The Majorana Demonstrator search for neutrinoless double beta decay
C. Cuesta, M. Buuck, J.A. Detwiler, J. Gruszko, I.S. Guinn, J. Leon, R.G.H. Robertson, N. Abgrall, A.W. Bradley, Y-D. Chan, S. Mertens, A.W.P. Poon, K. Vetter, I.J. Arnquist, E.W. Hoppe, R.T. Kouzes, J.L. Orrell, F.T. Avignone III, A.S. Barabash, S.I. Konovalov, V. Yumatov, F.E. Bertrand, A. Galindo-Uribarri, D.C. Radford, R.L. Varner, C.-H. Yu, V. Brudanin, M. Shirchenko, S. Vasilyev, E. Yakushev, I. Zhitnikov, M. Busch, T.S. Caldwell, T. Gilliss, R. Henning, M.A. Howe, J. MacMullin, S.J. Meijer, C. O'Shaughnessy, J. Rager, B. Shanks, J.E. Trimble, K. Vorren, W. Xu, C.D. Christofferson, C. Dunagan, A.M. Suriano, P.-H. Chu, S.R. Elliott, R. Massarczyk, K. Rielage, B.R. White, Yu. Efremenko, A.M. Lopez, H. Ejiri, A. Fullmer, G.K. Giovanetti, M.P. Green, V.E. Guiseppe, D. Tedeschi, C. Wiseman, B.R. Jasinski, K.J. Keeter, M.F. Kidd, R.D. Martin, E. Romero-Romero, J.F. Wilkerson
TThe M
AJORANA D EMONSTRATOR search forneutrinoless double beta decay
C. Cuesta ∗ , M. Buuck, J.A. Detwiler, J. Gruszko, I.S. Guinn, J. Leon, andR.G.H. Robertson Center for Experimental Nuclear Physics and Astrophysics, and Department of Physics,University of Washington, Seattle, WA, USAE-mail: [email protected]
N. Abgrall, A.W. Bradley, Y-D. Chan, S. Mertens, A.W.P. Poon, and K. Vetter † Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
I.J. Arnquist, E.W. Hoppe, R.T. Kouzes, and J.L. Orrell
Pacific Northwest National Laboratory, Richland, WA, USA
F.T. Avignone III
Department of Physics and Astronomy, University of South Carolina, Columbia, SC, USAOak Ridge National Laboratory, Oak Ridge, TN, USA
A.S. Barabash, S.I. Konovalov, and V. Yumatov
National Research Center “Kurchatov Institute” Institute for Theoretical and ExperimentalPhysics, Moscow, Russia
F.E. Bertrand, A. Galindo-Uribarri, D.C. Radford, R.L. Varner, and C.-H. Yu
Oak Ridge National Laboratory, Oak Ridge, TN, USA
V. Brudanin, M. Shirchenko, S. Vasilyev, E. Yakushev, and I. Zhitnikov
Joint Institute for Nuclear Research, Dubna, Russia
M. Busch
Department of Physics, Duke University, Durham, NC, USATriangle Universities Nuclear Laboratory, Durham, NC, USA
T.S. Caldwell, T. Gilliss, R. Henning, M.A. Howe, J. MacMullin, S.J. Meijer,C. O’Shaughnessy, J. Rager, B. Shanks, J.E. Trimble, K. Vorren, and W. Xu ‡ Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC, USATriangle Universities Nuclear Laboratory, Durham, NC, USA
C.D. Christofferson, C. Dunagan, and A.M. Suriano
South Dakota School of Mines and Technology, Rapid City, SD, USA
P.-H. Chu, S.R. Elliott, R. Massarczyk, K. Rielage, and B.R. White
Los Alamos National Laboratory, Los Alamos, NM, USA c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ a r X i v : . [ phy s i c s . i n s - d e t ] A ug he M AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta
Yu. Efremenko and A.M. Lopez
Department of Physics and Astronomy, University of Tennessee, Knoxville, TN, USA
H. Ejiri
Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka, Japan
A. Fullmer
Department of Physics, North Carolina State University, Raleigh, NC, USATriangle Universities Nuclear Laboratory, Durham, NC, USA
G.K. Giovanetti
Department of Physics, Princeton University, Princeton, NJ, USA
M.P. Green
Department of Physics, North Carolina State University, Raleigh, NC, USATriangle Universities Nuclear Laboratory, Durham, NC, USAOak Ridge National Laboratory, Oak Ridge, TN, USA
V.E. Guiseppe, D. Tedeschi, and C. Wiseman
