experimental searches for neutrinoless double-beta decays...
TRANSCRIPT
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Experimental Searches for Neutrinoless Double-Beta Decays in 76-Ge
Alan PoonInstitute for Nuclear and Particle AstrophysicsNuclear Science Division
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Alan Poon (LBNL), Erice 2017
Outline
• Introduction - 0νββ decay (see Agostini’s talk for a
comprehensive review)
• The MAJORANA DEMONSTRATOR (MJD)
• Next-generation 76Ge-based tonne-scale
experiment: LEGEND
• Summary
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Alan Poon (LBNL), Erice 2017
Zero-neutrino double beta decay (0νββ)
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Experimental goals for 0νββ search:
To establish/refute:• Neutrinos are Majorana fermion: • Lepton number violation (LNV): ΔL = 2
regardless of the dominant 0νββ mechanism.
⌫ = ⌫̄
“mass mechanism”
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Alan Poon (LBNL), Erice 2017
2νββ and 0νββ
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T1/2 ~ 1020 y T1/2 > 1026 y
(T 0⌫1/2)
�1 = G0⌫(Q�� , Z) |M0⌫ |2 hm��i2
form factor
nuclear matrix
element
effective Majorana
masshm��i =
�����
3X
i=1
U2eimi
�����
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Alan Poon (LBNL), Erice 2017
Considerations
• Preferably: • high isotopic abundance (a) • high efficiency (ε) • large mass (M) • long counting time (t) • low background (b) • good energy resolution (δΕ)
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There is not an obvious choice
of isotope or detector
technology
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Alan Poon (LBNL), Erice 2017
Many experimental ideas…
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Päs & Rodejohann, New J. Phys. 17 115010 (2015)
Zatschler Sisti
Agostini
AP
Hughes, Piepke
Zuber
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Alan Poon (LBNL), Erice 2017
76Ge experiments
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MAJORANA
Conventional design:
Vacuum cryostats in apassive graded shieldwith ultra-clean materials
GERDA
Novel design:
Direct immersion in active LAr shield
Agostini
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MAJORANA DEMONSTRATOR (MJD)
• Located underground at 4850’ Sanford Underground Research Facility
• 44-kg of Ge detectors in two independent cryostats – 29.7 kg of 88% enriched 76Ge crystals – 14.4 kg of natGe crystals
• Highest energy resolution among all 0νββ detector technology
• ~0.1% FWHM at Q(76Ge)=2039 keV
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.
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Alan Poon (LBNL), Erice 2017
P-type Point Contact (PPC) Detector
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- drift paths - Isochrones (Δt = 100 ns)
Hole vdrift (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).
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Alan Poon (LBNL), Erice 2017
MJD Detectors
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Detector Unit (DU)
Detector String3 - 5 DU / string
Detector Module7 strings / module x
2 modules
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Alan Poon (LBNL), Erice 2017
MJD ImplementationThree Steps
Prototype cryostat: 7.0 kg (10) natGe
Module 1: 16.9 kg (20) enrGe 5.6 kg (9) natGe
Module 2: 12.9 kg (14) enrGe 8.8 kg (15) natGe
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In-shield running
06/2014 - 06/2015
05/2015 - 10/2015Final Installations,12/2015 - on going
07/2016 - on going
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Alan Poon (LBNL), Erice 2017
MJD Data Sets
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Alan Poon (LBNL), Erice 2017
Exposure: 1.39 kg yDS3+4
Data Set 3+4 (M1+M2)
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• Only 1 event survived in 400 keV window. Background rate is 5.1+8.9-3.2 counts/(ROI t y) for a 3.1-keV ROI, (68% CL).
• Background index is (1.8+3.1-1.1)x10-3 counts/(keV kg y).
• We have 10x more exposure in hand. Analysis is in progress.
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Large Enriched Germanium Experiment for Neutrinoless ββ Decay
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Alan Poon (LBNL), Erice 2017
LEGENDMission: The collaboration aims to develop a phased, 76Ge-based double-betadecay experimental program with discovery potential at a half-life significantly longer than 1027 years, using existing resources as appropriate to expedite physics results.
