Pos(ICHEP2012)401 Decay Is Reviewed

PoS(ICHEP2012)401 http://pos.sissa.it/ decay is reviewed. The back- y of measurement is shown to be · νββ decay results in a preliminary half life νββ ∗ decay with the GERDA yr. The status of the preparations for the upgrade of GERDA 21 10 · νββ 0.10) y). The measurement of 2 ± · kg · Ge is given. Ge)=(1.88 76 cts/(keV 76 ( 006 004 . . 2 0 0 νββ / 87% in + − 1 2 [email protected] ≈ Speaker. ground index in the energy region of interest after 6.1 kg The status of the GERDA experiment for the0.02 search of 0 with additional 30 BEGe detectors withto a total mass of 20.8 kg made from germanium enriched of T ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

36th International Conference on High EnergyJuly Physics, 4-11, 2012 Melbourne, Australia MPI für Physik, Föhringer Ring 6,E-mail: 80805 München, Germany B. Majorovits for the GERDA collaboration The search for 0 experiment PoS(ICHEP2012)401 ]. 6 ],[ 5 yr can 25 ], respec- 3 ] it is clear 10 · 1 yr [ 25 10 · decay induced by the decay consistent with yr, respectively [ 25 νββ νββ 10 ] and 1.6 ], which would be consistent · 2 4 ], 0 4 yr [ yr [ 25 25 ). 0.12 eV – 0.25 eV, depending on the σ 10 10 > · · yr and 1.6 B. Majorovits for the GERDA collaboration ) 0 5 . . 25 3 0 eV (3 + − 10 · 14 20 ) decay would hence reveal Majorana character . . 0 0 + − =(1.2 2 ) decay is a second order weak process. Two neu- Ge with 1.9 νββ 2 Xe 1.9 76 νββ / 86%: The Heidelberg-Moscow and the IGEX collab- 0 1 136 νββ ≈ decay of decay of νββ =2040keV) to νββ ββ Ge with a half life of T Ge (Q 76 decay has been observed for many isotopes, the neutrinoless mode could not 76 νββ Left: Model of the GERDA experiment. Right: Three detector strings. Three detectors are decay of ] a combined analysis is done. It is claimed that a 90%C.L. lower limit of 3.4 While 2 The KamLAND-Zen experiment and the Enriched Xenon Observatory (EXO) have recently Since oscillations between neutrino flavors could be detected unambiguously [ Observation of neutrinoless double beta (0 Neutrino accompanied double beta (2 νββ 5 In [ be set, excluding an effectivenuclear matrix Majorana element neutrino calculation. mass While this of exchange refutes of with 97.5%C.L. a 0 light Majorana neutrino consistent with the claim [ published lower limits of 0 of neutrinos and implyexplanation Lepton of number the violation baryon asymmetry in of the our universe neutrino by sector, baryogenesis via a leptogenesis. keyyet be ingredient observed for without the doubt.experiments The using most High stringent Purity limits Germanium haveemitting (HPGe) come isotope detectors for that more than are a enrichedoration decade in give from the lower double limits beta for 0 tively. A subgroup of theof Heidelberg 0 Moscow collaboration has claimed evidencewith for an observation effective Majorana neutrino mass of 0.44 Figure 1: mounted per string. They are surrounded by a copper shroud. 1. Introduction trons coherently decay to twoneutrinos. protons In under case the neutrinos emission have ofof Majorana a two character, neutrino electrons the and decay and hence could two happen be electron without induced anti- the by emission an of exchange neutrinos. The GERDA experiment that neutrinos must have a tinyconjugates, but ie. non vanishing antiparticles, mass. soa Massive called neutrinos non Majorana vanishing could particles. Majorana be mass their This term ownmechanism. property to CP would neutrinos allow that to could be attribute naturally tiny thanks to the seesaw PoS(ICHEP2012)401 . Ge 76 yr (90% C.L.). 26 B. Majorovits for the GERDA collaboration Ca. If drifted close to the surface of the 10 · 42 K decays with a Q-value of 3.54 MeV via 42 Ar. 3 K can thus lead to background events in the energy 42 decay to 2 42 νββ right). The copper shroud is needed to shield the detectors 1 ]. The ultra clean cryogenic liquid acts as shield against external Ge detectors from the Heidelberg-Moscow and IGEX experiments. 7 . enr RG2: 2166g ANG4: 2372g ββ . The overall mass of the six working HPGe detectors is 14.6 kg corresponding Ge. 2 RG1: 2110g ANG3: 2391g . The array with HPGe detectors is submersed into the center of a liquid Argon 76 1 Left: Bare HP radiation, moderates and captures neutrons and serves as a muon Cerenkov veto in m copper foil (see Fig. K ions generated from the decay of γ µ 42 ANG2: 2833g ANG5: 2746g For the first phase eight reprocessed HPGe detectors of the Heidelberg-Moscow and IGEX The detectors are mounted together with a reference detector made from germanium with natu- The GERmanium Detector Array (GERDA) experiment is designed to confirm or refute the The design of the GERDA experiment bases on the concept to operate bare HPGe detectors decay with 81.9% probability to the ground state of region of interest around Q HPGe detectors by the present electrical fields