Department of Physics and Astronomy, University of South Carolina, Columbia, SC, USA
B.R. Jasinski
Department of Physics, University of South Dakota, Vermillion, SD, USA
K.J. Keeter
Department of Physics, Black Hills State University, Spearfish, SD, USA
M.F. Kidd
Tennessee Tech University, Cookeville, TN, USA
R.D. Martin
Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, ON,Canada
E. Romero-Romero
Department of Physics and Astronomy, University of Tennessee, Knoxville, TN, USAOak Ridge National Laboratory, Oak Ridge, TN, USA
J.F. Wilkerson
Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC, USATriangle Universities Nuclear Laboratory, Durham, NC, USAOak Ridge National Laboratory, Oak Ridge, TN, USA he M AJORANA
Collaboration is constructing the M
AJORANA D EMONSTRATOR , an ultra-lowbackground, modular, HPGe detector array with a mass of 44.8-kg (29.7 kg enriched ≥ Ge) to search for neutrinoless double beta decay in Ge. The next generation of tonne-scale Ge-based neutrinoless double beta decay searches will probe the neutrino mass scale inthe inverted-hierarchy region. The M
AJORANA D EMONSTRATOR is envisioned to demonstrate apath forward to achieve a background rate at or below 1 count/tonne/year in the 4 keV region ofinterest around the Q-value of 2039 keV. The M
AJORANA D EMONSTRATOR follows a modularimplementation to be easily scalable to the next generation experiment. First data taken with theD
EMONSTRATOR are introduced here.
XIII International Conference on Heavy Quarks and Leptons22-27 May, 2016Blacksburg, Virginia, USA ∗ Speaker. † Alternate address: Department of Nuclear Engineering, University of California, Berkeley, CA, USA ‡ Current address: Department of Physics, University of South Dakota, Vermillion, SD, USA c (cid:13) Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. he M
AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta
1. Introduction
In a number of even-even nuclei, β decay is energetically forbidden, but the second order weakprocess of 2 ν double beta decay is allowed, as first proposed by Goeppert-Mayer in 1935 [1]. Ifthe neutrino is a Majorana particle, neutrinoless double beta (0 νβ β ) decay could also occur viathe exchange of a light Majorana neutrino, or by other mechanisms [2, 3]. This decay violateslepton number and provides a model-independent test of the nature of the neutrino. The rate of0 νβ β -decay via light Majorana neutrino exchange is given by (cid:16) T ν / (cid:17) − = G ν | M ν | (cid:18) (cid:104) m ββ (cid:105) m e (cid:19) (1.1)where G ν is a phase space factor, M ν is a nuclear matrix element, and m e is the electron mass. (cid:104) m ββ (cid:105) is the effective Majorana neutrino mass of the exchanged neutrino. The latter is given by (cid:104) m ββ (cid:105) = (cid:12)(cid:12) ∑ i = U ei m i (cid:12)(cid:12) , where U ei specifies the admixture of neutrino mass eigenstate i in the electronneutrino. Because (cid:104) m ββ (cid:105) depends on the oscillation parameters, both the overall neutrino mass andthe mass hierarchy can impact the observed rate. Provided the nuclear matrix elements are are wellevaluated, 0 νβ β -decay experiments could establish an absolute scale for the neutrino mass.Experimentally, 0 νβ β -decay can be detected by searching the spectrum of the summed energyof the emitted betas for a monoenergetic line at the Q-value of the decay (Q ββ ). Recent sensitivesearches for 0 νβ β carried out in Ge (GERDA [4, 5]),
Xe (KamLAND-Zen [6, 7] and EXO-200 [8, 9]),
Te (CUORE-0 [10]), among others, set limits on the decay half-life.
