Eitel, K.

Schmidt, B.

Siebenborn, B.

English

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Journal of Physics: Conference SeriesPAPER • OPEN ACCESSFirst data from the CUPID-Mo neutrinoless double beta decayexperimentTo cite this article: B. Schmidt and on behalf of the CUPID-Mo Collaboration 2020 J. Phys.: Conf. Ser. 1468 012129 View the article online for updates and enhancements.This content was downloaded from IP address 129.13.72.198 on 30/04/2020 at 09:50Content 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 LtdTAUP 2019Journal of Physics: Conference Series 1468 (2020) 012129IOP Publishingdoi:10.1088/1742-6596/1468/1/0121291First data from the CUPID-Mo neutrinolessdouble beta decay experimentB. Schmidt on behalf of the CUPID-Mo Collaboration 1Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,USAE-mail: beschmidt@lbl.govAbstract.The CUPID-Mo experiment is searching for neutrinoless double beta decay in 100Mo, evaluatingthe technology of cryogenic scintillating Li2100MoO4 detectors for CUPID (CUORE Upgradewith Particle ID). CUPID-Mo detectors feature background suppression using a dual-readoutscheme with Li2MoO4 crystals complemented by Ge bolometers for light detection. Thedetection of both heat and scintillation light signals allows the efficient discrimination of α fromγ&β events. In this proceedings, we discuss results from the first 2 months of data taking inspring 2019. In addition to an excellent bolometric performance of 6.7 keV (FWHM) at 2615 keVand an α separation of better than 99.9% for all detectors, we report on bulk radiopurity for Thand U. Finally, we interpret the accumulated physics data in terms of a limit of T 0ν1/2 > 3×1023 yrfor 100Mo and discuss the sensitivity of CUPID-Mo until the expected end of physics data takingin early 2020.1. IntroductionIn 1937 Ettore Majorana published a seminal paper on a “Symmetrical theory of electronsand positrons” [1]. The theory predicts new lepton number violating processes and has beeninvestigated for its possibilty of producing the matter-antimatter asymmetry in the Universethrough leptogenesis [2]. It also predicts that neutrinos can be their own antiparticles givingrise to the hypothetical decay process of neutrinoless double beta decay (0νββ). In this decaythe (anti-) neutrino is reabsorbed in the second vertex and only two electrons and the nucleusremain in the final state. Hence, the full decay energy can be easily detected [3].Modern 0νββ searches are an extremely sensitive probe of the light Majorana neutrinoexchange mechanism in particular and of new lepton number violating physics in general.Bolometric detectors, such as CUORE, are among the leading present searches [4, 5, 6, 7, 8].These detectors feature one of the best energy resolutions in the field, very high signal efficiencyand they excel in the range of target isotopes that can be studied [9].A ton-scale cryogenic search has been demonstrated [7] and an additional backgroundsuppression by the discrimination of α versus γ&β events has been established in both R&D [10]and in medium scale demonstrator experiments [11, 12]. The Li2100MoO4 detectors operated inCUPID-Mo have been selected for the future upgrade for CUORE since they feature an excellentperformance and radiopurity [10, 13]. In the following we will present performance results as1 Collaboration list: http://cupid-mo.mit.edu/collaborationTAUP 2019Journal of Physics: Conference Series 1468 (2020) 012129IOP Publishingdoi:10.1088/1742-6596/1468/1/0121292well as an evaluation of the physics data from the first 2 months of the physics campaign fromApril to June 2019.2. Performance of scintillating Li2MoO4 bolometersThe CUPID-Mo experiment employs twenty ∼210 g Li2MoO4 crystals enriched to ∼ 97 % in100Mo. The detectors modules are arranged into 5 towers of 4 modules, where each modulecomprises a Li2100MoO4 crystal at the top and an adjacent thin Ge light detector below.This detector arrangement is housed inside of the low radioactivity infrastructure of theEDELWEISS-III experiment [14, 15] in the underground laboratory of Modane (France). Thesetup and the detector performance were characterized in an initial commissioning campaign inMarch 2019 and have been described in detail in [12]. All detectors were found to be operationaland only a single detector module with abnormal noise condition was discarded from the analysis.Of the remaining 19 detectors all achieved performances, compatible with better than 