After analysis of 5797 h of data from the detector NEMO3, new limits on neutrinoless double beta decay of Mo-100 (T-1/2 > 3.1 x 10(23) y, 90% CL) and Se-82 (T-1/2 > 1.4 x 10(23) y, 90% CL) have been obtained. The corresponding limits on the effective majorana neutrino mass are:    1.4 x 10(22) y (90% CL) for Mo-100 and T-1/2 > 1.2 x 10(22) y (90% CL) for Se-82. Corresponding bounds on the Majoron-neutrino coupling constant are  < (0.5-0.9) x 10(- 4) and <(0.7-1.6) x 10(- 4). Two-neutrino 2beta-decay half-lives have been measured with a high accuracy, (T1/2Mo)-Mo-100 = [7.68 +/- 0.02(stat) +/- 0.54(syst)] x 10(18) y and (T1/2Se)-Se-82 = [10.3 +/- 0.3(stat) +/- 0.7(syst)] x 10(19) y. (C) 2004 MAIK "Nauka/Interperiodica"

Arnold, R

Augier, C

Baker, J

Barabash, A

Brudanin, V

Caffrey, AJ

Egorov, V

Guyonnet, JL

Hubert, F

Hubert, P

Jenner, L

Jollet, C

Jullian, S

Klimenko, A

Kochetov, O

Konovalov, S

Kovalenko, V

Lalanne, D

Leccia, F

Linck, I

Longuemare, C

Lutter, G

Marquet, C

Mauger, F

Nicholson, HW

Ohsumi, H

Piquemal, F

Reyss, JL

Saakyan, R

Sarazin, X

Scheibling, F

Shitov, Y

Simard, L

Smolnikov, A

Stekl, I

Suhonen, J

Sutton, CS

Szklarz, G

Thomas, J

Timkin, V

Tretyak, V

Umatov, V

Vala, L

Vanyushin, I

Vasiliev, S

Vasilyev, V

Vorobel, V

Vylov, T

English

UCL Discovery

arXiv:hep-ex/0410021v1  7 Oct 2004Pis’ma v ZhETFStudy of 2β-decay of 100Mo and 82Se using the NEMO3 detectorR. Arnoldj, C. Augierh, J. Bakere, A. Barabashg 1), V. Brudaninc, A.J. Caﬀreye, V. Egorovc, J.L. Guyonnetj,F. Huberta, Ph. Huberta, L. Jennern, C. Jolletj, S. Jullianh, A. Klimenkoc, O. Kochetovc, S. Konovalovg,V. Kovalenkoc, D. Lalanneh, F. Lecciaa, I. Linckj, C. Longuemareb, G. Luttera, Ch. Marqueta, F. Maugerb,H.W. Nicholsoni, H. Ohsumim, F. Piquemala, J-L. Reyssd, R. Saakyann, X. Sarazinh, F. Scheiblingj, Yu. Shitovc,L. Simardh, A. Smolnikovc, I. ˇ Steklk, J. Suhonenf, C.S. Suttoni, G. Szklarzh, V. Timkinc, J. Thomasn,V. Tretyakc, V. Umatovg, L. V´ alak, I. Vanyushing, S. Vasilievc, V. Vasilyevg,V. Vorobell, Ts. VylovcaCENBG, IN2P3-CNRS et Universit´ e de Bordeaux, 33170 Gradignan, FrancebLPC, IN2P3-CNRS et Universit´ e de Caen, 14032 Caen, FrancecJINR, 141980 Dubna, RussiadCFR, CNRS, 91190 Gif sur Yvette, FranceeINEEL, Idaho Falls, ID 83415, U.S.A.fJYV¨ ASKYL¨ A University, 40351 Jyv¨ askyl¨ a, FinlandgITEP, 117218 Moscow, RussiahLAL, IN2P3-CNRS et Universit´ e Paris-Sud, 91405 Orsay, FranceiMHC, South Hadley, Massachusetts 01075, U.S.A.jIReS, IN2P3-CNRS et Universit´ e Louis Pasteur, 67037 Strasbourg, France.kIEAP CTU, Prague, Czech Republic.lCharles University, Prague, Czech Republic.mSAGA University, Saga, Saga 840-8502, JapannUniversity College London, London, UKSubmitted 30 July 2004Resubmitted 30 July 2004After analysis of 5797 h of data from the detector NEMO3, new limits on neutrinoless double beta decayof100Mo (T1/2 > 3.1 · 1023 y , 90% CL) and82Se (T1/2 > 1.4 · 1023 y , 90% CL) have been obtained. Thecorresponding limits on the eﬀective majorana neutrino mass are: < mν > < (0.8 − 1.2) eV and < mν >< (1.5−3.1) eV, respectively. Also the limits on double-beta decay with Majoron emission are: T1/2 > 1.4·1022y (90%CL) for100Mo and T1/2 > 1.2 · 1022 y (90%CL) for82Se. Corresponding bounds on the Majoron-neutrino coupling constant are < gee > < (0.5 − 0.9) · 10−4 and < (0.7 − 1.6) · 10−4. Two-neutrino 2β-decayhalf-lives have been measured with a high accuracy, T100Mo1/2 = [7.68 ± 0.02(stat) ± 0.54(syst)] · 1018 y andT82Se1/2 = [10.3 ± 0.3(stat) ± 0.7(syst)] · 1019 y.PACS: 23.40.