2. Overview of the M
AJORANA D EMONSTRATOR
The M
AJORANA D EMONSTRATOR [11] is an array of enriched and natural germaniumdetectors that will search for the 0 νβ β -decay of Ge. The specific goals of the M
AJORA - NA D EMONSTRATOR are several: to demonstrate a path forward to achieving a background rateat or below 1 count/(ROI-t-y) in the 4 keV region of interest (ROI) around the 2039 keV Q ββ ofthe Ge 00 νβ β -decay when scaled up to a tonne scale experiment; show technical and engineer-ing scalability toward a tonne-scale instrument; and perform searches for other physics beyond theStandard Model, such as dark matter and axions.The experiment is composed of 44.8 kg of high-purity Ge (HPGe) detectors which also act asthe source of Ge 0 νβ β -decay. The benefits of HPGe detectors are that Ge is an intrinsically low-background source material, with understood enrichment chemistry, excellent energy resolution,and event reconstruction capabilities. P-type point contact detectors [12, 13] were chosen afterextensive R&D by the collaboration for their powerful background rejection capabilities. TheD
EMONSTRATOR consists of a mixture of HPGe detectors including, 29.7 kg built from Ge materialthat is enriched to ≥
88% in Ge and 15.1 kg fabricated from natural Ge (7.8% Ge). The averagemass of the enriched detectors is ∼
850 g.A modular instrument composed of two cryostats built from ultra-pure electroformed copperis being constructed. Each module hosts 7 strings of 3-5 detectors. The modules are operated in apassive shield that is surrounded by a 4 π active muon veto. To mitigate the effect of cosmic raysand prevent cosmogenic activation of detectors and materials, the experiment is being deployed at2 he M AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta
AJORANA D EMONSTRATOR is shown in Figure 1.
Figure 1:
Picture of the M AJORANA D EMONSTRATOR . The main technical challenge of the M
AJORANA D EMONSTRATOR is to reach a backgroundrate of 3 counts/(ROI-t-y) after analysis cuts, which projects to a background level of 1 count/(ROI-t-y) in a large scale experiment after accounting for additional improvements from thicker shield-ing, better self-shielding, and if necessary, increased depth. To achieve this goal, backgroundsources must be reduced and offline background rejection must be maximized. The estimated ROIcontributions based on achieved assays [15] of materials and simulations for cosmic muon interac-tions sum to < AJORANA D EMONSTRATOR .
3. The M
AJORANA D EMONSTRATOR implementation
The M
AJORANA D EMONSTRATOR follows a modular implementation to scale easily to thenext generation experiment. The modular approach allows the independent assembly and commis-sioning of each module independently, providing a fast deployment and minimizing interferencewith already-operational detectors.As a first step, a prototype module was constructed using a commercial copper cryostat. It wasloaded with three strings of natural-abundance germanium and placed into shielding. Data was col-lected with this module from June 2014 through July 2015. It served as a test bench for mechanicaldesigns, fabrication methods, and assembly procedures for the construction of the electroformed-copper Modules 1 & 2. In addition, the prototype also tested DAQ, data building and analysistools.Following the prototype run, construction began on two modules with electroformed coppercryostats. The first, Module 1, was assembled in 2015. Module 1 houses 16.8 kg of enrichedgermanium detectors and 5.7 kg of natural germanium detectors. The strings were assembled andcharacterized in dedicated String Test Cryostats. Module 1 was moved into the shield, and data3 he M
AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta taking began, during 2015. The final stage, Module 2 supports 12.8 kg of enriched and 9.4 kg ofnatural Ge detectors. Module 2 has been assembled and will start taking data soon. Figure 2 showsthe installation of detectors into the cryostat of Module 2.
Figure 2:
Module 2 detectors being installed.
