A Search for various Double Beta Decay Modes of Cd

arXiv:nucl-ex/0301007v28Apr2003ASearchforvariousDoubleBetaDecayModesofCd,TeandZnIsotopesH.Kiel,D.M¨unstermannLehrstuhlf¨urExperimentellePhysikIV,Universit¨atDortmund,Otto–HahnStr.4,44221Dortmund,GermanyK.ZuberDenysWilkinsonLaboratory,Dept.ofPhysics,UniversityofOxford,KebleRoad,Oxford,OX13RH,UKAbstractVariousdoublebetadecaymodesofCd,ZnandTeisotopesareexploredwiththehelpofCdTeandCdZnTesemiconductordetectors.Thedatasetissplittedinanenergyrangebelow1MeVhavingastatisticsof134.5g·dandoneabove1MeVre-sultingin532g·d.Nosignalswereobservedinallchannelsunderinvestigation.Newimprovedlimitsfortheneutrinolessdoublebetadecayof70ZnofT1/21.3·1016yrs(90%CL),thelongeststandinglimitofalldoublebetaisotopes,and0νβ+ECof120TeofT1/22.2·1016yrs(90%CL)aregiven.Forthefirsttimealimitonthehalf-lifeofthe2νECECof120TeofT1/29.4·1015yrs(90%CL)isobtained.Inaddition,limitson2νECECforgroundstatetransitionsof106Cd,108Cdand64Znareimproved.TheobtainedresultsevenunderroughbackgroundconditionsshowthereliabilityofCdTesemiconductordetectorsforrarenucleardecaysearches.Keywords:Neutrinomass,doublebetadecay,leptonnumberviolationPACS:23.40.-s,21.10.Tg,27.60.+j,29.40.Wk1IntroductionConservationlawsplayacrucialroleinmodernparticlephysics,normallyresultingfrominvariancesundercertainsymmetrytransformations.However,therearesymmetriesintheStandardModel,wheresuchanunderlyingin-varianceisnotknown,themostpopularonesarebaryonandleptonnumberconservation.Therefore,theinvestigationofleptonnumberviolatingprocessesisoneofthemostpromisingwaysofprobingphysicsbeyondtheStandardPreprintsubmittedtoElsevierScience8February2008Model.Aparticularaspectofthistopicisleptonnumberviolationintheneu-trinosector,whichinthecaseofmassiveneutrinoswouldallowavarietyofnewphenomena,forarecentreviewsee[1].LeptonnumberviolationemergesimmediatelyincaseoftheexistenceofMa-joranamassesoftheneutrino,whichispredictedinmostGUT-theories,byusingthesee-sawmechanismtoexplainsmallneutrinomasses[2,3,4].Thegold-platedchanneltoprobetheMajoranacharacterofneutrinosistheΔL=2processofneutrinolessdoublebetadecay(0νβ−β−):(Z,A)→(Z+2,A)+2e−(1)Themeasuredhalf-lifeiscorrelatedwiththeneutrinomassviaT−11/2=G0νM20νhmeeime!2(2)withG0νasthephasespaceandM20νasthenucleartransitionmatrixelements.ThequantityhmeeiiscalledtheeffectiveMajoranamassandisgivenbyhmeei=|XU2eimiηCP|(3)wheremiarethemasseigenstates,ηCP=±1therelativeCP-phasesandUeithemixingmatrixelements.TheothereightpossibleeffectiveMajoranamasseshmαβiwithα,β=e,μ,τarecurrentlynotrestrictedatall[5].Inaddition,theStandardModelprocessof2νββ-decaycanoccur(Z,A)→(Z+2,A)+2e−+2¯νe(4)whosedetectionisimportanttocheckthereliabilityofnuclearmatrixelementcalculations.VariousothermechanismsbesidealightMajorananeutrinoex-changehavebeenproposedtomediate0νββ-decay,amongthemaree.g.right-handedweakcurrents[6]andR-parityviolatingSUSY[7,8].Todisentangletheunderlyingmechanismitisworthwhiletoexploreadditionalprocesses.IncaseoftheexistenceofrighthandedweakcurrentsthefullHamiltonianfordoublebetadecaycanbewrittenasH∼(jLJ†L+κjLJ†R+ηjRJ†L+λjRJ†R)(5)wherejdenotingleptoniccurrentsandJhadroniccurrentsandκ,η,λ≪1.ThesubscriptLdenotesleft-handedweakchargedcurrentsof(V-A)structureiiandRcorrespondstoright-handedweakchargedcurrentsof(V+A)type,whichhavenotbeenobservedyet.Inthiscaseeq.2hastobereplacedbyT−11/2=Cmm(hmνeime)2+Cηηhηi2+Cλλhλi2(6)+Cmη(hmνeime)hηi+Cmλ(hmνeime)hλi+Cηλhηihλi(7)wherethecoefficientsCcontainthephasespacefactorsandthematrixele-mentsandhηi=ηXjUejVejhλi=λXjUejVej(8)Vejcorrespondtothemixingmatrixelementsamongtheright-handedneu-trinostates.Thisresultsinanellipsoidinthethreeparametershmνei,hλiandhηi.Decaymodeswhicharedominantlydrivenbyright-handedweakcur-rentsaretransitionstoexcitedstatesandβ+β+-decays.Thelattercanoccurinthreevariants:(Z,A)→(Z−2,A)+2e+(+2νe)(β+β+)(9)e−B+(Z,A)→(Z−2,A)+e+(+2νe)(β+/EC)(10)2e−B+(Z,A)→(Z−2,A)(+2νe)(EC/EC)(11)Likein0νβ−β−theprocesses(9)-(11)canoccurwithandwithouttheemis-sionofneutrinos.TheQ-valueofthetransitionisreducedbymec2foranyproducedpositrons.Therefore,EC/EChasthelargestphasespace.ClearlyneutrinolessEC/ECinthestatedformviolatesenergyandmomentumcon-servation,thereforeadditionalparticleshavetobeemitted.Asdiscussedin[9]themostlikelymechanismisthecreationofanadditionalelectron-positronpairviainternalconversion.Howbothmodesmightdisentangletheunderlyingparametersisshowninfigure1followingthedescriptionof[10].InthispaperweexplorethevariousdecaychannelsofCd,TeandZnisotopes.ThisisdonebyusingCdZnTeandCdTesemiconductordetectorsasafirststeptowardstherealisationoftheCOBRA-project[13].Thecorrespondingdecayschemesofallinvestigatedisotopesareshowninfigure2,takenfrom[14].Forarecentreviewondoublebetadecaysee[15]andacompilationofcurrentresultscanbefoundin[16,17].iii-3-2-1123m-1.5·10-6-1·10-6-5·10-75·10-71·10-61.5·10-6lFig.1.Complementaryof0νββ-decayandβ+β+-decaymodes.Shownisthehλi-hmνeiplane(assuminghηi=0forsimplicity).Thesmallellipse(solidline)cor-respondstotheallowedparametervaluesoftherecentlyclaimedevidencefor0νββ-decay[11](seealso[12]).Alsoshownaretheparameterregions(dashedlines)