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Limit on the Radiative Neutrinoless Double Electron Capture of Ar from GERDA Phase I

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Limit on the Radiative Neutrinoless Double Electron Capture of Ar from GERDA Phase I Eur. Phys. J. C (2016) 76:652 DOI 10.1140/epjc/s10052-016-4454-5 Regular Article - Experimental Physics Limit on the radiative neutrinoless double electron capture of 36Ar from GERDA Phase I GERDA Collaboration1,a M. Agostini1, M. Allardt4, A. M. Bakalyarov13, M. Balata1, I. Barabanov11 , N. Barros4,20, L. Baudis19, C. Bauer7, E. Bellotti8,9, S. Belogurov11,12, S. T. Belyaev13, G. Benato19, A. Bettini16,17, L. Bezrukov11, T. Bode15, D. Borowicz3,5, V. Brudanin5, R. Brugnera16,17, A. Caldwell14, C. Cattadori9, A. Chernogorov12, V. D’Andrea1, E. V. Demidova12, A. di Vacri1, A. Domula4, E. Doroshkevich11, V. Egorov5, R. Falkenstein18, O. Fedorova11, K. Freund18, N. Frodyma3, A. Gangapshev7,11, A. Garfagnini16,17, C. Gooch14, P. Grabmayr18, V. Gurentsov11, K. Gusev5,13,15, J. Hakenmüller7, A. Hegai18,M.Heisel7, S. Hemmer17, G. Heusser7, W. Hofmann7,M.Hult6, L. V. Inzhechik11,21, J. Janicskó Csáthy15, J. Jochum18, M. Junker1, V. Kazalov11,T.Kihm7, I. V. Kirpichnikov12 , A. Kirsch7,A.Kish19, A. Klimenko5,7,22, R. Kneißl14, K. T. Knöpfle7, O. Kochetov5, V. N. Kornoukhov11,12, V. V. K u z m i n o v 11, M. Laubenstein1, A. Lazzaro15, V. I. Lebedev13, B. Lehnert4,H.Y.Liao14, M. Lindner7, I. Lippi17, A. Lubashevskiy5,7 , B. Lubsandorzhiev11, G. Lutter6, C. Macolino1,23, B. Majorovits14, W. Maneschg7, E. Medinaceli16,17, M. Miloradovic19, R. Mingazheva19, M. Misiaszek3, P. Moseev11, I. Nemchenok5, D. Palioselitis14, K. Panas3, L. Pandola2, K. Pelczar3, A. Pullia10, S. Riboldi10, N. Rumyantseva5, C. Sada16,17, F. Salamida9, M. Salathe7, C. Schmitt18, B. Schneider4, S. Schönert15, J. Schreiner7, A.-K. Schütz18, O. Schulz14, B. Schwingenheuer7, O. Selivanenko11, M. Shirchenko5,13, H. Simgen7, A. Smolnikov7, L. Stanco17, M. Stepaniuk7, L. Vanhoefer14, A. A. Vasenko12, A. Veresnikova11, K. von Sturm16,17, V. Wagner7,M.Walter19, A. Wegmann7, T. Wester4, C. Wiesinger15, H. Wilsenach4, M. Wojcik3, E. Yanovich11,I.Zhitnikov5,S.V.Zhukov13, D. Zinatulina5, K. Zuber4, G. Zuzel3 1 INFN Laboratori Nazionali del Gran Sasso and Gran Sasso Science Institute, Assergi, Italy 2 INFN Laboratori Nazionali del Sud, Catania, Italy 3 Institute of Physics, Jagiellonian University, Kra ków, Poland 4 Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany 5 Joint Institute for Nuclear Research, Dubna, Russia 6 European Commission, JRC-Geel, Geel, Belgium 7 Max-Planck-Institut für Kernphysik, Heidelberg, Germany 8 Dipartimento di Fisica, Università Milano Bicocca, Milan, Italy 9 INFN Milano Bicocca, Milan, Italy 10 Dipartimento di Fisica, Università degli Studi di Milano e INFN Milano, Milan, Italy 11 Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia 12 Institute for Theoretical and Experimental Physics NRC “Kurchatov Institute”, Moscow, Russia 13 National Research Centre “Kurchatov Institute”, Moscow, Russia 14 Max-Planck-Institut für Physik, Munich, Germany 15 Physik Department and Excellence Cluster Universe, Technische Universität München, Munich, Germany 16 Dipartimento di Fisica e Astronomia dell’Università di Padova, Padua, Italy 17 INFN Padova, Padua, Italy 18 Physikalisches Institut, Eberhard Karls Universität Tübingen, Tübingen, Germany 19 Physik Institut der Universität Zürich, Zurich, Switzerland 20 Present address:Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA, USA 21 Also at: Moscow Institute of Physics and Technology, Moscow, Russia 22 Also at: International University for Nature, Society and Man “Dubna”, Dubna, Russia 23 Present address: LAL, CNRS/IN2P3, Université Paris-Saclay, Orsay, France Received: 19 May 2016 / Accepted: 12 October 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Neutrinoless double electron capture is a process lation and the Majorana nature of neutrinos. A search for that, if detected, would give evidence of lepton number vio- neutrinoless double electron capture of 36Ar has been per- formed with germanium detectors installed in liquid argon a e-mail: [email protected] using data from Phase I of the GERmanium Detector Array 123 652 Page 2 of 6 Eur. Phys. J. C (2016) 76:652 Gerda ( ) experiment at the Gran Sasso Laboratory of INFN, p + n Italy. No signal was observed and an experimental lower limit W on the half-life of the radiative neutrinoless double electron e− 36 21 bound capture of Ar was established: T1/2 > 3.6 × 10 years at (A,Z+2) x (A,Z) 90% CI. − ebound + p W n 1 Introduction Fig. 1 Diagram for zero neutrino double electron capture with the The observation of neutrinoless double beta decay (0νββ): emission of one photon − (A, Z − 2) → (A, Z) + 2e , (1) geochemical measurement of 130Ba decay into 130Xe [8,9] can provide unambiguous information on lepton number and the second is a large-volume copper