The GERDA experiment in the search
for neutrinoless double-beta decay
Valentina Biancacci
Lomonosov Conference 2021

on behalf of the GERDA collaboration
22.08.21

Double-beta decay without neutrinos
The neutrinoless double-beta (0νββ) decay is ahypothetical nuclear transition.

For the standard interpretations, 0νββ can be mediated by theexchange of two massive Majorana neutrinos.
Motivation for 0νββ decay searches●●●
nuclear matrix element phase space factor●
effective neutrino mass

Valentina Biancacci●

Establish lepton number violation (LNV) → ΔL=2Only way to determine if neutrino is its own
antiparticle ()Important to understand the origin of the neutrino
mass
Probe the absolute neutrino mass scale and neutrinomass ordering
Provide important input to cosmology

Lomonosov Conference

222.08.210νββ signature and half-life
with backgroundQββ = 2039 keV for 76Ge

without background0νββ signal = monoenergetic peak

low background level and good energy resolution
Valentina Biancacci

Lomonosov Conference

322.08.21

The GERDA collaboration
The GERDA experiment (GERmanium Detector Array)is searching for the neutrinoless double beta decay of76Ge.

Collaboration meeting in Zurich (2019)
Valentina Biancacci

Lomonosov Conference

422.08.21

GERDA experiment
●●●

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The GERDA experiment has been proposed in 2004
as a new 76Ge double-beta decay experiment at
LNGS (Italy).
Up to 41 enriched detectors deployed from Dec 2015
to Dec 2019.
Two data taking periods with an upgrade in May
2018 in between.

The array of germanium detectors was placed in aliquid argon (LAr) cryostat.
A tank ﬁlled with 590 m3 pure water surrounded thecryostat.
Water tank equipped with PMTs detecting Cherenkov
light.

Valentina Biancacci

Lomonosov Conference

522.08.21

76

Why germanium?

●●●●High detection efﬁciency (detector = ββ source)
Enrichment from 7.7% to ~88%
Best proved energy resolution at the Q-value (0.13% FWHM)
Lowest background per FWHM energy resolution in the ﬁeld

Semi-coaxial detectors:
A. typically mass 2~3 kg
B. enriched to ~87% in 76Ge

II.
(b)

BEGe(Broad Energy Germanium) detectors:
A. average mass 670 g
B. small p+ contact at the bottom side
C. excellent energy resolution
Inverted-coaxial detectors:
A. same advantageous characteristics ofBEGe
B. larger mass

Valentina Biancacci

75 cm

65-80 cm
25-50 cm

III.

(c)

65-80 cm

80 cm

(a)I.

70-110 cm

Ge Detectors

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622.08.21

Data taking

new ﬁbers shrouds
5 new IC detectors●●

2011-2013: Phase I, 23.5 kg yr exposure
2015-2019: Phase II, 103.7 kg yr exposure, installation of LAr veto,upgrade with IC detectors in 2018, operation in background-free
regimeValentina Biancacci

Lomonosov Conference

722.08.21

Energy resolution●Peak ﬁtting algorithm to determine
each detector’s resolution●

Gaussian mixture models to determine
resolutions per detector type●

Excellent energy resolution leads tolower backgrounds and higher
discovery potential●

Well-understood peak shape, energy
scale stability, and linearity lead to
improved conﬁdence in results[GERDA, Eur. Phys. J. C (2021) 81, 682]

Valentina Biancacci

Lomonosov Conference

822.08.21

Active background reduction tools●

ββ decay signal:
single-site event energy
deposition in a 1 mm3volume●●

Anti-coincidence with the muon vetoAnti-coincidence between detectors (cutsmulti-site)
Active veto using LAr scintillation (LAr Veto)
Pulse shape discrimination (PSD)●●

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Lomonosov Conference

922.08.21

Pulse Shape Discrimination (PSD)●signal-like

Point-contact geometry allows
for MS event rejection based on
pulse shapes, current amplitude
over energy (A/E)[GERDA, Eur. Phys. J. C 73 2583 (2013)]
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Lomonosov Conference

