GERDA & the future of 76Ge-based experiments · GERDA & the future of 76Ge-based experiments Matteo...

GERDA & the future of

76Ge-based experiments

Matteo Agostini on behalf of the GERDA Collaboration

Technische Universitat Munchen (TUM), GermanyGran Sasso Science Institute (INFN), L’Aquila, Italy

25th International Workshop on Weak Interactions and Neutrinos (WIN2015)June 8–13, 2015, MPIK Heidelberg, Germany

Double-� decays

2-neutrino double-� decay (2⌫��):

• (A,Z) ! (A,Z + 2) + 2e� + 2⌫e

• allowed in the Standard Model

• measured in several isotopes

• T

2⌫1/2 in the range 1019 � 1024 yr

Neutrinoless double-� decay (0⌫��):

• (A,Z) ! (A,Z + 2) + 2e�

• lepton number violation (�L = 2)

• ⌫ has non-null Majorana mass component

• T

0⌫1/2 limits in the range 1021 � 1026 yr (1025 yr for 76Ge)

• claim for a signal (subgroup of HdM experiment)

Matteo Agostini (TU Munich & GSSI) 1

Double-� decays

2-neutrino double-� decay (2⌫��):

• (A,Z) ! (A,Z + 2) + 2e� + 2⌫e

• allowed in the Standard Model

• measured in several isotopes

• T

2⌫1/2 in the range 1019 � 1024 yr

Neutrinoless double-� decay (0⌫��):

• (A,Z) ! (A,Z + 2) + 2e�

• lepton number violation (�L = 2)

• ⌫ has non-null Majorana mass component

• T

0⌫1/2 limits in the range 1021 � 1026 yr (1025 yr for 76Ge)

• claim for a signal (subgroup of HdM experiment)


Neutrinoless double-� decay & neutrino physics

Assuming light-Majorana neutrino exchange as dominant 0⌫�� channel:

• (T 0⌫1/2)

�1 = G0⌫(Q�� ,Z)|M0⌫(A,Z)|2|m�� |2

• e↵ective Majorana mass:|m�� | ⌘

��Pi

U

2ei

m

i

�� =��c

212 c

213 m1 + s

212 c

213 m2 e

i2↵ + s

213 m3 e

i2��

10 4 0.001 0.01 0.11024

1026

1028

1030

1032

mlightest eV

T 120Ν

yrIH

NH

QD

76Ge

0vbb limits

[arXiv:1305.0056]


State of the art of 0⌫�� search with

76

Ge

GERDA + HdM + IGEX

PRL 111, 122503 (2013)

GERDA Collaboration

[Phys.Lett. B586, 198 (2004)]

Klapdor-Kleingrothaus et al.

[Phys.Rev.D65,092007 (2002)]

IGEX Collaboration

[Eur. Phys. J. A 12, 147 (2001)]

HdM Collaboration

0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 3.3 3.6 3.9

T1/20ν (Ge76) [1025 yr]

T1/2 > 1.9 1025 yr (90% CL)

T1/2 > 1.6 1025 yr (90% CL)

T1/2 = 1.19 -0.23+0.37 1025 yr

T1/2 > 2.1 1025 yr (90% CL)

T1/2 > 3.0 1025 yr (90% CL)


GERDA collaboration


Sensitivity and background goals

Phase I (Nov 2011 - May 2013):

• 15� 20 kg of target mass (87% 76Ge)

• bkg ⇠ 10�2 cts/(keV· kg· yr) at Q��

• exposure 21.6 kg·yr• sensitivity to scrutinize KK claim

Phase II (migration ongoing):

• new custom-produced BEGe detectors(additional 17 kg, 87% 76Ge)

• bkg .10�3 cts/(keV· kg· yr) at Q��

(active techniques for bkg suppression)

• exposure & 100 kg·yr• start exploring T

0⌫1/2 in the 1026 yr range

[Phys.Rev.D75, 092003 (2006)]


Detectors

• HPGe detectors from materialenriched in 76Ge (⇠87%)

• detectors well established technology

• optimal spectroscopy performance:� long-term stability� �E ⇡ 0.1% at Q��

� radio purity

p+ electrode(read-out)

0 V

n+ electrode3-4 kV

p-typeGe

60-80 mm

70-1

10 m

m

65-80 mm

25-50 mm

p-typeGe

E

he

he

Coaxial-type BEGe-type

��

ee

2�⇥⇥

2�⇥⇥: 76Ge -> 76Se + 2e + 2�0�⇥⇥: 76Ge -> 76Se + 2e

ee

0�⇥⇥

Energy'(keV)'

2νββ

0νββ

Ge-76: Qββ=2039 keV

arbitrary

units

Calorimeter detectors:• source=detector• high detection e�ciency• peak at Q-value (Q��)


Shielding strategy and apparatus

• bare Ge detectors in liquid Argon (LAr)• shield: high-purity LAr/H2O

• radio-pure material selection• deep underground (LNGS, 3800 m.w.e.)

