Neutrinoless Double Beta Decay of Ge76, Se82, Mo10

arXiv:nucl-th/0107016v16Jul2001NeutrinolessDoubleBetaDecayof76Ge,82Se,100Moand136Xetoexcited0+statesF.ˇSimkovic1,3)M.Nowak2),W.A.Kami´nski2),A.A.Raduta1,4)andAmandFaessler1)1InstituteofTheoreticalPhysics,UniversityofTuebingen,D–72076Tuebingen,Germany2DepartmentofTheoreticalPhysics,MariaCurie–SklodowskaUniversity,PL-20031Lublin,Poland3DepartmentofNuclearPhysics,ComeniusUniversity,Mlynsk´adolinaF1,SK–84215Bratislava,Slovakia4InstituteofPhysicsandNuclearEngineering,Bucharest,POBMG6,RomaniaandDept.ofTheoreticalPhysicsandMathematics,BucharestUniversity,POBMG11,Romania(February8,2008)AbstractTheneutrinolessdoublebetadecay(0νββ-decay)transitiontothefirstex-cited0+collectivefinalstateisexaminedforA=76,82,100and136nucleibyassuminglightandheavyMajorananeutrinoexchangemechanismsaswellasthetrilinearR-parityviolatingcontributions.RealisticcalculationsofnuclearmatrixelementshavebeenperformedwithintherenormalizedQuasi-particleRandomPhaseApproximation.Transitionstothefirstexcitedtwo-quadrupolephonon0+statearedescribedwithinabosonexpansionformalismandalternativelybyusingtheoperatorrecouplingmethod.Wepresentthesensitivityparameterstodifferentleptonnumberviolatingsignals,whichcanbeusedinplanningthe0νββ-decayexperiments.Thehalf-lifeof0νββ-decaytothefirstexcitedstate0+1isbyafactorof10to100largerthanthatofthetransitiontothegroundstate,0+g.s..PACSnumbers:23.40.BW,23.40.HC1I.INTRODUCTIONTheneutrinolessdoublebetadecay(0νββ-decay),whichviolatesthetotalleptonnumberbytwounits,isthemostsensitivelow-energyprobeforphysicsbeyondthestandardmodel(SM)[1–5].Theobservationofthe0νββ-decaywouldprovideunambiguousevidencethatatleastoneoftheneutrinosisaMajoranaparticlewithnon-zeromass[6].Thisconclusionisvalidwithoutspecifyingwhichfromtheplethoraofpossible0νββ-decaymechanismstrig-geredbyexchangeofneutrinos,neutralinos,gluinos,leptoquarksetcistheleadingone.Thecurrentexperimentalupperlimitsonthe0νββ-decayhalf-lifeimposestringentconstraints,e.g.,ontheparametersofGrandUnification(GUT)andsupersymmetric(SUSY)extensionsoftheSM.Thereisacontinuous,bothexperimentalandtheoretical,activityinthefieldof0νββ-decay.Aninterestingissueiswhataretheimplicationsoftheneutrinooscillationphe-nomenologytothe0νββ-decay.Wenotethattheresultsofthesolar[7],atmospheric[8]andterrestrial[9]neutrinoexperimentsprovideaconvincingevidenceofneutrinooscillations,whichrequirenon-vanishingmassesforneutrinosaswellasneutrinomixing[10].Theneu-trinooscillationsaresensitivetothedifferencesofthemassessquaredandcannotdistinguishbetweenDiracandMajorananeutrinos.Nevertheless,ifassumptionsaboutthecharacter(DiracorMajorananeutrinos),thephasesandtheneutrinomixingpatternareconsidered,onecanderiveestimatesfortheeffectiveMajoranaelectronneutrinomassmνresponsi-blefor0νββ-decay.Thecurrentviableanalysisimpliestheeffectiveneutrinomassmνtobewithintherange10−3eV≤mν≤1eV[11,12].Thepresentgenerationof0νββ-decayexperiments[13–15]achievesensitivitiesofT0ν1/2∼1024−1025yearsfordifferentisotopes[13–15].Itcorrespondstomν≈0.5−1.eV[16,17],i.e.,these0νββ-decayexperimentsallowalreadytodiscriminateamongvariousneutrinomixingschemes.Theneutrinooscillationsimplythatperhapsweareclosetotheobservationofthe0νββ-decay.Thiswouldbeamajorachievement.Maybeitisenoughtoincreasethesensitivitytomνbyaboutanorderofmagnitude,i.e.,the0νββ-decayexperimentsshouldbe2sensitivetohalf-lifesof1027−1028yearsforthegroundtogroundtransitions.Wehopethatthesedatawillstimulatenewexperimentalactivities.Ambitiousplansareunderwaytopushtheupperconstraintsonleptonnumberviolatingparametersfurtherdown.Byusingseveraltonsofenriched76Ge,theGENIUSexperimentisexpectedtoprobemνupto10−2eV[18].TheCUOREexperimentintendstosearchforrareeventswiththehelpofacryogenicTeO2detectorwithhighenergyresolution[19].TheongoingNEMO3experiment,nowunderconstructionintheFr´ejusundergroundlaboratory,willmeasureupto10kgofdifferentdoublebetadecayisotopes[20].BothCUOREandNEMO3haveachancetoreachasensitivitytotheeffectiveneutrinomassmνintheorderof0.1eV[17]Itisworthtoexaminealsootherpossibilitiestoincreasethesensitivityof0νββ-decayexperiments.Tillnow,theattentionwasconcentratedmostlytothe0νββ-decaytransitiontothegroundstateofthefinalnucleus.However,theremightbeachancethatthetransitionstotheexcited0+and/or2+finalstatesaremorefavorableexperimentally,atleastforaparticularmechanismforthe0νββ-decay[15,21].Generallyspeaking,transitionstotheexcitedstatesaresuppressedduetothereducedQββvalue.However,thisrestrictioncanbecompensatedbyapossiblelowerbackgroundduetoacoincidenceoftheβparticleswiththeγorγ’sfromtheexcitedfinalstate.Thepossibleadvantagedependsontheratioofcorrespondingnuclearmatrixelementstotheexcitedandtothegroundstate.Iftheirvaluesarecomparable,the0νββ-decayexperimentmeasuringtransitionstogroundandexcitedfinalstatescouldbeofasimilarsensitivity.Recently,thisissuereceivedanincreasingattentionfrommanyexpertsinthefield.The0νββ-decayof76Geand100Motothefirstexcited2+1finalstatehasbeeninvestigatedinRef.[22]byassumingmassiveMajorananeutrinosandright-handedweakcurrents.Itwasfoundthatthezeroneutrinodoublebetadeca