-s, 14.80.MzInterest in neutrinoless double beta decay (2β0ν) hasseen a signiﬁcant rebirth in recent years after evidencefor neutrino oscillations was obtained from the resultsof atmospheric [1] and solar [2, 3, 4, 5, 6] neutrino ex-periments (see, for example, the discussions in [7, 8, 9]).This observation of oscillations was recently con-ﬁrmed by the KamLAND experiment with reactor an-tineutrinos [10] and by the new SNO result [11]. Theseresults are an impressive proof that neutrinos have anon-zero mass. However, the experiments studying neu-trino oscillations are not sensitive to the nature of theneutrino mass (Dirac or Majorana?) and provide no in-formation on the absolute scale of the neutrino masses,1)Corresponding author, Institute of Theoretical and Experi-mental Physics, B. Cheremushkinskaya 25, 117218 Moscow, Rus-sia, e-mail: Alexander.Barabash@itep.ru, tel.: 007 (095) 129-94-68, fax: 007 (095) 883-96-01since such experiments are sensitive only to the diﬀer-ence, ∆m2. The detection and study of 2β0ν decaymay clarify the following problems of neutrino physics(see discussions in [12, 13, 14]): (i) neutrino nature; isthe neutrino a Dirac or a Majorana particle?, (ii) ab-solute neutrino mass scale (a measurement or a limiton m1), (iii) the type of neutrino mass hierarchy (nor-mal, inverted, or quasidegenerate), (iv) CP violation inthe lepton sector (measurement of the Majorana CP-violating phases).The main goal of the NEMO3 experiment is tostudy neutrinoless double beta decay of diﬀerent iso-topes (100Mo, 82Se etc.) with a sensitivity of up to∼ 1025 y, which corresponds to a sensitivity for the eﬀec-tive Majorana neutrino mass at the level of ∼ (0.1−0.3)eV [15]. The planned sensitivity for double beta decaywith Majoron emission is ∼ 1023 y (the sensitivity for12 R.Arnold et al.the coupling constant of Majoron to neutrino < gee >is at the order of ∼ 10−5). In addition, one of the goalsis a precise study of 2β2ν decay for a number of nuclei(100Mo, 82Se, 116Cd, 150Nd, 130Te, 96Zr and 48Ca) withhigh statistics and to study all major characteristics ofthe decay.NEMO3 is a tracking detector, which in contrast to76Ge experiments [16, 17], detects not only the totalenergy deposition but also other important parametersof the process. These include the energy of individualelectrons, their angular distribution, the event vertexcoordinates in the source plane, etc. This provides aunique opportunity to monitor and reject backgrounds.Since June of 2002 NEMO3 has been running in theFrejus Underground Laboratory (France) located at adepth of 4800 m.w.e.The detector has a cylindrical shape and consists of20 identical sectors (see Fig 1). A thin (∼30-60mg/cm2)source is placed in the center of the detector which con-tains 2β-decaying nuclei and has a total area of 20 m2and a mass of about 10 kg. In particular, it includes7.1 kg of enriched Mo (average enrichment is 98%, thetotal mass of 100Mo is 6.914 kg) and 0.962 kg of Se (en-richment is 97%, the total mass of 82Se is 0.932 kg). Toinvestigate the external background a part of the sourceis made of very pure natural materials (TeO2 - 0.767 kgand Cu - 0.621 kg). The level of contamination of thesources with radioactive impurities was obtained frommeasurements using low-background HPGe-detectors.The basic detection principles of NEMO3 are thefollowing. The energy of the electrons is measured withplastic scintillators coupled to PMTs (1940 individualcounters), while the tracks are reconstructed from theinformation obtained with drift Geiger cells (6180 cells).The tracking volume of the detector is ﬁlled with a mix-ture of ∼95% He, 4% alcohol, 1% Ar and 0.15% waterat 20 mbar above atmospheric pressure. In addition,a magnetic ﬁeld of 25 Gauss parallel to the detector’saxis is created by a solenoid surrounding the detector.The magnetic ﬁeld is used to identify electron-positronpairs to suppress the background associated with theseevents.The main characteristics of the detector’s perfor-mance are the following. The energy resolution ofthe scintillation