[EPJ C 73 (2013) 2330]


Backgrounds and mitigation techniques

Background sources:

• natural radioactivity (232Th and 238U chains):

� �-rays (e.g. 208Tl, 214Bi)

� ↵-emitting isotopes from surface contamination

(e.g. 210Po) or 222Rn in LAr

• long-lived cosmogenic Ar isotopes (39Ar,42Ar)

• cosmogenic isotopes activated in Ge (68Ge, 60Co)

Mitigation strategy:

• detector anti-coincidence

• time-coincidence (Bi-Po or 68Ge)

• pulse shape analysis

• detection of LAr-scintillation light


Backgrounds and mitigation techniques

Background sources:

• natural radioactivity (232Th and 238U chains):

� �-rays (e.g. 208Tl, 214Bi)

� ↵-emitting isotopes from surface contamination

(e.g. 210Po) or 222Rn in LAr

• long-lived cosmogenic Ar isotopes (39Ar,42Ar)

• cosmogenic isotopes activated in Ge (68Ge, 60Co)

Mitigation strategy:

• detector anti-coincidence

• time-coincidence (Bi-Po or 68Ge)

• pulse shape analysis

• detection of LAr-scintillation light


Phase I detector array configuration

• 3 + 1 strings• 8 enrGe coaxial detectors (2 not considered in the analysis)• 5 enrGe BEGe detectors (1 not considered in the analysis)• 1 natGe coaxial detectors

enrGe mass for physics analysis: 14.6 kg (coaxial) + 3.0 kg (BEGe)


Data taking of Phase I


Data taking of Phase I


Background modeling

even

ts/(3

0 ke

V)

-210

-110

1

10

210

310 datamodel

��⇥2K42K40Ac228Th228

AlphasCo60HCo60inGeBi214HBi214P

GE

RD

A 1

3-06

energy (keV)2000 2500 3000 3500

data

/mod

el ra

tio

012345

68%95%99.9%

data/model

Contribution at Q�� :

• �-rays (close sources):Bi-214, Tl-208, K-42

• ↵- and �-rays (surface decays):Ra-226 daughter, Po-210, K-42

more details in [EPJ C74 (2014) 2764]

• no line expected in the blinded window

• background flat between 1930-2190 keV(excluding peaks at 2104 and 2119 keV)

• mean FWHM at Q�� (mass/exposure weighted):

coax —> 4.8±0.2 keV

BEGe —> 3.2±0.2 keV

expectedco

unts

/keV

MC simulation


Background modeling

even

ts/(3

0 ke

V)

-210

-110

1

10

210

310 datamodel

��⇥2K42K40Ac228Th228

AlphasCo60HCo60inGeBi214HBi214P

GE

RD

A 1

3-06

energy (keV)2000 2500 3000 3500

data

/mod

el ra

tio

012345

68%95%99.9%

data/model

Contribution at Q�� :

• �-rays (close sources):Bi-214, Tl-208, K-42

• ↵- and �-rays (surface decays):Ra-226 daughter, Po-210, K-42

more details in [EPJ C74 (2014) 2764]

• no line expected in the blinded window

• background flat between 1930-2190 keV(excluding peaks at 2104 and 2119 keV)

• mean FWHM at Q�� (mass/exposure weighted):

coax —> 4.8±0.2 keV

BEGe —> 3.2±0.2 keV

expectedco

unts

/keV

MC simulation


Pulse shape discrimination

Coaxial detectors:

• artificial neural network

• 0⌫�� acceptance = 90+5�9%

• background acc at Q��= ⇠45%

BEGe detectors:• A/E parameter (mono-parametric PSD)

• 0⌫�� acceptance 92±2%

• background acc at Q��20%

[Eur.Phys.J C73 (2013) 2583]


Unblinding: spectrum around Q��

[PRL 111, 122503 (2013)] ⇤ w/o PSD Analysis cuts applied:

1) signals quality cuts

2) detector anti-coincidence

3) muon-vetoanti-coincidence

4) single-detectors timecoincidence (BiPo cut)

5) PSD

Survival fraction at Q�� :

1 ⇠99%2+3 ⇠60%4 ⇠100%

5 ⇠50%

w/o PSD

w/ PSD

exposure background expected cts observed ctsdata set [kg·yr] 10�2 cts/(keV· kg· yr) (Q��±5 keV) (Q��±5 keV)

golden 17.3 1.8 3.3 5BEGe 2.4 4.2 1.0 1


Unblinding: spectrum around Q��

[PRL 111, 122503 (2013)] ⇤ w/o PSD⌅ w/ PSD⇤

Analysis cuts applied:

1) signals quality cuts

2) detector anti-coincidence

3) muon-vetoanti-coincidence

4) single-detectors timecoincidence (BiPo cut)

5) PSD

Survival fraction at Q�� :

1 ⇠99%2+3 ⇠60%4 ⇠100%5 ⇠50%

w/o PSD

w/ PSD

exposure background expected cts observed ctsdata set [kg·yr] 10�2 cts/(keV· kg· yr) (Q��±5 keV) (Q��±5 keV)

golden 17.3 1.8 1.1 3.3 2.0 5 2BEGe 2.4 4.2 0.5 1.0 0.1 1 0


Statistical analysis

Profile likelihood analysis:

• ML fit(constant+Gauss in 1930-2190 keV range)

• multiple data sets (common T

0⌫1/2)

• T

0⌫1/2 � 0 (coverage tested)

Results (GERDA only):

• best fit for N0⌫�� = 0 signal cts

• T

0⌫1/2 > 2.1 · 1025 yr (90% C.L.)

• MC Median sensitivity (for no signal):T

0⌫1/2 > 2.4 · 1025 yr (90% C.L.)

Results (GERDA + IGEX [1] + HdM [2]):


• T

0⌫1/2 > 3.0 · 1025 yr (90% C.L.)

PRL 111, 122503 (2013); [1] Phys.Rev. D65, 092007 (2002); [2] Eur.Phys.J. A12, 147 (2001)


Statistical analysis

Profile likelihood analysis:

• ML fit(constant+Gauss in 1930-2190 keV range)

• multiple data sets (common T

0⌫1/2)

• T

0⌫1/2 � 0 (coverage tested)

Results (GERDA only):


• T

0⌫1/2 > 2.1 · 1025 yr (90% C.L.)

• MC Median sensitivity (for no signal):T

0⌫1/2 > 2.4 · 1025 yr (90% C.L.)

Results (GERDA + IGEX [1] + HdM [2]):


• T

0⌫1/2 > 3.0 · 1025 yr (90% C.L.)

PRL 111, 122503 (2013); [1] Phys.Rev. D65, 092007 (2002); [2] Eur.Phys.J. A12, 147 (2001)


Double-� decay with 2⌫ or Majorons emission

• Global fit of the energy spectrum

• Most accurate measurement of:T

2⌫1/2(

76Ge) = 1.926(95)⇥ 1021 yr

(68% probability)

• Most stringent limits on exoticprocesses:

T

0⌫�1/2 > 1023 yr for n=1,3,5,7

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

250 500 750 1000 1250 1500 1750 2000

energy [keV]

arb

itra

ry u

nit

s

0νββ2νββ

n=1n=3

n=5

n=7

[Acta Phys. Polon. B 37 (2006) 1905]

data

)ββνmodel (background + 2

68% interval

backgroundββν2

(n=1) (90% C.I.)χββν0 (n=2) (90% C.I.)χββν0 (n=3) (90% C.I.)χββν0 (n=7) (90% C.I.)χββν0

energy (keV)1000 1500 2000

even

ts/(3

0 ke

V)

10

210

310

yr)⋅golden data set (17.9 kg

GER

DA

14-

12

energy (keV)1000 1500 2000

even

ts/(3

0 ke

V)

1

10

210

yr)⋅BEGe data set (2.4 kg

GER

DA

14-

12

[arxiv:1501.02345]


Phase II upgrade

I Installation of additional 17 kg of BEGe detectors:

� increased array granularity (anti-coincidence cut)

� enhanced pulse shape discrimination performance

� excellent energy resolution

I PMT and fibers+SiPMto detect LAr scintillation light

I lower-mass holders


Broad Energy Germanium (BEGe) detectors

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000 2500

FW

HM

[ke

V]

energy [keV]

f(x) = (a2 + b2x)1/2

a = 0.386 (8)

b = 0.0433 (2)

