Double Beta Decay Experiments: Present Status and Prospects for the Future

Available online at www.sciencedirect.com

ScienceDirect Physics Procedia 74 (2015) 416 – 422

Conference of Fundamental Research and Particle Physics, 18-20 February 2015, Moscow, Russian Federation

Double beta decay experiments: present status and prospects for the future A.S. Barabash* SSC RF Institute for Theoretical and Experimental Physics of National Research Centre «Kurchatov Institute», Bolshaya Cheremushkinskaya 25, Moscow, 117218, Russia

Abstract

The review of modern experiments on search and studying of double beta decay processes is done. Results of the most sensitive current experiments are discussed. The main attention is paid to EXO-200, KamLAND-Zen, GERDA-I and CUORE-0 experiments. Modern values of T1/2(2Q) and best present limits on neutrinoless double beta decay and double beta decay with Majoron emission are presented. Conservative limits on effective mass of a Majorana neutrino ( < 0.46 eV) and a coupling constant of Majoron to neutrino ( < 1.3 u 10−5) are obtained. In the second part of the review prospects of search for the neutrinoless double beta decay in new experiments with sensitivity to  at the level of ~ 0.01-0.1) eV are discussed. The main attention is paid to experiments of CUORE, GERDA, MAJORANA, EXO, KamLAND-Zen-2, SuperNEMO and SNO+. Possibilities of low-temperature scintillating bolometers on the basis of inorganic crystals (ZnSe, ZnMoO4, Li2MoO4, CaMoO4 and CdWO4) are considered too. © Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) Keywords: double beta decay, neutrino mass, low-background measurements

* Corresponding author. Tel.: +7-916-933-93-50. E-mail address: [email protected]

1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.09.215

417

A.S. Barabash / Physics Procedia 74 (2015) 416 – 422

1. Introduction Interest in 0QEE decay has seen a significant renewal in recent 10-15 years after evidence for neutrino oscillations was obtained from the results of atmospheric, solar, reactor, and accelerator neutrino experiments. These results are impressive proof that neutrinos have a nonzero mass. The detection and study of 0QEE decay may clarify the following problems of neutrino physics (see recent review [1], for example): (i) lepton number non-conservation, (ii) neutrino nature: whether the neutrino is a Dirac or a Majorana particle, (iii) absolute neutrino mass scale, (iv) the type of neutrino mass hierarchy (normal, inverted, or quasidegenerate), (v) CP violation in the lepton sector (measurement of the Majorana CP-violating phases). Usually, three main modes of the 2E decay are considered: (A,Z) → (A,Z + 2) + 2e− + 2Q̃, (A,Z) → (A,Z + 2) + 2e−, (A,Z) → (A,Z + 2) + 2e− + F0(+F0),

(1) (2) (3)

where A is the atomic number, Z is the charge of the nucleus, e- is the electron, Q̃ is the antineutrino, and F0 is the Majoron. 2. Present status 2.1. Two neutrino double beta decay The 2QEE decay (process (1)) is a second-order process, which is not forbidden by any conservation law. The detection of this process provides the experimental determination of the nuclear matrix elements (NME) involved in the EE decay processes. This leads to the development of theoretical schemes for NME calculations both in connection with the 2QEE decays as well as the 0QEE decays. At present, this process has been registered for 11 nuclei (48Ca, 76Ge, 82Se, 96Zr, 100Mo, 116Cd, 128Te, 130Te, 136Xe, 150Nd and 238U). Moreover, the 2QEE decay has been registered for 100Mo and 150Nd to 0+1 excited states of daughter nuclei and the ECEC(2Q) process (double electron capture with the emission of two neutrinos) has been registered in 130Ba. Table 1 displays the present-day averaged and recommended values of T1/2(2Q) from [2]. Table 1. Averaged and recommended Т1/2(2Q) values (from [2]). Isotope

