinthispaper.MostSUSYmodelspredictnucleoncrosssectionsmuchsmallerthethepresentexperimentallimitσS≤10−5pbforthecoherentprocess.Asweshallseebelowtheconstraintonthespincross-sectionsislessstringent.Sincetheneutralinoisexpectedtobenonrelativisticwithaverageki-neticenergyT≈40KeV(mχ/100GeV),itcanbedirectlydetectedmainlyviatherecoilingofanucleus(A,Z)inelasticscattering.Insomerareinstancesthelowlyingexcitedstatesmayalsobepopulated[11].Inthiscaseonemayobservetheemittedγrays.Finallyithasalsorecentlybeensuggested[23]thatonemaybeabletodetecttheneutralinobyobservingtheionizationelectronsproducedduringitscollisionwiththenucleus.3Figure1.Therotationalvelocitydistance.Clearlyitdoesnotfalloutsidetheluminousmatter.ThisissuggestiveofthepresenceofadarkmatterhalooutsidetheM33galaxy.IneverycasetoextractfromthedatainformationaboutSUSYfromtherelevantnucleoncrosssection,onemustknowtherelevantnuclearmatrixelements[12]-[13].Thestaticspinmatrixelementsusedinthepresentworkhavebeenobtainedfromtheliterature[11].Anywaysinceinalltypesofexperimentstheobtainedratesareverylow,onewouldliketobeabletoexploitthemodulationoftheeventratesduetotheearth’srevolutionaroundthesun[14],[15].Inordertoaccomplishthisoneadoptsafoldingprocedure,i.eonehastoassumesomevelocitydistribution[14,15,17,18]fortheLSP.Onealsowouldliketoexploitothersignaturesexpectedtoshowupindirectionalexperiments[19].Thisispossible,sincethesunismovingwithrelativelyhighvelocitywithrespecttothecenterofthega