counters lies in the interval of 14-17%(FWHM for 1 MeV electrons). The time resolution is250 ps for an electron energy of 1 MeV. The reconstruc-tion accuracy of a two electron (2e) vertex is around1 cm. The characteristics of the detector are stud-ied in special calibration runs with radioactive sources.The energy calibration is carried out using 207Bi withconversion electrons at energies 0.482 and 0.976 MeV,and 90Sr with the end-point of the β spectrum at 2.283MeV. The vertex reconstruction accuracy for 2e− eventswas determined in measurements with 207Bi. The time-of-ﬂight properties were determined via measurementswith 60Co where two γ-quanta are emitted simultane-ously, 207Bi for which two electrons are emitted simulta-neously and neutron sources which provide high energyelectrons crossing the detector.The detector is surrounded by a passive shield madeof 20 cm of steel, 30 cm of water contained in tanks cov-ering the vertical exterior of the detector and wood andparaﬃn on the top and bottom. The level of radioactiveimpurities in the construction materials of the detectorand the passive shield was tested with low-backgroundHPGe detectors.From June to December of 2002 a number of cali-bration and test measurements were carried out, as wasthe ﬁrst data taken for double beta decay studies. Since14 February 2003 the detector has been routinely tak-ing double beta decay data . The calibrations with theradioactive sources are carried out every 1.5 months.Calorimeter stability is checked daily using a laser sys-tem between calibrations. A detailed description of thedetector and its characteristics is presented in [18].In this paper we present the results for 100Mo and82Se. Only part (5797 h of measurement) of the avail-able data has been analyzed.Double beta decay of 100Mo. 2e-events with acommon vertex at the source have been selected. For2e-events, an electron was deﬁned as a track betweenthe source foil and a ﬁred scintillation counter with theenergy deposited being greater than 200 keV. The trackcurvature must be consistent with a negatively chargedparticle. The time-of-ﬂight measurement should be con-sistent with the hypothesis of two electrons leaving thesource from a common vertex simultaneously. In orderto suppress the 214Bi background, which is followed bya 214Po α-decay, it is required that there are no delayedGeiger cell hits close to the event vertex or the elec-tron track (within a delay of up to 700 µs). A typical2e-event is shown in Fig 2.Fig 3 shows the 2β2ν energy spectrum for 100Mo.The total number of useful events (after backgroundsubtraction) is ∼141000. The signal-to-background ra-tio is 40/1, while it is 100/1 for energies above 1 MeV.This means that the background is negligible. Thedetection eﬃciencies which included the selection cutswere estimated by Monte Carlo (MC) simulations. Thiswas done for two models of the 100Mo decay. The ﬁrstone is the higher state dominance (HSD) mechanismand the second is the single state dominance (SSD)mechanism (see ref. [19]). For the SSD mechanism itStudy of 2β-decay of 100Mo and 82Se using the NEMO3 detector ... 3is assumed that the decay goes via the lowest 1+ stateof 100Tc [20]. The detection eﬃciencies calculated byMC simulations were 5.68% for the SSD mechanism and6.33% for the HSD mechanism. Correspondingly, thefollowing results were obtained for the 100Mo half-life:T1/2 = [7.68 ± 0.02(stat) ± 0.54(syst)] · 1018 y (SSD )T1/2 = [8.57 ± 0.02(stat) ± 0.6(syst)] · 1018 y (HSD )A preliminary analysis of the experimental singleelectron energy spectrum favors the SSD hypothesis.However, at the moment this question remains openand requires further studies. Both values are in agree-ment with the world mean value from other experi-ments: (8 ± 0.7) · 1018 y [21, 22].2β0ν-decay. The