Т1/2(2Q), yr

48

4.4+0.6 –0.5 u 1019

76

1.65+0.14-0.12 u 1021

82

(0.92 r 0.07) u 1020

96

(2.3 ± 0.2) u 1019

Са Ge Se Zr

100

Mo

100

(7.1 r 0.4) u 1018 100

+

Mo– Ru(0 1)

6.7+0.5-0.4 u1020

116

(2.87 ± 0.13) u 1019

128

(2.0 r 0.3) u 1024

130

(6.9 r 1.3) u 1020

136

(2.19 r 0.06) u 1021

Сd Те Те Xe

(8.2 r 0.9) u 1018

150

Nd

150

150

+

Nd– Sm(0 1)

238

1.2+0.3-0.2 u 1020

U

(2.0 r 0.6) u 1021

Ba, ECEC(2Q)

~ 1021

130

418

A.S. Barabash / Physics Procedia 74 (2015) 416 – 422

2.2. Neutrinoless double beta decay The 0QEE decay (process 2) violates the law of lepton-number conservation ('L = 2) and requires that the Majorana neutrino has a nonzero rest mass or that an admixture of right-handed currents be present in weak interaction. Also, this process is possible in some supersymmetric models, where 0QEE decay is initiated by the exchange of supersymmetric particles. This decay also arises in models featuring an extended Higgs sector within electroweak-interaction theory and in some other cases (see review [3], for example). The best present limits on 0QEE decay are presented in Table 2. In calculating constraints on , the NMEs from [4-10] and phase-space factors from [11, 12] have been used. Present conservative limit on can be set as 0.46 eV. Table 2. Best present results on 0QEE decay (limits at 90% C.L.). Isotope Т1/2 , yr E2E, keV

, eV

Experiment

2039.0

> 2.1 u 1025

< 0.26–0.62

GERDA-I [14]

3034.4

> 1.1 u 1024

< 0.33–0.62

NEMO-3 [15]

130

2527.5

> 2.8 u 10

< 0.31–1.1

CUORICINO [16]

136

2458.7

> 1.9 u 1025

< 0.14–0.46

KamLAND-Zen [17]

76

Ge

100

Mo Те Xe

24

2.3. Neutrinoless double beta decay with Majoron emission The 0QF0EE decay (process 3) requires the existence of a Majoron. It is a massless Goldstone boson that arises due to a global breakdown of (B-L) symmetry, where B and L are, respectively, the baryon and the lepton number. The Majoron, if it exists, could playa significant role in the history of the early Universe and in the evolution of stars. The best obtained limits on 0QF0EE decay (ordinary Majoron with spectral index n = 1) are presented in Table 3. Present conservative limit can be set as < 1.3 u 10−5. Experimental limits for other types of 2E decay with Majoron emission (n = 2, 3, 7; including decays with two Majorons) can be found in [13]. Table 3. Best present limits on 0QF0EE decay (ordinary Majoron) at 90% C.L. To calculate the NMEs from [4-10] and phase-space factors from [18] have been used. Isotope

Т1/2 , yr

76

> 4.2 u 10

82

> 1.5 u 1022

< (4.2–11.6) u 10-5

NEMO-3 [20]

Ge Se

23

< (2.1–6.2) u 10

Experiment -5

GERDA-I [19]

100

Mo

> 4.4 u 1022

< (1.6–3.0) u 10-5

NEMO-3 [15]

136

Xe

> 2.6 u 1024

< (0.4–1.3) u 10-5

KamLAND-Zen [21]

3. Current large-scale experiments 3.1. EXO-200 EXO–200 (Enriched Xenon Observatory) is operating at the Waste Isolation Pilot Plant (WIPP, 1585 m w.e.) since May 2011. The experiment consists of 175 kg of Xe enriched to 80.6% in 136Xe housed in a liquid time projection chamber (TPC). The mass of Xe in an active volume is ~ 98 kg. Both ionization and scintillation are used to measure the energy with a resolution of 3.9 % (FWHM) at 2.5 MeV. The detector is capable of effectively rejecting rays through topological cuts. Results obtained after 477.6 days of measurements (99.8 kg u yr of 136Xe exposure) are the following [22-24]: T1/2 (2Q, 136Xe) = [2.165 ± 0.016(stat) ± 0.059(syst)] u 1021 yr T1/2 (0Q, 136Xe) > 1.1 u 1025 yr T1/2 (0QF0, 136Xe) > 1.2 u 1024 yr

