Antideuterons as a Signature of Supersymmetric Dar

arXiv:hep-ph/9904481v128Apr1999AntideuteronsasaSignatureofSupersymmetricDarkMatter§FiorenzaDonatoa∗,NicolaoFornengobandPierreSalatia,ca)LaboratoiredePhysiqueTh´eoriqueLAPTH,BP110,F-74941Annecy-le-VieuxCedex,France.b)InstitutodeF´isicacorpuscular-C.S.I.C.,DepartamentodeF´isicaTe`orica,UniversitatdeVal´encia,C./DrMoliner50,E–46100Burjassot,Val´encia,Spain.c)Universit´edeSavoie,BP110473011Chamb´eryCedex,France.DraftVersionofFebruary1,2008Oncetheenergyspectrumofthesecondarycomponentiswellunderstood,measurementsoftheantiprotoncosmic–rayfluxattheEarthwillbeapowerfulwaytoindirectlyprobefortheexistenceofsupersymmetricrelicsinthegalactichalo.Unfortunately,itisstillspoiltbyconsiderabletheoreticaluncertainties.Asshowninthiswork,searchesforlow–energyantideuteronsappearinthemeantimeasaplausiblealternative,worthbeingexplored.AboveafewGeV/n,adozenspallationantideuteronsshouldbecollectedbythefutureAMSexperimentonboardISSA.Forenergieslessthan∼3GeV/n,the¯Dspallationcomponentbecomesnegligibleandmaybesupplantedbyapotentialsupersymmetricsignal.Ifafewlow–energyantideuteronsarediscovered,thisshouldbeseriouslytakenasacluefortheexistenceofmassiveneutralinosintheMilkyWay.I.INTRODUCTION.Cosmicrayfluxesareabouttobemeasuredwithunprecedentedprecisionbothbyballoonbornedetectorsandbyspaceinstruments.Thevariousongoingexperimentsarealsohuntingfortracesofantimatterinthecosmicradiation.TheBESScollaboration[1]planstopushthelimitonthe¯He/Heratiodownto10−8whereastheAMSspectrometershouldreachasensitivityof∼10−9onceitisinstalledontheInternationalSpaceStationAlpha(ISSA)[2].Thesearchforantinucleihasprofoundcosmologicalimplications.Thediscoveryofasingleantiheliumoranticarbonwouldactuallybeasmokinggunfortheexistenceofantimatterislandsinourneighborhood.However,lightantinuclei,mostlyantiprotonsbutalsoantideuterons,areactuallyproducedinourGalaxyassecondaries.Theyresultfromtheinteractionofhigh–energycosmic–rayprotonswiththeinterstellargasoftheMilkyWaydisk.Inapreviousanalysis,Chardonnetetal.[3]haveestimatedthefluxofantideuterium¯Dandantihelium¯3Hesecondaries.The¯DsignalisveryweakbutmaymarginallybedetectedbyAMSonboardISSA.Thecaseofantiheliumis,atleastforthemoment,hopeless.ThedarkmatteroftheMilkyWaycouldbemademostlyofelementaryparticlessuchastheheavyandneutralspeciespredictedbysupersymmetry.Themutualannihilationsoftheserelics,potentiallyconcealedinthehaloofourGalaxy,wouldthereforeproduceanexcessinthecosmicradiationofgammarays,antiprotonsandpositrons.Inparticular,supersymmetricantiprotonsshouldbeabundantatlowenergy,aregionwherethefluxof¯psecondariesisapriorinegligible.Thereisquiteanexcitementtryingtoextractfromtheobservationsapossible¯pexoticcomponentwhichwouldsignalthepresenceofsupersymmetricdarkmatterintheGalaxy.Unfortunately,ithasbeenrecentlyrealized[4–6]thatafewprocessesadduptogethertoflattenout,atlowenergy,thespectrumofsecondaryantiprotons.Ionisationlossesaswellasinelasticbutnon-annihilatingscatteringsonthehydrogenatomsofthegalacticdiskresultintothedecreaseoftheantiprotonenergy.Thelow–energytailofthe¯pspectrumisreplenishedbythemoreabundantpopulationfromhigherenergies.Thateffectisfurtherstrengthenedbysolarmodulationwhichalsoshiftstheenergyspectrumtowardslowerenergies.Asaresultoftheseeffects,thesecondary¯p’saremuchmoreabundantatlowenergy§ReportNos.IFIC/99-9,FTUV/99-9,LAPTH–722/99∗E–mail:[e-mail],fornengo@flamenco.ific.uv.es,[e-mail]ersymmetriccontributionfromtheconventionalcomponentofspallationantiprotonsmayturnouttobeaverydifficulttask.Theantiprotonsignalofsupersymmetricdarkmatteristhereforeinjeopardy.Antideuterons,i.e.,thenucleiofantideuterium,arefreefromsuchproblems.AsexplainedinSect.II,theyformwhenanantiprotonandanantineutronmergetogether.Thetwoantinucleonsmustbeatrestwithrespecttoeachotherinorderforfusiontotakeplacesuccessfully.Forkinematicreasons,aspallationreactioncreatesveryfewlow–energyparticles.Low-energysecondaryantideuteronsareevenfurthersuppressed.Energylossmechanismsarealsolessefficientinshiftingtheantideuteronenergyspectrumtowardslowenergies.Thecorrespondinginterstellar(IS)fluxisderivedinSect.III,forenergiesintherangeextendingfrom0.1upto100GeV/n.Itreachesamaximumof2−5×10−8¯Dm−2s−1sr−1GeV−1forakineticenergyof∼4GeV/n.AdozenofsecondaryantideuteronsshouldbecollectedbytheAMS/ISSAexperiment.Ontheotherhand,supersymmetric¯D’saremanufacturedatrestwithrespecttotheGalaxy.Inneutralinoannihilations,antinucleonsarepredominantlyproducedwithlowenergies.Thisfeatureisfurtherenhancedbytheirsubsequentfusionintoantideuterons,henceafairlyflatspectrumforsupersymmetricantideuteriumnucleiasshowninSect.IV.BelowafewGeV/n,secondaryantideuteronsarequitesuppressedwithrespecttotheirsupersymmetricpartners.Thatlow–energysuppressionisordersofmagnitudemoreeffectiveforantideuteronsthanforantiprotons.Thismakescosmic–rayantideuteronsamuchbetterprobeofsupersymmetricdarkmatterthanantiprotons.Unfortunately,antideuteronfluxesarequitesmallwithrespectto¯p’s.WeneverthelessshowinSect.Vthatasignificantportionofthesupersymmetricparamete