Experimental Searches for Neutrinoless Double-Beta Decays in 76-Ge Alan Poon

Transcription

1 Experimental Searches for Neutrinoless Double-Beta Decays in 76-Ge Alan Poon Institute for Nuclear and Particle Astrophysics Nuclear Science Division 1

2 Outline Introduction - 0νββ decay (see Agostini s talk for a comprehensive review) The MAJORANA DEMONSTRATOR (MJD) Next-generation 76 Ge-based tonne-scale experiment: LEGEND Summary 2

3 Zero-neutrino double beta decay (0νββ) mass mechanism Experimental goals for 0νββ search: To establish/refute: Neutrinos are Majorana fermion: Lepton number violation (LNV): ΔL = 2 regardless of the dominant 0νββ mechanism. 3 =

4 (T 0 1/2 ) 1 = G 0 (Q,Z) M 0 2 hm i 2 form factor nuclear matrix element effective Majorana mass 2νββ and 0νββ hm i = 3X i=1 U 2 eim i T1/2 ~ y T1/2 > y 4

5 Considerations Preferably: high isotopic abundance (a) high efficiency (ε) large mass (M) long counting time (t) There is not an obvious choice of isotope or detector technology low background (b) good energy resolution (δε) 5

6 Many experimental ideas Päs & Rodejohann, New J. Phys (2015) Zatschler Sisti Hughes, Piepke Agostini AP Zuber 6

7 76 Ge experiments MAJORANA Conventional design: Vacuum cryostats in a passive graded shield with ultra-clean materials Agostini GERDA Novel design: Direct immersion in active LAr shield 7

8 MAJORANA DEMONSTRATOR (MJD) Goals: - Demonstrate backgrounds low enough to justify building a tonne scale experiment. - Establish feasibility to construct & field modular arrays of Ge detectors. - Searches for additional physics beyond the standard model. Located underground at 4850 Sanford Underground Research Facility 44-kg of Ge detectors in two independent cryostats 29.7 kg of 88% enriched 76 Ge crystals 14.4 kg of nat Ge crystals Highest energy resolution among all 0νββ detector technology ~0.1% FWHM at Q( 76 Ge)=2039 kev 8

9 P-type Point Contact (PPC) Detector - drift paths - Isochrones (Δt = 100 ns) Hole v drift (mm/ns) PPC detectors have superb ability to distinguish between single-site events (ββ signal) and multi-site events (e.g. Compton-scattered γ) background. Pulse-shape discriminator (PSD): amplitude of the current pulse (A) vs event energy (E). 9

10 MJD Detectors Detector Unit (DU) Detector String 3-5 DU / string 10 Detector Module 7 strings / module x 2 modules

11 MJD Implementation Three Steps Prototype cryostat: 7.0 kg (10) nat Ge In-shield running 06/ /2015 Module 1: 16.9 kg (20) enr Ge 5.6 kg (9) nat Ge Module 2: 12.9 kg (14) enr Ge 8.8 kg (15) nat Ge 05/ /2015 Final Installations, 12/ on going 07/ on going 11

12 MJD Data Sets 12

13 Data Set 3+4 (M1+M2) DS3+4 Exposure: 1.39 kg y Only 1 event survived in 400 kev window. Background rate is counts/(roi t y) for a 3.1-keV ROI, (68% CL). Background index is ( )x10-3 counts/(kev kg y). We have 10x more exposure in hand. Analysis 13 is in progress.

14 Large Enriched Germanium Experiment for Neutrinoless ββ Decay 14

15 LEGEND Mission: The collaboration aims to develop a phased, 76 Ge-based double-beta decay experimental program with discovery potential at a half-life significantly longer than years, using existing resources as appropriate to expedite physics results. Select best technologies, based on what has been learned from GERDA and the MAJORANA DEMONSTRATOR, as well as contributions from other groups and experiments. First phase: Subsequent stages: (up to) 200 kg modification of existing GERDA infrastructure at LNGS BI goal (x5 lower) 0.6 c /(FWHM t y) start by kg (staged) timeline connected to U.S. DOE down select process BI goal (x30 lower) 0.1 c /(FWHM t y) 2-3 kg per detector Location: TBD. Required depth under investigation

