Neutrinoless double-beta decay is a uniquely sensitive probe of lepton number

violating processes, and its observation may answer fundamental questions in neutrino physics,

including the nature and the mass scale of the light neutrinos. Tracking calorimeter experiments

have particular strengths, including the ability to search for neutrinoless double-beta decay

amongst several different isotopes hosted in source foils. Full event reconstruction provides

powerful background rejection capability, and the ability to disentangle different mechanisms

for neutrinoless double-beta decay in the event of its discovery. These proceedings will give the

latest results from the NEMO-3 experiment, including new double-beta decay measurements

using the isotopes 48Ca and 150Nd. The current status and future prospects for the SuperNEMO

experiment will also be presented

Waters, D

Journal of Physics : Conference Series

UCL Discovery

Journal of Physics: Conference SeriesPAPER • OPEN ACCESSLatest Results from NEMO-3 & Status of theSuperNEMO ExperimentTo cite this article: David Waters and NEMO-3 and SuperNEMO Collaborations 2017 J. Phys.: Conf.Ser. 888 012033 View the article online for updates and enhancements.Related contentPredictions for neutrinoless double-betadecay in the 3+1 sterile neutrino scenarioC Giunti and E M Zavanin-Surface-alpha backgrounds for theMajorana neutrinoless double-beta decayexperimentT H Burritt, R A Johnson, J F Wilkerson etal.-Construction and commissioning of thetracker module for the SuperNEMOexperimentMichele Cascella, Ashwin Chopra, LaurenDawson et al.-This content was downloaded from IP address 128.41.9.202 on 02/05/2018 at 15:451Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd1234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012033  doi :10.1088/1742-6596/888/1/012033Latest Results from NEMO-3 & Status of theSuperNEMO ExperimentDavid Waters (for the NEMO-3 and SuperNEMO Collaborations)Department of Physics & Astronomy, University College London, London WC1E 6BT, UKE-mail: d.waters@ucl.ac.ukAbstract. Neutrinoless double-beta decay is a uniquely sensitive probe of lepton numberviolating processes, and its observation may answer fundamental questions in neutrino physics,including the nature and the mass scale of the light neutrinos. Tracking calorimeter experimentshave particular strengths, including the ability to search for neutrinoless double-beta decayamongst several diﬀerent isotopes hosted in source foils. Full event reconstruction providespowerful background rejection capability, and the ability to disentangle diﬀerent mechanismsfor neutrinoless double-beta decay in the event of its discovery. These proceedings will give thelatest results from the NEMO-3 experiment, including new double-beta decay measurementsusing the isotopes 48Ca and 150Nd. The current status and future prospects for the SuperNEMOexperiment will also be presented.1. Double-Beta DecayThe physics of double-beta decay and the motivation to search for neutrinoless double-beta decay(0νββ), is discussed in [1]. One of the motivations for NEMO-3 and subsequently SuperNEMOhas been isotope ﬂexibility, such that a putative signal can be conﬁrmed in multiple isotopes.Moreover, the opportunity to measure standard-model decay modes such as 2-neutrino doublebeta decay (2νββ) to the ground state, as well as excited states, provides valuable tests ofnuclear models.1.1. The Tracker-Calorimeter TechniqueNEMO-3 and SuperNEMO employ a tracker-calorimeter technique, depicted schematically inFigure 1 and further illustrated by means of a NEMO-3 double-beta candidate event in Figure 2.Double-beta decaying isotopes are hosted in thin foils surrounded by a tracking detector, whichin turn is surrounded by a calorimeter. The great strength of this technique lies in its particleidentiﬁcation capability; α-particles can be identiﬁed as short, straight tracks, even withoutdE/dx information, and e+/e− discrimination is possible in an externally applied magnetic ﬁeld.Unassociated calorimeter hits form γ candidates, although multiple Compton scatters make γcounting and energy reconstruction challenging. Timing information is also essential in suchdetectors, in order to reject particles of external origin through time-of-ﬂight measurements, aswell as to enable the identiﬁcation of sequential decays, for example “BiPo” decays that occurin 238U and 232Th decay chains.Full topological event reconstruction gives a very