Select best technologies, based on what has been learned from GERDA and theMAJORANA DEMONSTRATOR, as well as contributions from other groups andexperiments.
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First phase:
• (up to) 200 kg• modification of
existing GERDA infrastructure at LNGS
• BI goal (x5 lower)0.6 c /(FWHM t y)
• start by 2021
Subsequent stages:
• 1000 kg (staged)• timeline connected
to U.S. DOE downselect process
• BI goal (x30 lower)0.1 c /(FWHM t y)
• 2-3 kg per detector• Location: TBD.
Required depthunder investigation
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Alan Poon (LBNL), Erice 2017
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
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Alan Poon (LBNL), Erice 2017
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
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Alan Poon (LBNL), Erice 2017
LEGEND-1000 “baseline” design
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Alan Poon (LBNL), Erice 2017
LEGEND-1000
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Alan Poon (LBNL), Erice 2017
LEGEND-1000 optimization activities
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Alan Poon (LBNL), Erice 2017
LEGEND: 3σ discovery
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Phas
e II
: L-1
000
Phas
e I :
L-2
00
Detwiler 2017
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Alan Poon (LBNL), Erice 2017
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, 76Ge-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 76Ge-based neutrinoless double-beta decay experiment with a sensitivity of T1/2 that is significantly longer than 1027 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).
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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/cm2 s (arXiv:1602.07742)
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Module and shield
cryostatvacuum system preamplifier and
power distribution
cryogenic system
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Pulse-Shape DiscriminationP
4257
5A
P42
661C
P42
538A
B84
74
P42
664A
P42
665A
B84
80
P42
698A
P42
538B
P42
573A
P42
661A
P42
574C
P42
574A
P42
662A
B84
82
P42
574B
P42
662B
P42
537A
Acc
epta
nce
(%)
0
10
20
30
40
50
60
70
80
90
100
Tl DEP208 Tl DEP Mean208
Tl SEP208 Tl SEP Mean208
0vBB Cont. 0vBB Cont. Mean
208Tl DEP (single-site events) fixed to 90%
208Tl SEP (multiple-site events) reduced to 6%
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α background and delayed-charge recovery
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Pulse-Shape DiscriminationP
4257
5A
P42
661C
P42
538A
B84
74
P42
664A
P42
665A
B84
80
P42
698A
P42
538B
P42
573A
P42
661A
P42
574C
P42
574A
P42
662A
B84
82
P42
574B
P42
662B
P42
537A
Acc
epta
nce
(%)
0
10
20
30
40
50
60
70
80
90
100
Tl DEP208 Tl DEP Mean208
Tl SEP208 Tl SEP Mean208
0vBB Cont. 0vBB Cont. Mean
208Tl DEP (single-site events) fixed to 90%
208Tl SEP (multiple-site events) reduced to 6%
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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).
Exposure: 1.66 kg yDS1
Simulated spectrum uses half-life from Eur. Phys. J. C 75 (2015) 41629
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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).
• Only 5 events survived in 400 keV window. Background rate is 23+13-10 counts/(ROI t y) for a 3.1-keV ROI, (68% CL).
• Background index is (7.5+4.5-3.4)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
Energy (keV)0 500 1000 1500 2000 2500 3000
Cou
nts/
40 k
eV/k
g/da
y
3−10
2−10
1−10
No DCR cut
90% DCR cut
Exposure: 1.66 kg yDS1
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Cosmogenic backgrounds at low energy
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EDELWEISS-II JCAP11(2013)067
CDEX PR D93 092003
Low background in low-energy regime + low energy threshold: - extended low-energy physics program to search for physics beyond the Standard Model.
MJD 478 kg-d dataPhys. Rev. Lett. 118, 161801
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Pseudo-scalar axion-electron coupling
32Phys. Rev. Lett. 118, 161801 (2017)
90%
UL