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New Results from the NEMO 3 Experiment

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arXiv:0807.2157v1 [nucl-ex] 14 Jul 2008NEWRESULTSFROMTHENEMO3EXPERIMENT

V.I.TRETYAKonbehalfoftheNEMOCollaboration

InstitutePluridisciplinaireHubertCurien,

23RueduLoess,BP28,67037Strasbourg,France

onleaveofabsencefromJointInstituteforNuclearResearch,

JoliotCurie6,41980Dubna,Russia

NEMO3isacurrentlyrunningexperimenttosearchfortheneutrinolessdoublebetadecay

andtostudythetwo-neutrinodoublebetadecaywith10kgofenrichedisotopes.Onthebasis

ofthefirsttwoyearsofdatataking,alimitontheneutrinolessdecayT0ν

1/2>5.8·1023

yat90%

CLwasobtainedwith100

Mo.Thetwo-neutrinodoublebetadecayhalf-livesweremeasuredfor

allsevenββisotopeslocatedinNEMO3.Newresultsareobtainedfortwooftheseisotopes:

48

CaT2ν

1/2=[4.4+0.5

−0.4(stat)±0.4(syst)]·1019

y,96

ZrT2ν

1/2=[2.3±0.2(stat)±0.3(syst)]·1019

y.

1Introduction

Experimentalsearchfortheneutrinolessdoublebetadecay(0νββ)isofamajorimportancein

particlephysicsbecauseifobserved,itwillrevealtheMajorananatureoftheneutrino(ν≡¯ν)

andmayallowanaccesstotheabsoluteneutrinomassscale.The0νββ-decayviolatesthe

leptonnumberandisthereforeadirectprobeforthephysicsbeyondthestandardmodel.A

possibilityofthisprocessmayberelatedtoright-handedcurrentsinelectroweakinteractions,

supersymmetricparticleswithR-paritynonconservation,masslessGoldstonebosonssuchas

majorons.Inthecaseoftheneutrino-massmechanismthe0νββdecayratecanbewrittenas

[T0ν

1/2(A,Z)]−1

=󰀇m

ν󰀈2

·|M0ν

(A,Z)|2

·G0ν

(Q

ββ,Z),(1)

where󰀇m

ν󰀈istheeffectiveneutrinomass,M0ν

isthenuclearmatrixelement(NME),andG0ν

isthekinematicalfactorproportionaltothetransitionenergytothefifthpower,Q5

ββ.

Thespontaneoustwo-neutrinodoublebetadecay(2νββ)isararesecond-orderweakinter-

actionprocess.Theaccuratemeasurementofthe2νββ-decayisimportantsinceitconstitutes

theultimatebackgroundinthesearchfor0ν-decaysignal.Itisusefulforthetestofthenuclear

structureandprovidesvaluableinputforthetheoreticalcalculationsofthe0νββ-decayNME.

TheobjectiveoftheNEMO3experimentisthesearchforthe0νββ-decayandinvestigation

ofthe2νββ-decaywith10kgofββisotopes.

2NEMO3experiment

2.1TheNEMO3detector

TheNEMO3hasbeentakingdatasince2003intheModaneundergroundlaboratorylocated

intheFrejustunnelatthedepthof4800mw.e.Itsmethodofββ-decaystudyisbasedonthe

detectionoftheelectrontracksinatrackingdeviceandtheenergymeasurementinacalorimeter.The

detector1hasacylindricalshape.Thinsourcefoils(∼50mg/cm2

)arelocatedin

themiddleofthetrackingvolumesurroundedbythecalorimeter.Almost10kgofenriched

ββisotopes(listedinTable1)wereusedtoproducethesourcefoils.Thetrackingchamber

contains6180opendriftcellsoperatingintheGeigermode.Itprovidesavertexresolutionof

about1cm.Thecalorimeterconsistsof1940plasticscintillatorblockswithphotomultiplier

readout.Theenergyresolutionis14-17%/√

Mass(g)Signal/Background

100

Mo30340.711±0.002(stat)±0.054(syst)2

82

Se29959.6±0.3(stat)±1.0(syst)2

116

Cd28052.8±0.1(stat)±0.3(syst)

150

Nd33670.920+0.025

−0.022(stat)±0.062(syst)

96

Zr33502.3±0.2(stat)±0.3(syst)

48

Ca47724.4+0.5

−0.4(stat)±0.4(syst)

130

Te252976±15(stat)±8(syst)

Themeasurementofthe96

Zrhalf-lifewasperformedusingthedatacollectedwithin925

days.Atotalof678eventswereselected,withtheexpected328backgroundevents.Thelargestbackgroundcontributionisduetotheinternal40

Kcontaminationofthesample.The

distributionoftheenergysumoftwoelectronsandtheirangulardistributionareshowninFig.1,demonstratinggoodagreementofthedatawiththeMonteCarlosimulation.The2νββefficiency

isestimatedtobe7.6%.Themeasuredhalf-lifeisT2ν

1/2(96

Zr)=[2.3±0.2(stat)±0.3(syst)]·1019

y.

Figure1:Theenergysumandangulardistributionsfortwo-electroneventsfrom96

Zr.

The48

CasampleusedinNEMO3isknowntobecontaminatedby90

Sr(T

1/2=28.79y,

Q

β=0.546MeV).Itsdaughter90

Y(T

1/2=3.19h,Q

β=2.282MeV)isthemajorbackground

sourceinthiscase.Anactivityof1699±3mBq/kgwasmeasuredfor90

Yusingsingle-electron

events.Both90

Srand90

Yareessentiallypureβ−

emittersandimitateββeventsthrough

M¨ollerscattering.Tosupressthisbackgroundcontribution,eventswiththeenergysumgreater

than1.5MeVandcos(Θ

ee)<0areselected.Finally,with943daysofdatataking,there

areatotalof133two-electronevents,withanevaluatedresidualbackgroundcontributionof

17events.Theirtwo-electronenergysumdistributionandsingle-electronenergyspectrumare

presentedinFig.2.The2νββefficiencyis3.3%,andthemeasuredhalf-lifeisT2ν

1/2(48

Ca)=

[4.4+0.5

−0.4(stat)±0.4(syst)]·1019

y.

Figure2:Theenergysumandsingle-electronenergyspectrafortwo-electroneventsfrom48

Ca.

3.2Searchfor0νββdecay

Inthecaseofthemassmechanism,the0νββ-decaysignalisexpectedtobeapeakintheenergy

sumdistributionatthepositionofthetransitionenergyQ

ββ.Sincenoexcessisobservedat

thetailofthedistributionfor96

Zr,seeFig.1(left),norfor48

Ca,Fig.2(left),limitsareset

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