powerful means of rejecting backgroundswhich do not have a 2-electron origin. In addition, opening angle and individual electron energy21234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012033  doi :10.1088/1742-6596/888/1/012033Bβ−β−Figure 1. An illustration of the tracker-calorimeter technique, whereby the elec-trons from a double-beta decaying sourcetraverse a tracking detector, before hav-ing their individual energies measured ina calorimeter.inner calorimeterouter calorimetersource foil50 cm50 cmXY832 keV1256 keVFigure 2. A NEMO-3 2-electronevent in the search for 0νββ from100Mo. Two tracks have a commonorigin on the source foil, and projectto separate energy deposits in thecalorimeter (from [2]).distributions can be used to disentangle diﬀerent mechanisms behind neutrinoless double-betadecay in the event of a discovery.2. The NEMO-3 ExperimentThe NEMO-3 experiment ran from 2003 to 2011 in the Laboratoire Souterrain de Modane(LSM). It hosted 7 isotopes, with the largest masses comprised of 100Mo (7 kg) and 82Se (1 kg).5-inch low radioactivity PMTs coupled to polystyrene scintillator blocks provided an energyresolution of ΔE(FWHM)/E = 14%/√E(MeV). The tracking detector operated in Geigermode with a low-Z gas mixture of 95% helium, 1% argon and 4% ethanol quencher. An external25 Gauss magnetic ﬁeld allowed the reconstruction of track curvature for e+/e− discrimination.Calibration systems, shielding and an anti-radon enclosure completed the detector structure.2.1. Analysis TechniquesThe results presented here arise from several separate analyses of NEMO-3 data. Althoughdiﬀerent in their details, all analyses exploit the full event reconstruction capability of NEMO-3in order to constrain backgrounds in various control channels. Backgrounds originate from manysources, typically β + γ decays where Compton scattering, Mo¨ller scattering or pair productioncan give rise to 2-electron topologies. Backgrounds originating from outside the detector aremeasured in samples selected with the characteristic time signature of a particle crossing thedetector rather than originating in the source foil. One of the most important backgrounds,214Bi, can be measured in events containing 1 or more γ-rays in addition to a single electron,but also by looking for electron + α events with a time signature consistent with 214Bi-214Posequential decays. The spatial distribution of such events gives a further separation of thisbackground into contributions internal to the source, and contributions likely to have arisenfrom the decay of 222Rn inside the tracking volume. Another very important background from208Tl can be constrained, for example, by looking at the high γ−energy tail in e+ γ events.Typically, the various background contributions are ﬁtted simultaneously, or iteratively, inthe diﬀerent control samples. This gives rise to a highly constrained background predictionfor the 2-electron channel, where the search for double-beta decay processes subsequently takesplace.31234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012033  doi :10.1088/1742-6596/888/1/0120333. Latest Results from NEMO-33.1. Final Results from 100Mo.NEMO-3 recently published the results of the search for 0νββ in 100Mo with the full exposure ofapproximately 35 kg · yr [2, 3]. No signal is observed, and for the hypothesis of light-Majorananeutrino exchange, the following half-life limit is obtained :T 0νββ1/2 > 1.1× 1024 yr (90% C.L.) ,which corresponds to a limit on the eﬀective Majorana neutrino mass 〈mν〉 < 0.3 − 0.6 eV,where the range reﬂects nuclear matrix element (NME) uncertainties. This is the best half-lifelimit for the isotope 100Mo and the limit on 〈mν〉 is within a factor 2-3 of the best limits fromother experiments, yet obtained with a much smaller isotope mass.3.2. 2νββ measurements and 0νββ searches in 48Ca and 150Nd48Ca is a particularly interesting isotope in which to search for neutrinoless double-beta decaydue to its high Qββ value of 4.3 MeV, putting the visible energy above almost all backgroundprocesses. However it is diﬃcult to enrich, and there is some theoretical evidence that it mayhave a relatively suppressed NME for 0νββ. NEMO-3 has recently published its analysis of thesmall (7 g) sample it hosted (see Figures 3 and 4), obtaining the following measurement of thetwo-neutrino double-beta decay half-life :T 2νββ1/2 = [6.4+0.7−0.6(stat.)+1.2−0.9(syst.)]