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.65 1.7 1.75 1.8 1.85

mass

[kg

]

FWHM at 1332 keV [keV]

enricheddepleted

natural


Signal formation and development










Pulse shape discrimination technique

A/E method:

E: integral of the current signal (energy)A: maximum of the current signal

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600 700 800 900

curr

ent [a

.u.]

time [ns]

A

A

Multiple Interaction SiteSingle Interaction Site

A/E 0.96 0.98 1.00 1.02 1.04

no

rma

lize

d c

ou

nts

0

10

20

30

40

50

60ββν2

DEPGD32Bin Phase I

GERDA 13-06

A/E

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

cts

(a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0K n+ surface events, simulation42

Co, simulation60

Compton 1.8 - 2.2 MeV, calibration

FEP @ 1.6 MeV, calibration

GE

RD

A 1

3-0

6

[Budjas et al. JINST 4 P10007,M.A et al. JINST 6 P03005, Eur.Phys.J C73 (2013) 2583]


Detection of LAr scintillation light

Design:

• low-background photo-multipliers (9top, 7 bottom)

• wave-length-shifting fibers read-out bySiPMs

• wave-length-shifting nylon mini-shroud


Last commisioning results (Th-228 irradiation)

About two orders of magnitude suppression at Q�� !


Last commisioning results (Ra-226 irradiation)

Almost two orders of magnitude suppression at Q�� !


GERDA sensitivity projection for limit setting

• profile likelihood analysis

• MC-realizations of the data sets

• limit extraction performed for eachrealization

• global analyis:GERDA Phase IGERDA Phase II:(37 kg of 76Ge, 1e-3 cts/(keV· kg· yr))

• median sensitivity after 2 yr of datataking:

T

0⌫1/2& 1026 yr

|mee

| . 100meV

90

% C

.L.

limit

on

|m

ee| [

eV

]

time [yr]

4.6 < NME < 5.8

10-1

100

2012 2013 +0 +2 +4 / /

/ /9

0%

C.L

. lim

it o

n

T1

/2

[yr]

median

68% prob.

90% prob.

95% prob.

99% prob.

1025

1026

/ /

/ /

exp

osu

re [

kg •

yr]

101

102

/ /

/ /


sensitivity projection for GERDA + Majorana

• profile likelihood analysis

• MC-realizations of the data sets

• limit extraction performed for eachrealization

• global analyisi of:GERDA Phase IGERDA Phase II:(37 kg of 76Ge, 1e-3 cts/(keV· kg· yr))Majorana demonstrator(30 kg of 76Ge, 8e-4 cts/(keV· kg· yr))

• median sensitivity after 4 yr of datataking:

T

0⌫1/2& 3� 4 · 1026 yr

|mee

| . 60� 80meV

90

% C

.L.

limit

on

|m

ee| [

eV

]

time [yr]

4.6 < NME < 5.8

10-1

100

2012 2013 +0 +2 +4 / /

/ /9

0%

C.L

. lim

it o

n

T1

/2

[yr]

global analysis

GERDA Phase I

GERDA Phase II

Majorana Dem

1025

1026

/ /

/ /

exp

osu

re [

kg •

yr]

101

102

/ /

/ /


Conclusions

GERDA Phase I (21.6 kg·yr of exposure):• background order of magnitude lower than previous Ge experiments:

⇠0.01 cts/(keV· kg· yr) at Q�� (after PSD)

• blind analysis —> no positive 0⌫�� signal:

T 0⌫1/2 > 2.1 · 1025 yr at 90% C.L. (GERDA only)

• Long standing claim excluded at 99% C.L. (model-independent result)

• NEW: most accurate measurement of T 2⌫1/2

• NEW: stongest limits on T 0⌫�1/2 and T 2⌫

1/2 decay to excited states

GERDA Phase II:

• commissioning ongoing

• quasi background-free experiment

• start exploration of T 0⌫1/2 > 10

26yr in a ⇠2 yr of data taking


Collaboration

⇠100 members, 16 institutions, 6 countries


backup slides


Electric field and charge collection

Contributions to theelectric field (E):

1) electrodes potentials:�p+ = 0V, �

n+ = 4 kV

2) impurity concentration:negative charges fordepleted p-type Ge

Total field (1+2):holes are pushed to thedetector central slice (2)and then collected to thep+ electrode (1)