99.9 % αseparation at equally high γ&β acceptance. In the present analysis we use the same 19 detectorsand present results from an extended period of data taking between April and June 2019, about5 times longer than the comissioning. The data processing is based on the Diana processingtools developed by the CUORE and CUPID-0 collaborations [16, 17].Energy (keV)0 500 1000 1500 2000 2500 3000Counts / (2.5 keV)050010001500200025003000CUPID-Mo - First dataPreliminaryBi214DEAc228 Bi214Tl208Tl208Pb214Pb212Ac228Bi214Bi214+-Tl & e208Bi214SEBi214 Energy (keV)2540 2560 2580 2600 2620 2640 2660 2680 27001−10110210Energy (keV)2540 2560 2580 2600 2620 2640 2660 2680 2700Pull3−2−1−01234Energy (keV)2540 2560 2580 2600 2620 2640 2660 2680 27001−101102102540 2560 2580 2600 2620 2640 2660 2680321−01234Energy (keV)Counts / keVPullTl208Figure 1. Summed calibration data of19 detectors accumulated over 10 days ofU/Th calibration. Significant peaks havebeen labeled. The inset shows the result ofa simultaneous UEML fit of the 2615 keV208Tl peak.Energy (keV)0 500 1000 1500 2000 2500 3000 3500 4000FWHM (keV)012345678910Mo100ββQFigure 2. Energy dependence ofthe resolution of the CUPID-Mo detectors,modeled with σ(E) =√σ20 +(σE ∗√E)2.The fit result is drawn in red, with the 1σuncertainty band in green.In addition to the previous linear calibration [12] we perform a first non-linear calibrationof the Li2100MoO4 detectors. Using the most prominent lines from 208Tl (2615 keV) and 214Bi(609 keV, 1120 keV, 1764 keV) we fit a 2nd order polynomial through their respective meanpositions and obtain the summed calibration spectrum, shown in Fig. 1. We perform asimultaneous unbinned extended maximum likelihood (UEML) fit across the 19 detectors withindividual amplitudes and resolutions for the 208Tl peak and extract a resolution of 6.7 keVFWHM (harm. mean) at 2615 keV. We further model the spectrum with a common stepfunction that describes multi-Compton events in terms of the ratio of the continuum to the208Tl peak height and we include a common linear component to approximate 2νββ eventsand remaining background from pile-up. We use significant γ-peaks of the summed calibrationspectrum to obtain resolution estimates for various energies in order to extrapolate the energyresolution at 3034 keV (100Mo Qββ). We evaluate both a linear fit as well as an extrapolationadding the resolution from a fixed noise component and a statistical term that scales with√Ein quadrature. In Fig. 2 the preferred fit is drawn, which predicts a resolution of (7.7± 0.4) keV(FWHM) at Qββ .TAUP 2019Journal of Physics: Conference Series 1468 (2020) 012129IOP Publishingdoi:10.1088/1742-6596/1468/1/01212933. First physics dataThe accumulated exposure in physics data amounts to 0.5 kg×yr after removal of unstablecryogenic periods and periods with abnormal noise conditions (4%). In addition to the previousanalysis procedure of the commissioning data, we defined four pulse shape analysis cuts onnormalized pulse shape variables (rise-time, decay-time, trigger - optimum filter position,baseline slope). These cuts have been set at ±7 median absolute deviations (MAD), howeverthey presently dominate our efficiency at a level of ε = (81.1± 0.5)%. We thus plan to furtherstudy our pulse shape normalization procedure and thoroughly optimize our cuts for a futureblind 0νββ analysis. The resulting spectrum for γ&β events, dominated by the 2νββ decayshape is shown in Fig. 3.Energy (keV)0 500 1000 1500 2000 2500 3000 3500 4000Counts / (2.5 keV) 110210310CUPID-Mo - First dataPreliminaryTl208Pb212 Co60+-e Co60Pb214K40Mo99Bi2143000 3500 4000 4500 5000 5500110210RnCounts / (5 keV)Energy (keV)RaU 222210226234Po &Figure 3. Summed background spectrum for19 detectors and 0.5 kg × yr of exposure afteranalysis cuts. The inset shows the alpha region(3 MeV – 6 MeV) with basic data quality cutsonly. Significant peaks have been labeled.0 500 1000 1500 2000 2500 3000 3500Fit value - Energy (keV)1000200030004000/ ndf 2χ 6 / 2p0 0.06±0.6 p1 0.0003±1 p2 07−2e±07 −1eCUPID-Mo - First dataPreliminaryLiterature value - Energy (keV)0 500 1000 1500 2000 2500 3000 3500 4000Residuals (keV)3−2−1−01234Figure 4. Energy linearity of thebackground data with residuals from a2nd order polynomial fit with respectto the true value in the lower inset.The 1σ confidence band (green) and theextrapolated bias and uncertainty at Qββ(red) are shown.In order to prepare for the 0νββ analysis we evaluate the accuracy of our energy