(4) (5) (6)

419

A.S. Barabash / Physics Procedia 74 (2015) 416 – 422

With the present background, the predicted EXO–200 sensitivity after 5 years of data taking will be T1/2 ~ 4 u 1025 yr ( < 0.1–0.3 eV). 3.2. KamLAND-Zen The detector KamLAND–Zen is a modification of the existing KamLAND detector carried out in the summer of 2011. The EE source/detector is 13 tons of Xe-loaded liquid scintillator (Xe–LS) contained in a 3.08 m diameter spherical Inner Balloon (IB). The IB is suspended at the center of the KamLAND detector. The outer LS acts as an active shield for external J-rays and as a detector for internal radiation from the Xe–LS or IB. The Xe–LS contains (2.44 s0.01) % by weight of enriched xenon gas (full weight of xenon is ฏ320 kg). The isotopic abundances in the enriched xenon is (90.93 s0.05) % of 136Xe. Main obtained results after 213.4 days (89.5 kg u yr) of measurements (phase-1) are the following [21, 17]: T1/2 (2Q, 136Xe) = [2.30 ± 0.02(stat) ± 0.12(syst)] u 1021 yr T1/2 (0Q, 136Xe) > 1.9 u 1025 yr T1/2 (0QF0, 136Xe) > 2.6 u 1024 yr

(6) (7) (8)

After the phase-1 data-taking, several efforts for further improvements were done: (i) the removal of radioactive impurities by Xe-LS purification; (ii) increasing the Xe concentration from (2.44 ± 0.01)% to (2.96 ± 0.01)% (full weight of xenon is now 383 kg); (iii) developing a spallation background rejection method for 10C from muonspallation; (iv) optimization of the volume selection to minimize the effect of the mini-balloon backgrounds. The phase-2 has been started on 11 December 2013. After 114.8 days (27.6 kg u yr) new preliminary limit T1/2 (0Q, 136 Xe) > 1.3 u 1025 yr has been obtained [25]. The combined KamLAND-Zen result from the phase-1 and phase-2 data gives a 90% C.L. lower limit of T1/2 (0Q, 136Xe) > 2.6 u 1025 yr (still is not published). With the present background, the sensitivity after 2 years of data taking will be T1/2 ~ 3 u 1025 yr using the phase-2 data along. 3.3. GERDA-I, GERDA-II GERDA is a low-background experiment which searches for the neutrinoless double beta decay of 76Ge, using an array of high-purity germanium (HPGe) detectors isotopically enriched in 76Ge [26]. The detectors are operated naked in ultra radio-pure liquid argon, which acts as the cooling medium and as a passive shielding against the external radiation. The experiment is located in the underground Laboratori Nazionali del Gran Sasso of the INFN (Italy, 3500 m w.e.). The Phase I of GERDA use eight enriched coaxial detectors refurbished from HeidelbergMoscow and IGEX experiments (corresponding to approximately 18 kg of 76Ge). The energy resolution is ≈ 4.5 keV at 2.039 MeV. GERDA-I measurements have been started in November 2011 and stopped in May of 2013. Results of the measurement are [14, 27, 19]: T1/2 (2Q, 76Ge) = 1.84+0.14-0.10 u 1021 yr T1/2 (0Q, 76Ge) > 2.1 u 1025 yr T1/2 (0QF0, 76Ge) > 4.2 u 1023 yr

(9) (10 ) (11)

In the upcoming second phase additionally to the Phase I detectors 30 BEGe detectors with a total mass of ~ 20 kg will be used (full mass of Ge will be ~ 35 kg). The background index (BI) of the experiment will be reduced by using pulse shape discrimination (PSD) of the BEGe detectors and the additional readout of scintillation light in the liquid argon surrounding the detectors. With a total exposure of 100 kg u yr and a BI of 10−3 counts/(keVu kg u yr) the limit setting sensitivity of the experiment can be improved to ~ 1.4 u 1026 yr (90% C.L). Commissioning of Phase II (GERDA-II) has been started on March 2015.

