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”

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

�����

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

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

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

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).

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

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

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.

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

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

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

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

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

Pseudo-scalar axion-electron coupling

32Phys. Rev. Lett. 118, 161801 (2017)

90%

UL