4. Module 1 preliminary results
All open data from Module 1 are divided in two datasets: dataset 0 (DS0) and dataset 1 (DS1).DS0 was a set of commissioning runs used to test analysis and data production corresponding todata taken from June 2015 to October 2015. In fall 2015, we implemented planned improvements: • Installed the electroformed inner copper shield. Built with the final pieces of electroformedcopper, the inner shield was not yet ready for installation during the initial construction ofModule 1. • Added additional shielding within the vacuum of the cross arm. • Replaced the cryostat Kalrez seal with PTFE which has much better radiopurity and muchlower mass and entails a 3 orders of magnitude reduction in the ROI contribution. • Repaired non-operating channels.These changes define the difference between DS0 and DS1. Hence, DS1 is the dataset that isbeing used to determine the background. DS1 data described here are taken from December 2015to April 2016. Data taking continues, but after that date data blinding began. Table 1 summarizesthe distribution of the total time elapsed during DS0 and DS1. The exposure evolution, taking intoaccount the total active mass is shown in Figure 3.The state-of-the-art data analysis techniques that further enable the D
EMONSTRATOR (cid:48) s physicsreach are still being developed. Double beta decay events are characterized as single-site eventsbecause the range of the electron is small compared to that of a typical Compton-scattering back-ground gamma. Using pulse shape discrimination methods [16], it is possible reject >
90% ofmulti-site events while retaining 90% of single-site events and reducing the Compton continuum atQ ββ by >
50% in case of backgrounds from the
Th calibration source, as seen in Figure 4.4 he M
AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta
DS0 DS1Total 103.15 d 104.68 dTotal acquired 87.93 d 97.52 dPhysics 47.70 d 54.73 dHigh radon 11.76 d 7.32 dDisruptive commissioning tests 13.10 d 28.61 dCalibration 15.44 d 6.86 dDown time 15.21 d 7.16 d
Table 1:
DS0 and DS1 duty cycles. The run time is distributed as follows: Total: total - time elapsed fromthe beginning until the end of the dataset; total acquired - total livetime of the dataset; physics - backgroundruns to be used in the physics analysis; high radon - runs during which the radon purge for the shield wascompromised, either intentionally or accidentally; disruptive commissioning tests - data corresponding toDAQ tests and electronics calibrations; calibration - calibration runs taken with a
Th or Co source;down time - difference between the total and the acquired time, where the dominant period correspond todetectors being biased down to troubleshoot some detectors. (a) (b)
Figure 3:
DS0 and DS1 exposure plots of the physics data.
5. Next generation Ge experiment
The M
AJORANA
Collaboration is working cooperatively with GERDA and others towards theestablishment of a single Ge 0 νβ β -decay collaboration to build a large experiment to explore theinverted hierarchy region. Periodic joint meetings take place with this purpose. Leading a tonne-scale 0 νβ β experiment is one of the highest priorities new activity for the US Nuclear Physics com-munity as indicated in the latest Long Range Plan for nuclear science in the US recently releasedby the Nuclear Science Advisory Committee [17]. We anticipate down-select of best technologies,based on results of the two experiments. Moving forward is predicated on demonstration of pro-jected backgrounds. There is on-going work to go from a conceptual design to a viable, competitiveproposal including R&D on: robust signal and high voltage connectors, ultra-clean materials, alter-native detector designs, detector signal readout, cryostat and detector mount designs, enrichment,cooling and shielding, and required depth. 5 he M
AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta P A P C P A B P A P
A B P A P
B P
A P
A P C P A P
A B P B P
B P A A cc ep t an c e ( % ) Tl DEP
Tl DEP Mean
Tl SEP
Tl SEP Mean
Figure 4:
Acceptance efficiency of the pulse shape discrimination cuts at the double-escape peak (single-site events), the single-escape peak (multi-site events), and at 0 νβ β
ROI (Compton scattering events) of theModule 1 detectors evaluated with
Th calibration data.
Summary
The M
AJORANA D EMONSTRATOR will search for the 0 νβ β -decay with and array of Gedetectors. A modular instrument composed of two cryostats built from ultra-pure electroformedcopper is being constructed. Module 1 has been taking data since June 2015, and before datablinding started in April 2016, 185 days of data were taken from which 102 days are used in thephysics analysis. The data are divided in two datasets: DS0 and DS1. In Fall 2015, we implementedplanned improvements: installed the inner copper shield, added additional shielding within thevacuum of the cross arm, exchanged the cryostat seal for one with a low background component,and repaired non-operating channels. These changes define the difference between DS0 and DS1,and DS1 is being used to determine the background. Data analysis is in progress and includes,for instance, pulse shape discrimination to reject multi-site events. The D
EMONSTRATOR aims toreach a background rate of 3 counts/(ROI-t-y) after analysis cuts. This projects to a backgroundlevel of 1 count/(ROI-t-y) in a large scale experiment that is already being planned.
