Experiments for Double-Beta Decay

arXiv:nucl-ex/0301011v38Feb20038February200821:12WSPC/INSTRUCTIONFILEElliott˙BBDKInternationalJournalofModernPhysicsAcWorldScientificPublishingCompanyExperimentsforNeutrinolessDouble-BetaDecayStevenR.ElliottPhysicsDivision,LosAlamosNationalLaboratoryMSH803,P-23,LosAlamos,NM,87545,USAReceived(DayMonthYear)Revised(DayMonthYear)Therecentneutrinooscillationexperimentalresultsindicatethatatleastoneneutrinohasamassgreaterthan50meV.Thenextgenerationofdouble-betadecayexperimentswillverylikelyhaveasensitivitytoaneffectiveMajorananeutrinomassbelowthistarget.Thereforethisisaveryexcitingtimeforthisfieldofresearchasevennullresultsfromtheseexperimentshavethepotentialtoelucidatethenatureoftheneutrino.Keywords:Double-betadecay,Neutrinomass.1.IntroductionDouble-betadecay(ββ)isanuclearprocesswherethenucleusincreasesinchargeby2whileemitting2electrons.Two-neutrinodouble-betadecay(ββ(2ν))isanallowedsecondorderweakprocessthatalsoemitstwoneutrinos.Zero-neutrinodouble-betadecay(ββ(0ν))canonlyoccurifneutrinosaremassiveparticlesthatareselfconjugate(i.e.massiveMajoranaparticles).Therecentexperimentalresultsfromatmospheric1,2,3,4,solar5,6,7,8,9,andreactor10,11,12neutrinosourcesindicateneutrinoshavemassandthattheymix.Inparticular,atleastoneneutrinomusthaveamassgreaterthanabout50meV.Thisveryexcitingresulthasgreatlyrenewedtheinterestinββ(0ν)becausethenextgenerationexperimentsaimtoreachthistarget.Infactofalltheexperimen-taltechniquestosearchforneutrinomass,onlyββ(0ν)experimentscanreachanabsolutemassvaluenearthe50-meVmassscaleindicatedbytheoscillationex-periments.Furthermore,onlyββ(0ν)hasthepotentialtodeterminewhethertheneutrinoisitsownanti-particle.Thisexcitingsituationandthepresentexperimen-talandtheoreticalstatushasbeendescribedinarecentreview13andthemanyreferencestherein.Althoughanyobservationofββ(0ν)hasalwaysbeenanexcitingpossibility,inthenot-to-recentpast,limitsontheMajoranamassfromthenon-observationofββ(0ν)couldbeinterpretedintwodistinctways.Perhapstheneutrinomasswasjustsmallerthanthelimit.Or,sinceββ(0ν)isonlysensitivetoMajorananeutrinos,itcouldbethattheneutrinowasaDiracparticlethathasamassgreaterthanthe18February200821:12WSPC/INSTRUCTIONFILEElliott˙BBDK2StevenR.Elliottlimit.Now,however,thereisaknownminimummassscalefortheneutrino.Anullresultbelowthismassscalecouldeliminatepresently-possiblemassspectrafromhavingaMajoranacomponent.Thereforesucharesultwouldbeanon-trivialconstraintonneutrinoproperties.Theoscillationresultsconvincinglyindicatetwomass-differencescales:Oneatδm2atm∼(50meV)2atapproximatelymaximalmixingandoneatδm2sol∼(7meV)2atamixinganglenear30degrees.TheseresultsindicatethatneutrinosdohavemassbutdonotgiveanindicationoftheabsoluteneutrinomassscaleoritsCPnature.Furthermore,wehavelittledataonthehierarchyoftheneutrinomasses.Thatiswedonotknowifνeispredominatelycomposedofthelightestmasseigenstateornot.ββ(0ν)canprovidedatatoaddresstheseneutrinoproper-ties,butbecauseofpossiblenon-trivialMajoranaphasesinthemixingmatrix,wewillalsorequireinputfromkinematicneutrinomassexperimentssuchasKatrin14.Therefore,allthreetypesofexperiments,oscillation,ββ(0ν),andkinematic,arerequiredtocompletelyunderstandtheneutrino.Notethatindicationsofathirdmass-differencescalenear1eV2fromtheLSNDexperiment15areasyetuncon-firmed.AlthoughIhavechosentoignorethatresultinthissimplediscussion,thelargemassscalewouldonlyaddtotheinterestinββexperiments.Thedecayrate(Γ)ofdoublebetadecayisproportionaltothesquareoftheeffectiveMajorananeutrinomass(hmββi),aneasilycalculablephasespacefactor(G),andadifficult-to-calculatenuclearmatrixelement(M0ν);Γ=GM0νhmββi2.(1)Thevalueforhmββiinturn,dependsonthevaluesoftheindividualneutrinomasseigenstates(mi),themixingmatrixelements(Uei)andtheMajoranaphases(αi);hmββi2=NXi|Uei|2eαimi2.(2)InthecaseofCPconservationeαi=±1.IfwetakeUe3=0,theatmosphericmixingtobemaximal,andthesolarmixingangletobeabout30degrees(U2e1=0.25,U2e2=0.75),onecansimplifythisequationforhmββiandgetafeelingastothesensitivitythatββ(0ν)hastotheabsolutemassscale.hmββi∼|Ue1|2m1±|Ue2|2qm21+δm212.(3)ForMajorananeutrinosinthenormalhierarchy,onefindshmββivaluesgreaterthanseveralmeVandintheinversehierarchyonefindsvaluesgreaterthanseveraltensofmeVevenwhenthesmallestmasseigenstateisnearzero.Simplystated,pδm2atmisaveryenticingtargetforββ(0ν)experimentsandtherecentexcitementinββ(0ν)isthatthenextgenerationofexperimentswillbesensitivetohmββinearthistarget.8February200821:12WSPC/INSTRUCTIONFILEElliott˙BBDKExperimentsforNeutrinolessDouble-BetaDecay32.BackgroundstoDouble-BetaDecayToobserveββ(0ν),onemustreducebackgroundssothatanultra-low-ratepeakcanemergefromacontinuumofotherprocesses.Toreachthedesired50-meVgoal,significantbackgroundimprovementsareneeded.Thebackgroundsthatcurrentlyafflictββ(0ν)experimentscanbeclassifiedinto3groups:ββ(2ν),naturalorman-maderadioactivities,andcosmogeniclyproducedradioactivities.2.1.Two-NeutrinoDouble-BetaDecayasaBackgroundUnlikeββ(0ν),thetwoelectronssharetheavailableenergywithtwoneutrinosintheββ(2ν)process.Thustheirsumenergyspectrumisadistributionuptotheendpoint.(SeeFig.1.