420

A.S. Barabash / Physics Procedia 74 (2015) 416 – 422

3.4. CUORE-0 This experiment is running at the Gran Sasso Underground Laboratory (3500 m w.e.) since March 2013. CUORE-0 is a CUORE-style tower built using the same materials, assembly devices and procedures developed for CUORE. It is therefore identical in all the aspects to the 19 towers of CUORE (52 TeO 2 crystals, 39 kg of TeO2 mass and 11 kg of 130Te). Actually, the main goal of CUORE-0 is to provide a proof of concept of the CUORE detector in all stages. Given the sizable mass and the anticipated background level, CUORE-0 is operating as an independent 0QEE experiment while CUORE is under construction. The results of a search for neutrinoless doublebeta decay in a 9.8 kg u yr exposure of 130Te was recently presented [28]. The characteristic detector energy resolution and background level in the region of interest are (5.1 ± 0.3) keV (FWHM) and ~0.058 counts/(keVukg uyr), respectively. Lower bound on the decay half-life has been obtained, T1/2 (0Q, 130Te) > 2.7 u 1024 yr at 90% C.L. Combining CUORE-0 data with the 19.75 kg u yr exposure of 130Te from the CUORICINO experiment [16] authors obtain T1/2 (0Q , 130Te) > 4 u 1024 yr at 90% C.L. The CUORE experiment data taking is expected to start at the end of 2015. 4. Future large-scale experiments Seven of the most developed and promising experiments which can be realized within the next few years are presented in Table 4. The estimation of the sensitivity to  is done using NMEs from [4-10] and phase-space factor values from [11, 12]. CUORE-0, EXO-200 and KamLAND-Zen are the current experiments and continue the data taking. CUORE, GERDA-II and MAJORANA-DEMANSTRATOR will start data taking in 2015. During 2016-2017 SuperNEMO-Demonstrator and SNO+ will start to take data too. Full GERDA/MAJORANA, SuperNEMO, nEXO and KamLAND2-Zen will start to have data after 2020. Table 4. Seven most developed and promising projects. Sensitivity at 90% C.L. for three (1-st step of GERDA and MAJORANA, first step of SuperNEMO, CUORE-0 and KamLAND-Zen), five (EXO-200, SuperNEMO, SNO+, and CUORE) and ten (EXO, full-scale GERDA and MAJORANA) years of measurements is presented. M - mass of isotopes.

11 200

Sensitivity Т1/2, yr 5 u 1024 1026

Sensitivity , eV 0.2-0.8 0.05-0.18

35 1000

1.5 u 1026 6 u 1027

0.09–0.2 0.01–0.04

in progress R&D

30 1000

1.5 u 1026 6 u 1027

0.09–0.2 0.01–0.04

in progress R&D

Experiment

Isotope

M, kg

CUORE [28,29]

130

Te

GERDA [26]

76

Ge

MAJORANA [30]

Status current in progress

EXO [31]

136

200 5000

4 u 1025 1027-1028

0.1–0.3 0.01–0.05

current R&D

KamLAND-Zen [25]

136

380 1000

4 u 1025 6 u 1026

0.1–0.3 0.03–0.08

current

SuperNEMO [32]

82

7 100200

6.5 u 10 (12) u 1026

0.2–0.6 0.04–0.14

in progress R&D

SNO+ [33]

130

800 8000

1026 1027

0.050.18 0.020.06

in progress R&D

Xe

Xe

Se Te

24

R&D

Actually the experiments specified in Table 4 don’t settle all variety of the offered experimental approaches to

EE decay search. There is a set of other propositions (see reviews [3, 34, 35]. The part from them already is at a

stage of development of prototypes with a mass of the studied isotope ~(110) kg (NEXT [36], CANDLESS [37], LUCIFER [38], AMORE [39], LUMINEU [40], LUCINEU). Researches with a prototype are a necessary stage before realization of full-scale experiment.