energy range 2.65-3.2 MeV hasbeen investigated. The information about the electron’senergies and their angular distribution in 2e-events canbe used to improve the selection of candidate events andthus, improve the NEMO3 sensitivity [24]. Introducinga higher threshold of 900 keV for each electron, one gets13 candidate events in this energy window. This is inagreement with our expectations from the background(18.8 events) coming mainly from 222Rn (13 events) plusthe 2β2ν contribution (5.8 events). The calculated 2β0νeﬃciency is 8.2%. As a result a limit has been set atT1/2 > 3.1 · 1023 y (90% CL), which is better than thebest previous measurement (T1/2 > 5.5·1022 y [23]). Us-ing the nuclear matrix elements (NME) from [25, 26, 27],the following range of limits on the neutrino mass hasbeen derived: < mν > < (0.8 − 1.2) eV. This rangecan be compared with that of the 76Ge experiment [16](using the same nuclear matrix elements [25, 26, 27]),< mν > < (0.33 − 0.84) eV.For neutrinoless double beta decay caused by theright current admixture ( λ term) in weak interactionsthe 2.8-3.2 MeV energy window was used. An addi-tional cut on the electron energies, |E1 − E2| > 800keV [24], allows an extra background suppression. Inthis case there are only 3 candidate events in the 2.8-3.2 MeV window with 2.8 (2.0+0.8) expected. With aneﬃciency of 4.2% this gives T1/2 > 1.8·1023 y (90% CL)and < λ > < (1.5 − 2.0) · 10−6, using the NME from[28].2β0νχ decay. The energy interval 2.65-3.2 MeV hasbeen studied. Introducing a threshold of 750 keV foreach electron one gets only 18 events (13.7+8 are ex-pected with an eﬃciency of 0.49%) and the half-lifelimit yields T1/2 > 1.4 · 1022 y (90% CL). This valueis better than the previous one (T1/2 > 5.8·1021 y [29])and the corresponding limit on the Majoron-neutrinocoupling constant (NME from [25, 26, 27]) is < gee >< (0.5 − 0.9) · 10−4 . This is one of the best limits forthe majoron coupling constant.Double beta decay of 82Se. The energy spec-trum of 2β2ν-events for 82Se is shown in Fig 4. A higherthreshold of 300 keV was used in order to improve thesignal-to-background ratio. The total number of usefulevents after the background subtraction is ∼ 1800.The signal-to-background ratio is approximately 4 to1. The detection eﬃciency has been calculated byMC calculations to be 6.02%. The 82Se half-life valueobtained is,T1/2 = [10.3 ± 0.3(stat) ± 0.7(syst)] · 1019y.This value is in agreement with our previous mea-surement with NEMO2 [30] and with the world averagevalue of (9 ± 1) · 1019 y [22].2β0ν-decay. Three events have been detected in theinterval 2.65-3.2 MeV. The expected background in thisenergy windows is 5.3 events. Again this background ismainly due to 222Rn. Eﬃciency of 15.8% gives a limitfor the 82Se 0ν-decay of T1/2 > 1.4 · 1023 y (90% CL),which is better than the previous value by one order ofmagnitude (T1/2 > 1.4 · 1022 y at 90% CL [31]). Thecorresponding limit on the eﬀective neutrino mass, us-ing NME from [25, 26, 27] is mν < (1.5 − 3.1) eV.For neutrinoless double beta decay caused by theright current admixture ( λ term) in weak interactions,the 2.65-3.2 MeV energy window was used and a cuton the electron energy, |E1 − E2| > 800 keV [24] wasapplied. The number of events is zero and the eﬃciencyis 8.5%. This gives T1/2 > 1.1 · 1023 y (90% CL) and< λ > < (3.2−3.8)·10−6, using the NME from [32, 33].2β0νχ-decay. The energy interval 2.3-3.2 MeV wasstudied. The number of selected events is 39, while 40is the estimated background. Using the eﬃciency es-timated by MC calculations (4.2%) the following limitwas obtained, T1/2 > 1.2·1022 y. (90% CL). This is bet-ter than the previous best limit (T1/2 > 2.4·1021 y [30]).Using this result and the NME from [25, 26, 27] one getsthe corresponding limit for the Majoron-neutrino