scaleon five significant peaks in the background data (Fig. 4), fitting a 2nd order polynomial inestimated versus true peak position. The quadratic term is compatible with 0 and the estimatedbias at Qββ is ∆E = (0 ± 1.6) keV. Relaxing all pulse shape and pile-up cuts we performa preliminary investigation of the bulk contamination of the Li2100MoO4 crystals countingevents in a ± 15 keV window from the alpha decays in the U-series and the Th-series. Weobtain results of 238U - (0.5 ± 0.2)µBq/kg, sum of 234U+226Ra - (1.4 ± 0.3)µBq/kg , 230Th -(0.3± 0.14)µBq/kg, 222Rn - (0.6± 0.2)µBq/kg, 218Po - (0.5± 0.2)µBq/kg for the U-series andsimilarly 232Th - (0.3± 0.14)µBq/kg, 228Th - (0.4± 0.2)µBq/kg, 224Ra - (0.25± 0.12)µBq/kg,212Bi (0.3± 0.2)µBq/kg for the Th-series.4. OutlookBased on these first two months of physics data we can confirm the high degree of reproducibilityand ease of operation of the detectors. Thanks to the very low background with a single eventbetween 3 MeV and 4 MeV and no event close to Qββ of 100Mo at 3034 keV we can translatethese first results into a limit on T 0ν1/2 using a one sided 90% Poisson confidence bound of 2.3events. We fold in the systematic uncertainty from the efficiency evaluation following Cousinsand Highland [18] and obtain a limit of T 0ν1/2> 3× 1023 yr (see Fig. 5).TAUP 2019Journal of Physics: Conference Series 1468 (2020) 012129IOP Publishingdoi:10.1088/1742-6596/1468/1/0121294Livetime (yr)0 0.2 0.4 0.6 0.8 1(yr)ν0 1/290% exclusion sensitivity on T00.511.522.52410×cts/keV/kg/yr-210cts/keV/kg/yr-310Mo), TAUP 2019100CUPID-Mo preliminary (Mo), Phys. Rev. D 92, 072011100NEMO-3 limit (Figure 5. Bayesian exclusion sensitivity at 90% C.I. for 5 keV resolution (FWHM) at Qββand different background levels. Both the present limit after 2 months of data taking and theNEMO-3 limit after 5 years of data taking with ∼7 kg of 100Mo are shown.We compare this result to our projected sensitivity calculated for a 5 keV energy resolution(FWHM), 75% 0νββ containment efficiency, a ∼ 90% analysis efficiency and for the backgroundindex of either 10−2 counts/(keV×kg×yr) or 10−3 counts/(keV×kg×yr). Since the presentexposure is in the background free regime for both hypothesis, there is no impact of the slightlyworse energy resolution and we fall within 10% of the expected exclusion limit. We note thatthis limit is a factor 4 worse than the leading limit of NEMO-3 for 100Mo of T 0ν1/2> 1.1× 1024 yr[19], achieved after 5 years of data taking with ∼7 instead of ∼2 kg of 100Mo. Based on thesevery encouraging results from a preliminary analysis we started working on the development ofa detailed background model and to optimize the procedure for a blind 0νββ analysis with morethan half a year (2 kg × yr) of physics exposure in early 2020.Acknowledgements:We gratefully acknowledge the help of the technical staff of the Laboratoire Souterrain de Modane andother participating laboratories as well as our supporting funding agencies. For the full Acknowledgementplease see http://cupid-mo.mit.edu/collaboration.References[1] Majorana E 1937 Nuovo Cim. 14 171[2] Fukugita M and Yanagida T 1986 Phys. Lett. B 174 45[3] Dell’Oro S, Marcocci S, Viel M and Vissani F 2016 Adv. High Energy Phys. 2016 2162659[4] Gando A et al. (KamLAND-Zen) 2016 Phys. Rev. Lett. 117 082503[5] Albert J B et al. (EXO-200) 2018 Phys. Rev. Lett. 120 072701[6] Aalseth C E et al. (Majorana) 2018 Phys. Rev. Lett. 120 132502[7] Alduino C et al. (CUORE) 2018 Phys. Rev. Lett. 120 132501[8] Agostini M et al. (GERDA) 2018 Phys. Rev. Lett. 120 132503[9] Giuliani A et al. 2018 Eur. Phys. J. C 78 272[10] Armengaud E et al. (LUMINEU) 2017 Eur. Phys. J. C 77 785[11] Azzolini O et al. (CUPID-0) 2019 Eur. Phys. J. C 79 583[12] Armengaud E et al. (CUPID-Mo) 2019 arXiv:1909.02994 (Preprint 1909.02994)[13] Poda D V et al. (LUMINEU) 2017 AIP Conf. Proc. 1894 020017[14] Armengaud E et al. (EDELWEISS) 2017 JINST 12 P08010[15] Hehn L et al. (EDELWEISS) 2016 Eur. Phys. J. C 76 548[16] Alduino C et al. (CUORE) 2016 Phys. Rev. C 93 045503[17] Azzolini O et al. (CUPID-0) 2018 Eur. Phys. J. C 78 734[18] Cousins R D and Highland V L 1992 Nucl. Instrum. Meth. A 320 331[19] Arnold R et al. (NEMO-3) 2015 Phys. Rev. D 92 072011

First data from the CUPID-Mo neutrinoless double beta decay experiment

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First data from the CUPID-Mo neutrinoless double beta decay experiment

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