× 1019 yr .This result constitutes the most precise measurement of the two-neutrino half-life for 48Ca andis in mild tension with nuclear shell model predictions of ∼ 4× 1019 yr [4].Another of the most promising isotopes in which to search for 0νββ is 150Nd. It has a highQββ value (3.4 MeV), as well as a favourable phase space factor for double-beta decay due toits large atomic number. NEMO-3 has recently published the ﬁnal analysis of a 36.6 g sampleof 150Nd, obtaining a two-neutrino half-life in agreement with previous measurements :T 2νββ1/2 = [9.34 ± 0.22(stat.)+0.62−0.60(syst.)]× 1018 yr .A new technique has been developed in the search for 0νββ, using a “Boosted Decision Tree”(BDT) to exploit the full event reconstruction in order to optimise the signal and backgroundseparation [5]. Figure 5 shows an example of how variables other than the summed-energy oﬀergood separation between backgrounds and diﬀerent signal models, and Figure 6 shows the resultof combining several such variables in a BDT. The result is a limit on the 0νββ half-life in 150Ndof :T 0νββ1/2 > 2.0× 1022 yr (90% C.L.) ,corresponding to a limit on the eﬀective Majorana neutrino mass 〈mν〉 < 1.6 − 5.3 eV. Theexpected (observed) half-life limit is 11% (34%) better than obtained with the summed-energydistribution alone, proving the beneﬁt of using multi-variate techniques in experiments such asNEMO-3 for which full topological event information is available [5].3.3. Other NEMO-3 AnalysesSeveral other NEMO-3 analyses are currently underway and mentioned brieﬂy here.150Nd is the best candidate isotope in which to search for neutrinoless quadruple-beta decay,a lepton-number violating process that can occur even if neutrinos are Dirac particles [6]. TheNEMO-3 dataset is unique in its ability to explicitly identify such events, and the search for41234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012033  doi :10.1088/1742-6596/888/1/012033Rφ(cm)270 275 280 285 290 295 300 305Z(cm)-100-50050100100Mo 100Mo 150Nd96Zr48Ca82Se 82SeFigure 3. An image of the NEMO-3 source foils in the vicinity of the48Ca source, showing the relative rateof single-electron events from the nineCaF2 disks (adapted from [4]). Events / 0.1 MeV110210Ca - 7 g, 5.25 y48NEMO-3 - Data 1368 EntriesββνCa 248 Y90Sr / 90External RadonββνOther 2 Other internal〉ββm〈  < 2.68obsN〉η〈  < 2.70obsN〉λ〈  < 2.65obsNn = 10χ  < 9.35obsN (MeV)TOT E1 2 3 4 5Data / MC0.511.5Figure 4. The distribution of the summedelectron energy in 2-electron events in 48Ca. The2νββ-signal is clearly visible above backgrounds.Open histograms represent limits on non-standard model 0νββ processes. The ratio ofdata events to the total Monte Carlo predictionis shown in the bottom panel (from [4]).Events / 0.1 MeV-110110210Data (3095) ββν2Other Backgrounds〉 ν m〈 〉 λ 〈〉 η 〈n = 10χNd - 36.6 g, 5.25 y150NEMO-3 -  (MeV)eMax E0 0.5 1 1.5 2 2.5 3 3.5 4Data/MC0.511.5Figure 5. The maximum electronenergy in 2-electron events from theanalysis of 150Nd. The diﬀerent 0νββ-mechanisms all have a diﬀerent shapefrom the 2νββ and other backgroundprocesses. See [5] for details.BDT score-0.4 -0.2 0 0.2 0.4 0.6Events-110110210Data (3095)ββνNd 2150Tl internal208Other backgrounds〉 ν m〈 ββν0 < 3.29obsN = 0.71b1 - CL Nd - 36.6 g, 5.25 y150NEMO-3 - Figure 6. The Boosted Decision Tree(BDT) output, which combines 9 kinematicand detector parameters into a single score.The BDT provides better separation betweenbackgrounds and (in open black histogram)0νββ mediated by light-Majorana neutrinoexchange. See [5] for details.this process is currently underway. Figure 7 shows a 4-electron event in 100Mo which, due to itslarger mass and lack of 0ν4β decay mode, is an ideal sample in which to test the background51234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012033  doi :10.1088/1742-6596/888/1/012033X8X888Figure 7. A 4-electron event origi-nating from the 100Mo foil in NEMO-3.This is a background control channel inthe search for quadruple-beta decay in150Nd. No. Events / 0.1 MeV110210310 No. Events / 0.1 MeV / MeV2 + E1E1.5 2 2.5 3 3.5 4 4.5 5Data/MC0.511.52.6 2.8 3 3.2 3.402468101214 Data (8936)External BGsRadon BGsPa234mInternal Bi214Internal Tl208Internal Se82 ββν2 Limitββν0	Figure 8. The distribution of the summedelectron energy in 2-electron events in 82Se.The shape of the 0νββ signal normalised tothe 90% C.L. limit on the half-life, is indicatedby the dashed open histogram.modelling for this process.NEMO-3 has recently released the ﬁnal analysis of 116Cd, providing the world’s