Acknowledgments
This material is based upon work supported by the U.S. Department of Energy, Of-fice of Science, Office of Nuclear Physics under Award Numbers DE-AC02-05CH11231, DE-AC52-06NA25396, DE-FG02-97ER41041, DE-FG02-97ER41033, DE-FG02-97ER41042, DE-SC0012612, DE-FG02-10ER41715, DE-SC0010254, and DE-FG02-97ER41020. We acknowl-edge support from the Particle Astrophysics Program and Nuclear Physics Program of the Na-tional Science Foundation through grant numbers PHY-0919270, PHY-1003940, 0855314, PHY-1202950, MRI 0923142 and 1003399. We acknowledge support from the Russian Foundation for6 he M
AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta
Basic Research, grant No. 15-02-02919. We acknowledge the support of the U.S. Departmentof Energy through the LANL/LDRD Program. This research used resources of the Oak RidgeLeadership Computing Facility, which is a DOE Office of Science User Facility supported underContract DE-AC05-00OR22725. This research used resources of the National Energy ResearchScientific Computing Center, a DOE Office of Science User Facility supported under Contract No.DE-AC02-05CH11231. We thank our hosts and colleagues at the Sanford Underground ResearchFacility for their support.
References [1] M. Goeppert-Mayer, Double Beta-Disintegration, Phys. Rev. 48 (2014) 512.[2] F. T. III Avignone, S. R. Elliott and J. Engel, Double beta decay, Majorana neutrinos, and neutrinomass, Rev. mod. Phys. 80 (2008) 481.[3] J. D. Vergados, H. Ejiri and F. Simkovic, Theory of neutrinoless double-beta decay, Rep. Prog. Phys.75 (2012) 1063013.[4] M. Agostini, et al., Results on Neutrinoless Double- β Decay of Ge from Phase I of the GERDAExperiment, Phys. Rev. Lett. 111 (2013) 122503.[5] B. Schwingenheuer, et al., First data release GERDA Phase II: Search for 0 νβ β of Ge.URL [6] A. Gando, et al., Limit on Neutrinoless β β
Decay of
Xe from the First Phase of KamLAND-Zenand Comparison with the Positive Claim in Ge, Phys. Rev. Lett. 110 (2013) 062502.[7] A. Gando, et al., Search for Majorana Neutrinos near the Inverted Mass Hierarchy region withKamLAND-Zen, arXiv 1605.02889.[8] J. B. Albert, et al., Improved measurement of the 2 νβ β half-life of
Xe with the EXO-200 detector,Phys. Rev. C 89 (2014) 015502.[9] J. B. Albert, et al., Search for Majorana neutrinos with the first two years of EXO-200 data, Nature510 (2014) 229.[10] A. Gando, et al., Search for Neutrinoless Double-Beta Decay of
Te with CUORE-0, Phys. Rev.Lett. 115 (2015) 102502.[11] N. Abgrall, et al., The M
AJORANA D EMONSTRATOR
Neutrinoless Double-Beta Decay Experiment,Adv. High Energy Phys. 2014 (2014) 365432.[12] P. S. Barbeau, et al., Large-mass ultralow noise germanium detectors: performance and applicationsin neutrino and astroparticle physics, JCAP 9 (2007) 009.[13] P. N. Luke, et al., Low capacitance large volume shaped-field germanium detector, IEEE Transactionson Nuclear Science 36 (1989) 926.[14] J. Heise, The Sanford Underground Research Facility at Homestake, J. Phys.: Conf. Ser. 606 (2015)012015.[15] N. Abgrall, et al., The M
AJORANA D EMONSTRATOR radioassay program, NIM A 828 (2016) 22.[16] D. Budjas, et al., Pulse shape discrimination studies with a Broad-Energy Germanium detector forsignal identification and background suppression in the GERDA double beta decay experiment,JINST 4 (2009) 10007. he M AJORANA D EMONSTRATOR search for neutrinoless double beta decay
C. Cuesta[17] The 2015 Nuclear Science Advisory Committee, The 2015 Long Range Plan for Nuclear Science.URL http://science.energy.gov/~/media/np/nsac/pdf/2015LRP/2015_LRPNS_091815.pdfhttp://science.energy.gov/~/media/np/nsac/pdf/2015LRP/2015_LRPNS_091815.pdf