A.S. Barabash / Physics Procedia 74 (2015) 416 – 422

421

Here it should be noted prospects of the direction founded on use of low-temperature scintillating bolometers on the basis of ZnSe, ZnMoO4, Li2MoO4, CaMoO4 and CdWO4 crystals (it means that search for EE decay of 82Se, 116 Cd, 100Mo and 48Ca could be one). Operability of individual detectors on the basis of these crystals has been shown earlier [38, 39, 41-47], including crystals made of the enriched material [46, 47]. And possibility of essential suppression of a background due to simultaneous registration of thermal and scintillation signals was also shown. In addition, the ability to recognize the interacting particle from the different pulse shapes of the thermal signal was demonstrated for ZnMoO4 and ZnSe crystals. All this allows to hope for development in the future of the installations which will contain ~ 1001000 kg of the studied 2E isotope, will have very low level of the background (~ 10−4 counts /(keV u kg u yr)) and sensitivity on the level ~ n u 1027 yr ( ~ 0.01 eV) after 5 years of measurements (see also discussions in [48, 49]). 5. Conclusion Thus, at present, the 2QEE decay of 11 nuclei (48Ca, 76Ge, 82Se, 96Zr, 100Mo, 116Cd, 128Te, 130Te, 136Xe, 150Nd and U) has been registered. Moreover, the 2QEE decay of 100Mo and 150Nd to 0+1 excited states of daughter nuclei and the ECEC(2Q) process in 130Ba (geochemical experiment) have been detected too. The 0QEE decay has not been observed yet, and the best limits on have been obtained in experiments with 136 Xe, 76Ge, 100Mo, and 130Te. Using most reliable NME calculations it is possible to set the present conservative limit as < 0.46 eV. Conservative present limit on decay with Majoron emission has been obtained as < 1.3 u 10−5 (ordinary Majoron with n = 1). Sensitivity to on the level ~ (0.10.2) eV will be reached by next generation experiments in a few years from now and on the level ~ (0.020.05) eV (inverted hierarchy region) after 2020. One of very promising approach is the experiments which will use low-temperature scintillating bolometers, based on inorganic crystals (ZnSe, ZnMoO4, Li2MoO4, CaMoO4 and CdWO4). 238

