Background Free Search for 0 Decay of 76Ge with GERDA · Victoria Wagner (MPIK) GERDA Phase II...
Transcript of Background Free Search for 0 Decay of 76Ge with GERDA · Victoria Wagner (MPIK) GERDA Phase II...
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Victoria Wagnerfor the GERDA collaboration
Max-Planck-Institut für Kernphysik
Rencontres de Moriond, Electro Weak La Thuile, March 24 2017
Background Free Search for 0 Decay of 76Ge with GERDA
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GERDA Phase II Victoria Wagner (MPIK) Moriond, 24.03.2017 2
The GERDA Collaboration: searching for neutrinoless double beta decay of 76Ge
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GERDA Phase II Victoria Wagner (MPIK) Moriond, 24.03.2017 3
Signature & Experimental Challenges
● Measure sum energy of electrons
Need to achieve● < 1 bck event in ROI● excellent energy resolution
T 1/20ν ∝M⋅t
T 1/20ν ∝√ M⋅tΔ E⋅BI
● zero background regime
● background, i.e. statistical fluctuationlimited scenario
M·t: exposure [kg yr], ΔE: energy resolution, BI: background index [counts/(keV kg yr)]
Observable:
(T 1/20ν )−1∝N0 ν
T1/22ν = 1.9⋅1021 yr
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High Purity Germanium (HPGe) Detectors
● 3-4 keV FWHM at Q = 2039 keV (0.2%)
● HPGe detectors isotopically enriched in 76Ge (~87%)● high detection efficiency of ββ: source = detector● “no” intrinsic background [Astropart.Phys. 91 (2017) 15-21] ● discrimination of signal- from background like events using pulse shape analysis
Germanium Detectors
● Measure sum energy of electrons
2νββ- - - - - -
- - - - - -
+ + +
h+e-
0νββ
- - - - - -
+ + +
h+ e-e- e-
e- e-
νν-
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T1/22ν = 1.9⋅1021 yr
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GERDA @ LNGS
cosmic muon fluxreduced by a factor ~ 106 → 1 μ/m2/h
1.4
km o
f ro
ck35
00 m
w.e
.
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The Germanium Detector Array
590m3 pure watermuon veto & neutron absorber
HPGedetectors
concept: operate bare HPGe detectors in LAr which serves as coolant & (active) shielding
Eur. Phys. J. C (2013)73:2330
GERDA Phase I (Nov 2011- May 2013)● 17.8 kg enriched semi-coaxial +
3.6 kg enriched BEGe● exposure 21.6 kg·y● BI ~ 10-2 cts/(keV·kg·yr)● T1/2 > 2.1 · 10
25 yr (90% C.L.)PRL 111, 122503 (2013)
GERDA Phase II (Dec 2015 - )● 30 enriched BEGe (= 20.0 kg)
+ 7 enriched semi-coaxial (= 15.6 kg) ● LAr instrumentation● goal: BI ~ 10-3 cts/(keV·kg·yr)
0ν
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GERDA Phase II Array
low radioactivity electronics
new low mass holderswith reduced massand Cu → Si
~8cm
~8cm
wire bonding for contacting
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Discriminating Signal from Background Events
background event● energy deposition in multiple
locations (MSE) in single detector
or on detector surface ()
→ pulse shape discrimination ● coincident energy deposition in
more than one detector
→ detector anti-coincindence● additional energy deposition in LAr
→ LAr veto
ββ event● local energy deposition (SSE)
in single detector 2νββ
ν ν
0νββγ
γ
γγ
γ
/β α
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LAr Instrumentation – Hybrid Design
16 photomultiplier tubes (PMTs) 810 wavelength shifting fibers coupled to SiPMs
Cu cylinder with wavelength shiftingreflector foil
49cm
60cm
100c
m60
cm
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Start of GERDA Phase II
Full Integration of Phase II Array
finished in December 2015 ● all Ge and LAr detector channels
working
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Start of GERDA Phase II
First data release in June 2016
● 35 out of 37 detectors used for analysis
● blinded region: Q ± 25 keV
● quality cuts (phys. acc. > 99.9%)
● events in coincidence with muon veto ( phys. acc.~ 99.9 %)
Full Integration of Phase II Array
finished in December 2015 ● all Ge and LAr detector channels
working
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Energy Scale and Resolution
FWHM @ Q: ● BEGe's:
3.0(2) keV
● Coax:4.0(2) keV
energy [keV]500 1000 1500 2000 2500
FWH
M [k
eV]
1
1.5
2
2.5
3
3.5