proportional counter 78 violation and indicate the Majorana nature of neutrinos, searching for double K-shell capture in Kr [10]. Sev- regardless the physics mechanism responsible for the decay. eral experiments including the latter established limits on Currently many experiments are searching for this decay both neutrino accompanied and neutrinoless double elec- considering different isotopes. Among these there is the tron capture of different isotopes (see Refs. [10–17]). For Gerda (GERmanium Detector Array) experiment [1]imple- some isotopes the possibility of a resonant enhancement ν menting bare germanium detectors enriched in 76Ge. This of the 0 ECEC decay has been predicted in case of mass experiment searches for neutrinoless double beta decay degeneracy between the initial state and an excited final of 76Ge. Recently the best limit on 0νββ decay half- state [4,18]. 36 life of 76Ge has been published by the Gerda collabora- Ar is expected [19] to undergo double electron cap- 36 tion [2]. ture to the ground state of S. The available energy [20] . ± . Another lepton number violating process that can provide of the decay is 432 58 0 19 keV and, therefore, both the the same information as neutrinoless double beta decay is radiative and the internal conversion modes are energetically the double capture of two bound atomic electrons without allowed. A resonance enhancement of the decay is not pos- the emission of neutrinos (0νECEC): sible for this isotope. Calculations based on the quasiparticle random-phase approximation (QRPA) predict a half-life for − 36 38 2e + (A, Z + 2) → (A, Z) + Q, (2) Ar in the order of 10 years for an effective Majorana neutrino mass of 1 eV [21]. So far, an experimental limit on where the quantity Q corresponds to the energy difference the radiative mode obtained during detector characterizations Gerda between the ground state atoms (A, Z +2) and (A, Z) [3,4]. in the Detector Laboratory has been published (T1/2 > . × 18 While in the corresponding process where two neutrinos 1 9 10 years at 68% CL) [22]. ν 36 are emitted (2νECEC) the available energy of the decay is The radiative mode of 0 ECEC in Ar with the emission carried away by neutrinos plus X-rays or Auger electrons, in of one photon provides a clear signature through the discrete the neutrinoless double electron capture the decay must be value of its energy and allows the detector to be separate accompanied by the emission of at least another particle to from the source of the decay. Two cascades of characteristic = . = . ensure energy and momentum conservation. Different modes X-rays with energies of EK 2 47 keV and EL 0 23 keV can be considered in which 0νECEC decay is associated with are emitted [23], corresponding to the capture of the elec- the emission of different particles like e+e− pairs, one or trons from the K - and the L-shell, respectively. The uncer- two photons, or one internal conversion electron. A detailed tainties for the energies of the X-rays amount to 0.4eV. discussion about double electron capture processes can be The corresponding energy for the monochromatic photon is = − − = . ± . found in Refs. [5–7]. Eγ Q Ek EL 429 88 0 19 keV. γ For 0+ → 0+ transitions the capture of two K -shell This paper reports the search for the 429.88 keV line ν 36 Gerda electrons with the emission of only one photon is forbidden from 0 ECEC decay of Ar with Phase I germanium because of angular momentum conservation. Therefore, the detectors and the determination of a limit on its half-life. most likely process is the capture from the K - and the L-shell. The diagram of this mode is depicted in Fig. 1. The unsta- ble daughter atom relaxes by emission of X-rays or Auger 2 The GERDA experiment electrons. At present, only two experiments found an indication of The Gerda experiment [1] is located at the Laboratori two neutrino double electron capture. The first is based on a Nazionali del Gran Sasso (Lngs) of the INFN. It was 123 Eur. Phys. J. C (2016) 76:652 Page 3 of 6 652 GERDA 16-03 3 GERDA 16-03 4 10 enr 10 39 Coax Ar enrCoax, 17.9 kg yr enrBEGe enr BEGe, 2.3 kg yr nat 103 Coax natCoax, 5.9 kg yr 2 2νββ counts / keV 10 102 counts/(20 keV kg yr) 10 10 1 -1 10 1 500 1000 1500 2000 2500 360 380 400 420 440 460 480 energy [keV] energy [keV] Fig. 2 Energy spectra from the three data sets collected during Gerda region between 360 and 500 keV. The shaded area corresponds to the Phase I. The left panel shows the energy spectra weighted with the prod- ROI defined between 410 and 450 keV uct of life time and detector mass. The right panel displays the energy designed in two phases. During Phase I reprocessed p- fore, data collected from these detectors were discarded. The type semi-coaxial High-Purity Germanium (HPGe) detec- total collected data used for the search for 0νECEC of 36Ar tors enriched in 76Ge (enrGe)toupto86%[24]fromthe correspond to a life time of about 460d.
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