E

E

E

Interplay between (1) and (2)results in the funnel e↵ect:

anodecathodeelectronsholesinteraction point

final part of hole tra-jectories independent ofinteraction positions

[JINST 6 (2011) P03005]


Background model – 2⌫�� half-life

energy (keV)600 800 1000 1200 1400 1600 1800

even

ts/(3

0 ke

V)

100

200

300

400

500 experimental energy spectrum model 68%

ββν 2K42 K40 Bi214

GER

DA

12-1

2

energy (keV)600 800 1000 1200 1400 1600 1800

even

ts/(3

0 ke

V)

1

10

210

energy (keV)600 800 1000 1200 1400 1600 1800

data

/mod

el ra

tio

0.51.01.52.02.5 data/model

68%95%99.9%

I Binned maximum likelihood (5 kg·yr)I Nuisance parameters:

• Active detector masses (6+1)

• Ge-76 fractions (6)

• Background contributions (3x6)

I T

2⌫1/2 common to all detectors

I After marginalizing:

T

2⌫1/2 = (1.84+0.09

�0.08 fit+0.11�0.06 syst) · 1021

[J.Phys.G 40 (2013) 035110]

publication year1990 1995 2000 2005 2010

yr)

21 (1

01/

2T

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

ITEP

-YPI

PNL-

USC

PNL-

USC

-ITEP

-YPI PN

L-U

SC-IT

EP-Y

PI

HdM

IGEX

HdM

HdM

-K

HdM

-B

this work

Barabash

NNDC

GER

DA

12-1

2


Background model – 2⌫�� half-life

energy (keV)600 800 1000 1200 1400 1600 1800

even

ts/(3

0 ke

V)

100

200

300

400

500 experimental energy spectrum model 68%

ββν 2K42 K40 Bi214

GER

DA

12-1

2

energy (keV)600 800 1000 1200 1400 1600 1800

even

ts/(3

0 ke

V)

1

10

210

energy (keV)600 800 1000 1200 1400 1600 1800

data

/mod

el ra

tio

0.51.01.52.02.5 data/model

68%95%99.9%

I Binned maximum likelihood (5 kg·yr)I Nuisance parameters:

• Active detector masses (6+1)

• Ge-76 fractions (6)

• Background contributions (3x6)

I T

2⌫1/2 common to all detectors

I After marginalizing:

T

2⌫1/2 = (1.84+0.09

�0.08 fit+0.11�0.06 syst) · 1021

[J.Phys.G 40 (2013) 035110]

energy [keV]

cou

nts

/20

keV

0

2000

4000

6000

8000

10000

12000

500 1000 1500 2000

[HdM, EPJA12 (2001) 147]


Background model – ↵-emitting isotopes

I fit window 3500-7500 keV

Ip-value of the fit: 0.7

I 80 bins of width 50 keV:

79% in the green band

98% in the yellow band


Comparison with Phys.Lett. B586 198 (2004)

Hypothesis test: H0 (bkg only) vs H1 (T 0⌫1/2 = 1.19+0.37

�0.23 · 1025 yr + bkg)

In Q�� ± 2�E

(after PSD):

• expected 2.0±0.3 bkg cts

• expected 5.9±1.4 signal cts(assuming H1)

• observed 3 cts

energy (keV)2025 2030 2035 2040 2045 2050 2055 2060

coun

ts/k

eV

0

1

2

3 GERDA 13-07

HT = 2.1x1025 yr(90% C.L. lower limit)

1/2 1

T = 1.19x1025 yr1/2

[PRL 111, 122503 (2013)]

GERDA only:I Frequentist p-value (N0⌫�� = 0|H1) = 0.01

I Bayes factor P(H1)/P(H0)=2.4 · 10�2

GERDA + IGEX + HdM: I Bayes factor P(H1)/P(H0)=2 · 10�4

Long standing

claim strongly

disfavoured!

T

0⌫1/2 from Mod. Phys. Lett. A 21 (2006) 1547 is not considered because of inconsistencies (i.e. missing e�ciency

factors, problem in the conversion from counts to T

0⌫1/2) pointed out in Ann. Phys. 525 (2013) 269.


Date post:	21-Oct-2019
Category:	Documents
Upload:	others
View:	10 times
Download:	0 times

GERDA & the future of 76Ge-based experiments · GERDA & the future of 76Ge-based experiments Matteo...

Documents