Acknowledgements Portions of this work were supported by grants from RFBR (no 15-02-0291915). References [1] Bilenky S.M., Giunti C. Int. J. Mod. Phys. A. 2015;30:1530001. [2] Barabash A.S. Nucl. Phys. A. 2015;935:52. [3] Vergados J.D., Ejiri H., Simkovic F. Rep. Prog. Phys. 2012;75:106301. [4] Hyvarinen J., Suhonen J. Phys. Rev. C. 2015;91:024613. [5] Simkovic F., Rodin V., Faessler A., Vogel P. Phys. Rev. C. 2013;87:045501. [6] Barea J., Kotila J., Iachello F. Phys. Rev. C. 2015;91:034304. [7] Rath P.K., Chandra R., Chaturvedi K. et al. Phys. Rev. C. 2013;88:064322. [8] Rodriguez T.R., Martinez-Pinedo G. Phys. Rev. Lett. 2010;105:252503. [9] Menendez J., Poves A., Caurier E., Nowacki F. Nucl. Phys. A. 2009;818:139. [10] Neacsu A., Horoi M. Phys. Rev. C. 2015;91:24309. [11] Kotila J., Iachello F. Phys. Rev. C. 2012;85:34316. [12] Stoica S, Mirea M. Phys. Rev. C. 2013;88:37303; updated in arXiv:1411.5506. [13] Barabash A.S. Phys. Part. Nucl. 2011;42:613. [14] Agostini M., Allardt M., Andreotti E. et al. Phys. Rev. Let. 2013;111:122503. [15] Arnold R., Augier C., Baker J.D. et al. Phys. Rev. D. 2014;89:11101. [16] Andreotti E., Arnaboldi C., Avignone F.T. et al. Astropart. Phys. 2011;34:822. [17] Gando A., Gando Y., Hanakago H. et al. Phys. Rev. Let. 2013;110:062502. [18] Doi M., Kotani T., Takasugi E. Phys. Rev. C. 1988;37:2575. [19] Agostini M., Allardt M., Bakalyarov A.M. et al. arXiv: 1501.02345. [20] Arnold R., Augier C., Baker J. et al. Nucl. Phys. A. 2006;765:483. [21] Gando A., Gando Y., Hanakago H. et al., Phys. Rev. C. 2012;86:021601(R). [22] Albert J.B., Auger M., Auty D.J. et al., Phys. Rev. C. 2014;89:015502. [23] Albert J.B., Auty D.J., Barbeau P.S. et al. Phys. Rev. C. 2014;90:092004. [24] Albert J.B., Auty D.J., Barbeau P.S.et al. Nature 2014;510:229. [25] Asakura K., Gando A., Gando Y. et al., arXiv:1409.0077. [26] Ackermann K.-H., Agostini M., Allardt M. et al. Eur. Phys. J. C. 2013;73:2330.

422

A.S. Barabash / Physics Procedia 74 (2015) 416 – 422

[27] Agostini M., Allardt M., Andreotti E. et al. J. Phys. G. 2013;40:035110. [28] Aflonso K., Artusa D.R., Avignone F.T. et al. arXiv:1504.02454. [29] Artuza D.R., Avignone F.T., Azzolini O. et al. Adv. HEP 2015;2015:879871. [30] Abgrall N., Aguayo E., Avignone F.T. et al. Adv. HEP 2014;2014:365432. [31] Albert J. EPJ Web of Conf. 2014;66:08001. [32] Bongrand M.. Phys. Proc. 2015;61:211. [33] Maio A. J. Phys. Conf. Ser. 2015;587:012030. [34] Avignone F.T., Elliott S.R., and Engel J. Rev. Mod. Phys. 2008;80:481. [35] Elliott S.R. Mod. Phys. Lett. A. 2012;27:1230009. [36] Hernando J.A. Phys. Proc. 2015;61:251. [37] Umehara S., Kishimoto T., Nomachi M. et al. Phys. Proc. 2015;61:283. [38] Beeman J.W., Bellini F., Benetti P. et al. Adv. HEP 2013;2013:237973. [39] Lee S.J., Choi J.H., Danevich F.A. et al. Astropart. Phys. 2011;34:732. [40] Tenconi M. Phys. Proc. 2015;61:782. [41] Arnaboldi C., Capelli S., Cremonesi O. et al. Astropart. Phys. 2011;34:344. [42] Arnaboldi C., Brofferio C., Cremonesi O. et al. Astropart. Phys. 2011;34:797. [43] Arnaboldi C., Beeman J.W., Cremonesi O. et al. Astropart. Phys. 2010;34:143. [44] Cardani L., Gironi L., Ferreiro Iachellini N. et al. J. Phys. G. 2014;41:075204. [45] Bekker T.D., Ciron N., Danevich F.A. et al. arXiv:1410.6933. [46] Poda D.V., Armengaud E., Arnaud Q. et al. arXiv:1502.01161. [47] Barabash A.S., Chernyak D.M., Danevich F.A. et al. Eur. Phys. J. C. 2014;74:3133. [48] Artusa D.R., Avignone F.T., Azzolini O. et al. Eur. Phys. J. C. 2014;74:3096. [49] Wang G., Chang C.L., Yefremenko V. et al. arXiv: 1504.03599.