most precisemeasurement of the 2-neutrino half-life as well as a limit on the 0νββ half-life [7]. Oneparticularly interesting aspect of this analysis is a test of the mechanism behind 2νββ. It isfound that single-state dominance (SSD) whereby 2-neutrino decay proceeds exclusively throughthe lowest-lying intermediate Jπ = 1+ state, is favoured over higher-state dominance (HSD) inwhich higher-energy intermediate 1+ states are also involved.The second largest-mass isotope hosted in NEMO-3 was 82Se and it is especially importantto make precise measurements of this, the baseline isotope for SuperNEMO. A preliminary ﬁnalmeasurement of the 2νββ half-life has been performed :T 2νββ1/2 = [10.07 ± 0.14(stat.) ± 0.54(syst.)]× 1019 yr ,which is more precise that the current world-average value. Figure 8 shows the regionof interest in the summed-energy distribution for the 0νββ search. A half-life limit of2.5 × 1023 yr (90% C.L.) is obtained, corresponding to 〈mν〉 < 1.2 − 3.0 eV, which is notablefor being only 4 times worse than the half-life limit for 100Mo yet obtained with only 15% ofthe isotope mass. The signiﬁcantly longer 2νββ half-life is one of the important reasons for thebetter performance of 82Se.4. SuperNEMOSuperNEMO is a successor to the NEMO-3 experiment, which at the 100 kg scale can start toprobe neutrinoless double-beta decay half-lives of 1026 yr and Majorana mass scales down to∼ 50 meV. In order to achieve this, the energy resolution of the calorimeter needs to be improvedby almost a factor of 2 and the radon background inside the tracker reduced by a factor of 30,with similarly large reductions in the internal background contaminations inside the source foil.A number of other technical improvements, for example to the calibration systems and stabilitymonitoring, are being made. In the ﬁrst phase a Demonstrator Module containing 7 kg of 82Se,depicted in Figure 9, is in the ﬁnal stages of construction.61234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012033  doi :10.1088/1742-6596/888/1/012033Figure 9. An exploded drawing ofthe SuperNEMO Demonstrator Module,showing the central source-foil frame (inred) sandwiched between two trackermodules, which are in turn sandwichedbetween the two calorimeter main walls.Figure 10. A photograph of the couplingbetween the ﬁrst tracker module and the ﬁrstcalorimeter main wall, which took place inOctober 2016 at the LSM. This completesthe installation of half of the instrumentedSuperNEMO Demonstrator Module detector.4.1. SuperNEMO StatusThe production of the calorimeter, employing 8-inch high-QE radio-pure PMTs coupled topolystyrene scintillator, is now complete, with the typical energy resolution meeting therequirements for SuperNEMO of 8%(FWHM) at 1 MeV. The production of the tracker modules,containing over 2000 individual Geiger cells, is also complete, and dedicated measurementsindicate that radon activity levels at the required level of 150 μBq/m3 can be achieved withreasonable gas-ﬂow rates. In October 2016 the ﬁrst half of the detector was integrated at LSM(see Figure 10) and the remaining parts of the detector will be added over the next 6 months,such that data-taking can commence in mid-2017.5. ConclusionsNEMO-3 has been an extremely productive experiment, yielding competitive bounds onneutrinoless double-beta decay from 100Mo, while at the same time producing a wealth of othermeasurements from the six other isotopes that it hosted. The SuperNEMO DemonstratorModule, which will have signiﬁcantly better performance and lower backgrounds, is in anadvanced stage of preparation at the LSM.6. AcknowledgmentsNEMO-3 and SuperNEMO have been made possible by the hard-work of the staﬀ at the ModaneUnderground Laboratory. We acknowledge support by the grants agencies of the Czech Republic,CNRS/IN2P3 in France, RFBR in Russia, STFC in the U.K. and NSF in the U.S.References[1] F. Deppisch, these proceedings.[2] R. Arnold et al., Phys. Rev. D 89 (2014) 111101.[3] R. Arnold et al., Phys. Rev. D 92 (2015) 072011.[4] R. Arnold et al., Phys. Rev. D 93 (2016) 112008.[5] R. Arnold et al., Phys. Rev. D 94 (2016) 072003.[6] J. Heeck and W. Rodejohann, Europhys. Lett. 103 (2013) 32001.[7] R. Arnold et al., arXiv:1610.03226 [hep-ex].

Latest Results from NEMO-3 & Status of the SuperNEMO Experiment

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Latest Results from NEMO-3 & Status of the SuperNEMO Experiment

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