enriched Coaxenriched BEGe
calibration data
physics data GE
RD
A 1
607
energy [keV]2610 2620
coun
ts /
(0.1
keV
)
210
310
410
energy [keV]1460 1480 1500 1520 1540
coun
ts /
(1.0
keV
)
0
50
100
150
Dec '15 – June '16
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Performance of the LAr Veto
energy [keV]500 1000 1500 2000 2500 3000 3500 4000 4500 5000
coun
ts /
25 k
eV
1
10
210
310
410prior liquid argon (LAr) vetoafter LAr veto
yr)2110= 1.92 1/2
(TMonte Carlo 2
yrenriched Coax 5.0 kg
GE
RD
A 1
607
energy [keV]500 1000 1500 2000 2500 3000 3500 4000 4500 5000
coun
ts /
25 k
eV
1
10
210
310
410 yrenriched BEGe 5.8 kg
Po2102
Ar39
K
● 2:bck = 96:4 (1.0-1.3 MeV)
2νββ MC with T1/2 = 1.9 · 1021 yr from Phase I EPJC 75 (2015) 416
2νββ
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Pulse Shape Analysis: A/E for BEGe
α-region
γ-lines
A E
A/E ≡ 1
(A/E
– 1
)/σ
A/E
Signal region
detector current time profile used to discriminate signal from background events
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A/E Analysis for BEGe's
2νββ acceptance85+2 %
A E
A/E ≡ 1
(A/E
– 1
)/σ
A/E
80% of background events rejected at Q and keep high signal efficiency = 87(2) %
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Background Suppression LAr + PSD
BEGe, 5.8 kg · yr
BI AC ,MV , LAr=4.5−1.6+2.2⋅10−3 counts
keV⋅kg⋅yr
BI AC ,MV=15.7−3.1+3.8⋅10−3 counts
keV⋅kg⋅yr
BI AC ,MV , LAr, PSD=0.7−0.5+1.1⋅10−3 counts
keV⋅kg⋅yr
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Spectrum at Q
Extended unbinned profile likehood: ● flat background in 1930-2190 keV
● signal = Gaussian with mean at Q and standard deviation E
● 7 parameters: 6 BI + common T1/2energy [keV]1950 2000 2050 2100 2150
yr )
kg
coun
ts /
( keV
310
210
110
1Phase I yr23.6 kg
prior cuts after all cuts after LAr veto (Phase II) limit (90% C.L.)
Q
energy [keV]1950 2000 2050 2100 2150
yr )
kg
coun
ts /
( keV
310
210
110
1Phase II enriched Coax yr5.0 kg
energy [keV]1950 2000 2050 2100 2150
yr )
kg
coun
ts /
( keV
310
210
110
1Phase II enriched BEGe yr5.8 kg
● best fit for N = 0
● lower limit T1/2 > 5.3 · 1025 yr ┼
with T1/2 sensitivity 4.0 · 1025 yr
(90 % C.L.)
┼Frequentist approach after Cowan et al., EPJC 71 (2011) 1554
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Current Status of GERDAPreliminary● since summer 2016 additional (blinded) data
● after LAr veto and PSD: BI=0.6−0.4+0.6⋅10−3 countskeV⋅kg⋅yr
BEGe
BI=2.2−0.8+1.1⋅10−3 countskeV⋅kg⋅yr
Coax
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Conclusion
● GERDA sets new limit on the half-life of 0 decay of 76Ge
● best energy resolution: FWHM = 3.0 keV (4.0 keV) BEGe (Coax) at Q
● flat background in ROI
● lowest background at Q: 10-3 counts/ (keV·kg·yr)
will stay background-free
→ important ingredients for discovery
mββ5.3⋅1025 yr@90 C . L.
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Beyond GERDA
● LEGEND (Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay)
● new collaboration formed in Oct 2016 (=GERDA+Majorana+new groups)
● Goals:
- 1 t enriched Ge
- first phase: 200 kg in existing infrastructure @ LNGS
- reduce background with respect to GERDA → remain background-free
→ best discovery potential
Eur.Phys.J.C76 (2016) 176
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Bonus Slides
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Duty Cycle
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Performance of the LAr Veto
energy [keV]1460 1480 1500 1520 1540
coun
ts /
1 ke
V
50
100
150 K42K40
● random coincidences: 2.3%
● 42K line suppressed by factor 5-642K → 42Ca
+ γ (1.5 MeV) in Ge+ β- (≤ 2 MeV) in LAr
40K → 40Ar (EC)no energy in LAr
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42K Background
● solution: transparent nylon cylinder coated with wave length shifter
● tested in test cryostat LArGe ● nylon from BOREXINO
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Signals of BEGe's
figure taken from JINST 6 P03005, 2011
● final drift paths of holes nearly independent of interaction point
● high gradient of weighting potential
→ single site events (SSE) have similar pulse shape
time [ns]81000 81500 82000
time [ns]81000 81500 82000
curre
nt [a.
u.]