against β connection with the PMTs. On topthe of HPGe the detectors tank can a be clean manipulated room and houses lowered the into lock the system tank. throughexperiments which are used. The available HPGeare detectors shown in after Fig. dismounting from theto vacuum 165 cryostats moles of ral abundance in three strings. Inwere a second installed. string two more The HPGefrom individual detectors with strings 60 natural with abundance enriched detectors are surrounded by shrouds made (LAr) cryostat with 4 mdiameter diameter. that is The instrumented cryostatexternal with itself Photo is Multiplier surrounded Tubes by (PMTs). a water The tank with water 10 shields m against claim for evidence in the first phaseget of mass the and GERDA experiment. the In background athe identification second overall possibilities phase sensitivity will the to available be the tar- significantly half increased, life increasing of 0 The calibration peaks at 2615 keV and 583 keV are shown in separate windows. the claim induced by other scenariosby germanium such based as experiments. SUSY, leptoquarks, compositness can be tested only radiation and cooling mediumshown for in the Fig. HPGe detectors simultaneously. A sketch of the setup is Figure 2: directly in a cryogenic liquid [ The GERDA experiment Right:Energy spectra of a calibration measurement of the six working HPGe detectors enriched in 2. The experimental setup PoS(ICHEP2012)401 Ge ββ 76 decay of Ge (red) and HPGe K) and 1764 keV yr can be obtained 76 42 νββ 25 10 · decay of cosmogenically β yr and of the HPGe detectors · K), 1525 keV ( (left). In the energy region below 40 . The energy resolution (FWHM) at 3 2 20) keV around the Q-value of the B. Majorovits for the GERDA collaboration ± decay is blinded (green line). Right: Background ]. ββ 9 4 decay is seen between 600 keV and 1500 keV in the νββ yr are shown in Fig. · ]. 8 yr). · yr [ Ge. A preliminary analysis results in a half-life for 2 kg 21 · 76 10 · Po on the surface of the detectors. 0.10) cts/(keV right shows a decomposition of the energy spectrum in the energy range from 600 keV 228 ± Ar. A clear excess due to 2 3 Ge. No visible features are contained in the energy windows 1940 to 2140 keV. The BI 006 004 . . Left: Background spectra taken with the phase I HPGe detectors enriched in 39 yr of data has been taken with the enriched detectors. 0 0 76 · + − =(1.88 Th and Under the current conditions a 90% lower limit on the half life of 2 Figure The energy spectrum of the enriched detectors after 6.1 kg The data is blinded in an energy region of (2040 Since November 2011 the GERDA experiment is running in its phase I configuration. Cali- The BI reached with the GERDA experiment is by a factor of 8 (13) lower than the ones from Bi). Other contributions to the background index (BI) in the energy region of interest are due 2 228 νββ / 500 keV both spectra are dominated by events resulting from the 2 1 214 T detectors enriched in to 1800 keV into differentis observed inferred components: from peaks The in presence the of energy spectrum the at background 1460 components keV ( detectors made from natural germaniumare (blue). labelled. The Prominent energy lines region around from the identified Q-value background of contributions 2.6 MeV of the detectors isof between 6.1 4.5 kg keV (0.17%) and 5.1 keV (0.20%). Until June 2012 a total 3. First results from GERDA phase I with natural abundance after≈ 3.7 kg produced ( to decay of is 0.020 after 20 months of total running time. bration spectra of all enriched detectors are shown in Fig. Figure 3: spectrum of the enriched detectorsfitting in simulated the energy spectra region to 600 thenot keV considered prominent – as 1800 features keV. they A is are decomposition shown. negligible obtained in Ohter by the shown identified energy background range. components are the Heidelberg Moscow and IGEX experiments [ The GERDA experiment PoS(ICHEP2012)401 3 26 yr). This would · ] have been trans- kg 9 · yr) in spring 2013, at · kg · cts/(keV 3 . The second design foresees − 4 ]. If event topologies can be dis- cts/(keV 10 3 11 20 kg. During all processing steps < − Th source. The option to insert both ≈ ] a significant fraction of background 10 . 228 · 4 ]. The field configurations of BEGe de- 13 B. Majorovits for the GERDA collaboration decay are deposited mostly within 1 mm 14 87 % procured in 2005 [ yr). additional efforts are necessary. Hence, ββ ≈ · 10 day ship voyage. At all production sites the ≈ kg · middle, thus ensuring optimal shielding of the en- 5 800 mm above and below the detector array at the =270.8 days). 4 ≈ 2 Ge to / ]. 1 76 10 1080 wavelength shifting fibers surrounding the detector ]. cts/(keV Co at a level of 1.8 ] or segmentation [ 3 ≈ Ge (T 15 60 − 68 12 Ge and 68 Co. All transport has been performed in a specially designed container. 