cou-pling constant < gee > < (0.7 − 1.6) · 10−4.Presently, the NEMO3 tracking detector is continu-ing to collect data. At the same time, work is in progressto improve the detector’s performance, background con-ditions and data analysis techniques. In particular aradon barrier tent has been installed and a radon trap-ping factory will be in operation in autumn of 2004,which will help reduce the 222Rn background by a fac-tor of 10-100. It is believed that the sensitivity of the4 R.Arnold et al.experiment can be increased signiﬁcantly in the nearfuture.The authors would like to thank the Modane Under-ground Laboratory staﬀ for their technical assistance inrunning the experiment. Portions of this work were sup-ported by a grant from INTAS N 03051-3431and a grantNATO PST.CLG.980022.1. H. Sobel et al., Nucl. Phys. B (Proc. Suppl.) 91, 127(2001).2. B.T. Cleveland et al., Astrophys. J. 496, 505 (1998).3. S. Fukuda et al., Phys. Rev. Lett. 86, 5655 (2001); 86,5656 (2001).4. V.N. Gavrin et al., Nucl. Phys. B (Proc. Suppl.) 91, 36(2001).5. M. Altman et al., Phys. Lett. B 490, 16 (2000).6. Q.R. Ahmad et all., Phys. Rev. Lett. 89, 011301 (2002);89, 011302 (2002).7. P.C. de Holanda, A.Yu. Smirnov, Phys. Rev. D 66,113005 (2002).8. J.N. Bahcall et al., JHEP 0207, 054 (2002);hep-ph/0204314.9. M. Maltoni et al., Phys. Rev. D 67, 013011 (2003);hep-ph/0207227.10. K. Eguchi et al., Phys. Rev. Lett. 90, 021802 (2003).11. S.N. Ahmed et al., Phys. Rev. Lett. 92, 181301 (2004).12. S.M. Bilenky, hep-ph/0403245.13. S. Pascoli, S.T. Petcov, W. Rodejohann, Phys. Lett., B558, 141 (2003).14. S. Pascoli, S.T. Petcov, Phys. Lett., B 580, 280 (2004).15. NEMO3 Proposal, preprint 94-29, LAL Orsay, (1994).16. H.V. Klapdor-Kleingrothaus et al., Eur. Phys. J. A 12,147 (2001).17. C.E. Aalseth et al., Phys. Rev. D 65, 092007 (2002).18. R. Arnold et al., preprint LAL 04-05, February 2004;hep-ph/0402115.19. F. Simkovic, P. Domin, S. Semenov, J. Phys. G 27, 2233(2001).20. J. Abad et al., Journal of Physique, 45, 147 (1984); Ann.Fis. A 80, 9 (1984).21. V.D. Ashitkov et al., JETP Lett., 76, 529 (2001).22. A.S. Barabash, Czech. J. Phys. 52, 567 (2002).23. H. Ejiri et al., Phys. Rev. C 63, 065501 (2001).24. R. Arnold et al., Nucl. Instr. Meth. A 503, 649 (2003).25. F. Simkovic et al., Phys. Rev. C 60, 055502 (1999).26. S. Stoica, H.V. Klapdor-Kleingrothaus, Nucl. Phys. A694, 269 (2001).27. O. Civatarese, J. Suhonen, Nucl. Phys. A 729, 867(2003).28. J. Suhonen, O. Civitarese, Phys. Rev. C 49, 3055(1994).29. H. Ejiri et al., Phys. Lett. B 531, 190 (2002).30. R. Arnold et al., Nucl. Phys. A 636, 209 (1998).31. S.R. Elliott et al., Phys. Rev. C 46, 1535 (1992).32. T. Tomoda, Rep. Prog. Phys. 54, 53 (1991).33. J. Suhonen, S.B. Khadkikar, A. Faessler, Nucl. Phys. A535, 509 (1991).Study of 2β-decay of 100Mo and 82Se using the NEMO3 detector ... 5Fig.1. The NEMO3 detector. 1 - source foil; 2 - plastic scintillator; 3 - low radioactive PMT; 4 - tracking chamber(6,180 octagonal Geiger cells).Fig.2. A view of a reconstructed 2e-event in NEMO3. Sum energy of the electrons is 2024 keV, energy of electrons inthe pair is 961 keV and 1063 keV.Fig.3. 2β2ν spectrum of100Mo, background subtracted.Fig.4. 2β2ν spectrum of82Se, background subtracted.This figure "fig1.png" is available in "png"￿ format from:http://arXiv.org/ps/hep-ex/0410021v1This figure "Fig2.png" is available in "png"￿ format from:http://arXiv.org/ps/hep-ex/0410021v1This figure "Fig3.png" is available in "png"￿ format from:http://arXiv.org/ps/hep-ex/0410021v1This figure "Fig4.png" is available in "png"￿ format from:http://arXiv.org/ps/hep-ex/0410021v1

Study of 2 beta-decay of Mo-100 and Se-82 using the NEMO3 detector

Study of 2 beta-decay of Mo-100 and Se-82 using the NEMO3 detector

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