16 LEGEND - Best of both worlds MAJORANA Radiopurity of nearby parts (front-end electronics, cables, Cu mounts, etc.) Low noise electronics Low energy threshold (cosmogenic and low-e background) GERDA LAr active veto Low-A shield, no Pb Both Clean fabrication techniques Control of surface exposure Development of large point-contact detectors 16

17 LEGEND-200 MAJORANA Radiopurity of nearby parts (front-end electronics, cables, Cu mounts, etc.) Low noise electronics Low energy threshold (cosmogenic and low-e background) GERDA LAr active veto Low-A shield, no Pb Both Clean fabrication techniques Control of surface exposure Development of large point-contact detectors 17

18 LEGEND-1000 baseline design 18

19 LEGEND

20 LEGEND-1000 optimization activities 20

21 LEGEND: 3σ discovery Phase I : L-200 Phase II : L Detwiler 2017

22 Summary The MAJORANA DEMONSTRATOR is running. Statistics are still very low, and detailed analyses are proceeding to study the backgrounds and to search for new physics (such as axions). Among all detector technologies, 76 Ge-based experiments demonstrated to have the best energy resolution and the lowest backgrounds in the ROI. A new international collaboration LEGEND has been formed to pursue a phased, tonne-scale 76 Ge-based neutrinoless doublebeta decay experiment with a sensitivity of T1/2 that is significantly longer than years. Based on current backgrounds, LEGEND 1000 goal requires only a factor of x30 improvement from demonstrated backgrounds (x5 for LEGEND 200 and another x6 for LEGEND 1000). 22

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24 MAJORANA Underground Laboratory 4850 level, Sanford Underground Research Facility (SURF) in Lead, SD 4300 mwe Class-100 clean room conditions Muon flux: 5 x 10-9 µ/cm 2 s (arxiv: ) 24

25 Module and shield cryogenic system vacuum system preamplifier and power distribution cryostat 25

26 Pulse-Shape Discrimination Acceptance (%) Tl DEP 208 Tl SEP 0vBB Cont. 208 Tl DEP Mean 208 Tl SEP Mean 0vBB Cont. Mean 208 Tl DEP (single-site events) fixed to 90% P42575A P42661C P42538A B8474 P42664A P42665A B8480 P42698A P42538B P42573A P42661A P42574C P42574A P42662A B8482 P42574B P42662B P42537A 208 Tl SEP (multiple-site events) reduced to 6% 26

27 α background and delayed-charge recovery 27

28 Pulse-Shape Discrimination Acceptance (%) Tl DEP 208 Tl SEP 0vBB Cont. 208 Tl DEP Mean 208 Tl SEP Mean 0vBB Cont. Mean 208 Tl DEP (single-site events) fixed to 90% P42575A P42661C P42538A B8474 P42664A P42665A B8480 P42698A P42538B P42573A P42661A P42574C P42574A P42662A B8482 P42574B P42662B P42537A 208 Tl SEP (multiple-site events) reduced to 6% 28

29 Data Set 1 Applied cuts to remove instrumental background events, events with multiple-detector-hit events (granularity cut), multi-site events (PSD), and surface α events (DCR). DS1 Exposure: 1.66 kg y Simulated spectrum uses half-life from Eur. Phys. J. C 75 (2015)

30 Data Set 1 (M1 only, with inner Cu shield) Applied cuts to remove instrumental background events, events with multiple-detector-hit events (granularity cut), multi-site events (PSD), and surface α events (DCR). No DCR cut 1 90% DCR cut DS1 Exposure: 1.66 kg y 2 Only 5 events survived in 400 kev window. Background rate is counts/(roi t y) for a 3.1-keV ROI, (68% CL) Energy (kev) Background index is ( )x10-3 counts/(kev kg y). Better than all other currently-running detector technologies (but higher than GERDA-II). Low statistics results. All analysis cuts are still being optimized. 30

31 Cosmogenic backgrounds at low energy MJD 478 kg-d data Phys. Rev. Lett. 118, CDEX PR D EDELWEISS-II JCAP11(2013)067 Low background in low-energy regime + low energy threshold: - extended low-energy physics program to search for physics beyond the Standard Model. 31

32 Pseudo-scalar axion-electron coupling 90% UL Phys. Rev. Lett. 118, (2017) 32