1022.08.21

Total active background suppression●●●±25 keV around Qββ blinded
Spectrum well understood:○≲ 0.5 MeV: 39Ar○∼ [0.5 - 2] MeV: 2νββ○
≳4 MeV: αFlat background in 0νββ signal region

Valentina Biancacci

Lomonosov Conference

GERDA, Science 365, 1445–1448 (2019)1122.08.21

Results●●

Combined unbinned maximum likelihood fit○ Combined fit of Gaussian signal &uniform background
Best fit for null signal strength[GERDA , Nature 544 (2017), 47–52]
Valentina Biancacci

Lomonosov Conference

1222.08.21

Conclusion●

GERDA employed an array of HPGe
detectors enriched in 76Ge to search for 0νββ●

GERDA ran in background-free regime forthe entire duration of its data taking.

●Provides the most stringent constraints on
the half-life of 0νββ decay:
T1/2 > 1.8 × 1026 yr → |mββ| ≲ 79 − 180 meV●

Next step…

Valentina Biancacci

stay tuned!

Lomonosov Conference[GERDA , Nature 544 (2017), 47–52]

1322.08.21

backupValentina Biancacci

Lomonosov Conference

1422.08.21

Liquid Argon Veto●

Stable operations over 4 years ofdata taking:○ 16 PMts○ 1.5 km light guiding ﬁbers +SiPM readout○ Vetoes events incoincidence withGermanium●●Acceptance (0νββ) ~ 98%
Crucial role in backgroundsuppression after PSD: ÷ 6 in theROI[GERDA, European Phys J C 78 (2018), 388]
Valentina Biancacci

Lomonosov Conference

1522.08.21

Searching in 76Ge
Why using Germanium?
Advantage● High efﬁciency → Source=Detector● Small intrinsic BI = high purity Ge● Excellent ΔE● Well-established technology
Disadvantages● Small Q-value● Small● Small abundance 0νββ of isotope
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HPGedetector

1622.08.21

HPGe detectors
Advantages of Inverted Coaxial Point Contact (ICPC) detectors:●

Enriched detectors, 92% of detector material is 76Ge●

Excellent resolution and pulse shape discrimination●

Signiﬁcantly larger w.r.t. BEGe or PPC (up to 3 kg)●Less channels, less background●

Better surface to volume-ratio (30-40%)●

Production started early 2019●

About 60 detectors expected by fall 2021

Valentina Biancacci

Lomonosov Conferencen+ electrodep+ electrodepassivation

1722.08.21

Performance on the energy scale

Valentina Biancacci

Lomonosov Conference

1822.08.21

Background model[GERDA , J High Energy Phys, 2020 (2020), no. 3, 139]

All the background components are well known
Valentina Biancacci

Lomonosov Conference

1922.08.21

Details on PSD

Valentina Biancacci

Lomonosov Conference

2022.08.21

LEGEND experimentFirst Stage (LEGEND-200):● Upgrade of the existing infrastructure of GERDA experiment● Reduction of the BI of a factor 5 w.r.t. GERDA Phase II goal
● ~200 kg of detector mass: 35 kg from GERDA + 30 kg from MJD+ 140 kg which are new.
Further Stage (LEGEND-1000):● 1000 kg detector mass (staged)● Background reduction of a factor 20 w.r.t. LEGEND-200● To reach beyond 1028 yrs half-life discovery sensitivity● Location to be deﬁned (SNOLAB or LNGS)
L200 Goals
half life discovery sensitivity
mass sensitivity
background index
cts/(keV⋅kg⋅yr)

1027 yrs
30-70 meV2⋅10-4
21

Valentina Biancacci

Lomonosov Conference

22.08.21

Discovery sensitivity for 76Ge, 130Te, and 136Xe
Comparison of rough sensitivity between ongoing & planned experiments

Agostini, Benato, Detwiler, Phys. Rev. D 96 053001 (2017)
Valentina Biancacci

Lomonosov Conference

2222.08.21