00.20.40.60.8
1 SSEMSE = superposition of SSE
current signal = q · v · q: charge, v: velocity
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PSD with Coaxial HPGe
current signal = q · v · q: charge, v: velocity
● To identify signal like events artificial neural networkalgorithm TMlpANN from TMVA is used
● Input variables: times when charge pulse reach1%, 3%, … , 99% of maximum amplitude
● DEP events of at 1503 keV serve as signal sample● FEP events at 1621 keV as multi site event sample
45% of background events rejected at Qbb with a 0 efficiency of 90+5-9 %
● 2 efficiency 85 ± 2 %
more detail in Eur.Phys.J C73 (2013) 2583
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Coax PSD
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Background Suppression LAr + PSD
Coax, 5.0 kg · yr
BI AC ,MV , LAr=10.4−2.7+3.5⋅10−3 counts
keV⋅kg⋅yr
BI AC ,MV=16.5−3.5+4.2⋅10−3 counts
keV⋅kg⋅yr
BI AC ,MV , LAr , PSD=3.5−1.5+2.1⋅10−3 counts
keV⋅kg⋅yr
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Background Model
Background Model BEGe
Preliminary resultsbefore PSD & LAr veto
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Background Composition at Q
enrBEGe enrCoax
~ 1/3 ~ 1/3214Bi and 208Tl ~ 1/3 ~ 1/342K LAr ~ 1/3 ~ 1/3
BI counts/(keV kg yr) 0.014 0.015
Preliminary resultsbefore PSD & LAr veto
Monte Carlo Simulation of BEGe Background
expect flat background in ROI
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Effective Majorana Neutrino Mass
(T 1/20ν )−1∝|mee|
2
observable
Access to ● absolute neutrino mass scale ● mass hierarchy
Modified figure from Nucl. Part. Phys. Proc., 260:188–193, 2015
Assuming light Majorana neutrino exchange●
● effective Majorana mass:
|mee|≡|∑i U ei2 mi|
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Phase I + II Data Sets
(T 1/20ν )−1∝N0 ν=
ln 2⋅N Am76
M⋅tT 1/2
0ν ⋅ϵ⋅ϵPSD⋅ϵLAr
data set exposure [kg yr]
signal eff Energy resolution (keV, FWHM)
Background index 0.001 cnts/(keV kg
yr)
Phase I gold 17.9 0.57 (3) 4.3 (1) 11 ± 2
Phase I silver 1.3 0.57 (3) 4.3 (1) 30 ± 10
Phase I BEGe 2.4 0.66 (2) 2.7 (2) 5+4
Phase I extra 1.9 0.58 (4) 4.2 (2) 5+4
Phase II coax 5.0 0.53 (4) 4.0 (2) 3+3
Phase II BEGe 5.8 0.60 (1) 3.0 (2) 0.7+1.3-1
-0.5
NA: Avogadro’s constant, m76: molar mass of 76Ge
M·t: exposure [kg yr], T1/2: half-life of 0 decay,LArLAr efficiency, PSDPSD efficiency, exposure averaged efficiency incl. active volume,enrichement, FEP
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Results from GERDA Phase I
Claim: T1/2 =1.19×1025 yr [Phys. Lett. B 586 198(2004)]0ν
GERDA: 90% lower limit (T1/2
) [Phys. Rev. Lett. 111 (2013) 122503]0ν
BI=1.0 (1)⋅10−2 countskeV kg yr
number of events in Qββ±2σE after cuts (gray):
● 2.0 ± 0.3 expected from background
• 3 observed
no signal observed at Qββprofile likelihood: best fit for N0 = 0→ limit on the half-life
(90% C.L.)
→claim rejected with 99% probability
T1/20ν >2.1⋅1025 yr
● 21.6 kg · y exposure
● blind analysis: events in ROI not available for analysis
● background index (BI) after pulse shape discrimination
● 10 times better BI than previous experiments Q = 2039 keV
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