60 cm causing a multi site event (MSE) [ ≈ Ge and 500 mm diameter, lined with VM2000 reflecting foil is surrounding the detector 68 radiation can be rejected in the analysis. For phase II of the GERDA experiment ≈ γ Ensuring very low background surrounding, shielded transports together with the exploitation The second phase of GERDA will exploit this background recognition technique. Two al- The 37.5 kg of germanium enriched in When energy is deposited in LAr it scintillates at 128 nm. If LAr scintillation light can be To bring the BI to a level of 10 Complementarily background can be recognized for events depositing energy inside the detec- germanium was stored in underground sitesto while minimize no processing exposure was of going the on.techniques) material The expected allows rigorous due to effort to keep thethis BI time (without dominated veto by from the the decay of discrimination of the described background recognitionBI techniques in the will energy allow region to of further interest. significantly The goal reduce is the to reach a BI of tinguished by pulse shape analysisevents [ due to tectors in conjunction with theSSEs surface from properties MSEs allow and for surface very events efficient [ distinction of signal like riched germanium against cosmic rays during the allow to push the 90%years within C.L. three half years life of measurement exclusion as limit shown in sensitivity Fig. of the experiment to above 10 array. The PMTs are mounted at a distance of detectors with special field configuration will be used [ top and the bottom of the shroud. A test setup is shown in Fig. ternative designs are being prepared.a In shroud the with low background photo multiplier tube (PMT) design a containment created by aarray. curtain The of light transported byfibers. the fibers It is has been detected shown using in silicon100 simulations photo can that multipliers be with reached coupled both for designs to background background the rejection induced efficiencies by of an external in a single site event (SSE) whilelikely for be Compton scattered separated background by events the energy deposits will formed into 29 working BEGe diodes with a total mass of detected coincident with an energy deposit in aevent. HPGe This detector, the allows event for is very very likelyshown efficient a that background rejection this of technique external works background reliably [ events. Measurements have active veto technologies will be used for the second phase of GERDA. tor only: The energy of the two electrons emitted in The GERDA experiment 4. Preparations for GERDA phase II designs simultaneously is also considered. great care has been takenisotopes to such minimize as the activation of the enriched germanium with cosmogenic Transport between Europe and the USAlowest was possible done container by slot ship, as shown taking in care Fig. to store the container at the PoS(ICHEP2012)401 (2000)638 (2011)225 455 654 yr. Preparations for phase 21 10 · 0.10) ± B. Majorovits for the GERDA collaboration (2004)371 yr of data has been taken until June 2012. · (2012)010001 522 (20 01)147 86 12 6 decay is (1.88 (2003)634 (1995)543 515 45 (1993)107, B. Majorovits and H. Klapdor-Kleingrothaus, Eur. νββ (2012)032505 (2007)479 332 yr). A total of 6.1 kg · 109 570 kg (2007)19 · 52 (2011)P04005 (2011)P03005 6 6 (2008)P08007, J. Janicskó-Csáthy et al, Nucl. Instr. Meth. A 3 cts/(keV Ge with a total of 20.8 kg are planned for 2013. 76 004 006 . . 0 0 − + (1999)463 , J. Hellmig and H. Klapdor-Kleingrothaus, Nucl. Instr. Meth. A 6 Left: LAr instrumentation test setup with PMTs. Middle: Shielding container being loaded into Phys. J. A Phase I of GERDA started data taking in late 2011. The experiment is running stably since [5] A. Gando et al., arXiv:1211.3863 [6] M. Auger et al., Phys. Rev. Lett. [8] GERDA collaboration, M. Agostini et al.,[9] submitted toGERDA Eur. collaboration, Phys. K.H. J. Ackermann A, et arXiv:1212.3210 al, submitted to Eur. Phys. J. C, arXiv:1212.4067 [3] D. González et al., Nucl.[4] Instr. Meth. A H. Klapdor-Kleingrothaus et al., Nucl. Instr. Meth. A [7] G. Heusser, Annu. Rev. Nucl. Part. Sci. [2] H.V. Klapdor-Kleingrothaus et al., Eur. Phys. J. A [1] Particle Data Group, J. Beringer et al. Phys. Rev. D [10] P. Peiffer et al, JINST [11] I. Abt et al., Eur. Phys. J. C [12] F. Petry et al, Nucl. Instr. Meth. A The measured preliminary half life of 2 with a BI of 0.020 the bottom of a transatlanticthe ship. GERDA The experiment shielding in terms container of is 90%C.L. shown half in life the limit. inlet. Right:5. Sensitivity Conclusions reach of and outlook II of the experiment aredetectors ongoing enriched and in the upgrade with LAr instrumentation, additional 30References BEGe [13] I. Abt et al, Nucl.[14] Instr. Meth. A M. Agostini et al, JINST [15] M. Agostini et al, JINST Figure 4: The GERDA experiment