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{{Short description|Neutrinoless double beta decay experiment}}
The '''[[Chemistry:Germanium|Germanium]] Detector Array''' (or '''GERDA''') experiment was searching for [[Physics:Neutrinoless double beta decay|neutrinoless double beta decay]] (0νββ) in Ge-76 at the underground [[Organization:Laboratori Nazionali del Gran Sasso|Laboratori Nazionali del Gran Sasso]] (LNGS). Neutrinoless beta decay is expected to be a very rare process if it occurs. The collaboration predicted less than one event each year per kilogram of material, appearing as a narrow spike around the 0νββ Q-value (Q<sub>ββ</sub> = 2039 keV) in the observed energy spectrum. This meant [[Physics:Radiation protection|background shielding]] was required to detect any rare decays. The [[Organization:Laboratori Nazionali del Gran Sasso|LNGS facility]] has 1400 meters of rock overburden, equivalent to 3000 meters of water shielding, reducing [[Astronomy:Cosmic ray|cosmic radiation]] [[Physics:Background radiation|background]]. The GERDA experiment was operated from 2011 onwards at LNGS.<ref name="gerdafinal"/>

After completing the GERDA experiment, the GERDA collaboration merged with [[Physics:MAJORANA|MAJORANA]]-collaboration to build a new experiment LEGEND.

GERDA reported its final results in December 2020 in the [[Physics:Physical Review Letters|Physical Review Letters]]. The experiment reached all the goals that it set to itself, but no detection of any 0νββ events was made.<ref name="gerdafinal">{{Cite web|url=https://www.appec.org/news/another-milestone-in-the-search-for-neutrinoless-double-beta-decay-final-results-of-gerda|title = Another milestone in the search for neutrinoless double-beta decay – Final results of GERDA » APPEC | date=18 January 2021 }}</ref>

The experience from GERDA led to the expectation that further background reduction was in reach so that a background-free experiment with an even larger source strength, respectively exposure, became possible. The LEGEND collaboration, continuing GERDA's work, was aiming at increasing the sensitivity to the half-life of 0νββ decay up to <math>10^{28} yr</math>. In a first phase, it planned to deploy a mass of 200&nbsp;kg of enriched germanium detectors in the slightly modified infrastructure of GERDA with the start of data taking planned for 2021.<ref name="gerdafinal"/>

== Design ==
The experiment used high purity enriched [[Chemistry:Germanium|Ge]] crystal diodes ([[Physics:Semiconductor detector#Germanium detectors|HPGe]]) as a beta decay source and [[Physics:Particle detector|particle detector]]. The detectors from the HdM (Heidelberg-Moscow<ref name="article2021">{{Cite journal|doi = 10.1140/epjc/s10052-021-09403-2|title = Calibration of the Gerda experiment|year = 2021|last1 = Agostini|first1 = M.|last2 = Araujo|first2 = G.|last3 = Bakalyarov|first3 = A. M.|last4 = Balata|first4 = M.|last5 = Barabanov|first5 = I.|last6 = Baudis|first6 = L.|last7 = Bauer|first7 = C.|last8 = Bellotti|first8 = E.|last9 = Belogurov|first9 = S.|last10 = Bettini|first10 = A.|last11 = Bezrukov|first11 = L.|last12 = Biancacci|first12 = V.|last13 = Bossio|first13 = E.|last14 = Bothe|first14 = V.|last15 = Brudanin|first15 = V.|last16 = Brugnera|first16 = R.|last17 = Caldwell|first17 = A.|last18 = Cattadori|first18 = C.|last19 = Chernogorov|first19 = A.|last20 = Comellato|first20 = T.|last21 = d'Andrea|first21 = V.|last22 = Demidova|first22 = E. V.|last23 = Marco|first23 = N. Di|last24 = Doroshkevich|first24 = E.|last25 = Fischer|first25 = F.|last26 = Fomina|first26 = M.|last27 = Gangapshev|first27 = A.|last28 = Garfagnini|first28 = A.|last29 = Gooch|first29 = C.|last30 = Grabmayr|first30 = P.|journal = The European Physical Journal C|volume = 81|issue = 8|page = 682|pmid = 34776783|pmc = 8550656|bibcode = 2021EPJC...81..682A|display-authors = 1 | arxiv=2103.13777 }}</ref>) and IGEX<ref name="article2021"/> experiments were reprocessed and used in phase 1. The detector array was suspended in a liquid [[Chemistry:Argon|argon]] [[Physics:Cryostat|cryostat]] lined with copper and surrounded by an ultra-pure water tank. [[Physics:Photomultiplier|PMTs]] in the water tank and plastic scintillators above detected and excluded background muons. Pulse-shape discrimination (PSD) was applied as a cut to discriminate between particle types.

GERDA followed in the footsteps of other 0νββ experiments using germanium; already more than 50 years ago (that is, around 1970), a 0.1&nbsp;kg germanium detector was used by a Milano group in the first 0νββ decay search with a germanium detector. Since then, the sensitivity had been increased by a factor of one million.<ref name="gerdafinal"/>

Phase 2 increased the active mass to 38&nbsp;kg using 30 new broad energy germanium (BEGe) detectors. A magnitude reduction in background was planned to 10<sup>−3</sup> counts/(keV·kg·yr) using cleaner materials. This increased the half-life sensitivity to 10<sup>26</sup> years once 100&nbsp;kg·yr of data was taken and enabled evaluation of possible ton-scale expansion.

== Results ==
Phase I collected data November 2011 to May 2013, with 21.6&nbsp;kg·yr exposure. No neutrinoless decays were observed, yielding a 0νββ 90% CL half-life limit of <math>T_{0 \nu \beta \beta} > 2.1 \cdot 10^{25} yr </math>. This limit could be combined with previous results, increasing it to 3·10<sup>25</sup> yr, disfavoring the Heidelberg-Moscow detection claim. A bound on the effective neutrino mass was also reported: m<sub>ν</sub> < 400 meV.

The double beta decay (with two neutrinos) half-life was also measured: T<sub>2νββ</sub> = 1.84·10<sup>21</sup> yr.

Phase II had additional enriched Ge detectors and reduced background, raising the sensitivity about one order of magnitude.

Phase II (7 strings, 35.8&nbsp;kg of enriched detectors) was started in Dec 2015.<ref name=G2-07>{{cite conference |title=First results from GERDA Phase II |author=GERDA collaboration |author2=M.Agostini|display-authors=0<!--|collaboration=GERDA collaboration--> |date=8 July 2016 |url=https://www.mpi-hd.mpg.de/gerda/public/2016/t16_neutrino_gerda_ma.pdf |conference=XXVII International Conference on Neutrino Physics and Astrophysics (Neutrino 2016) |conference-url=http://neutrino2016.iopconfs.org/programme |location=London}}</ref>{{rp|10}}

Preliminary results of Phase II have been published in Nature.<ref>{{citation |author=GERDA collaboration |author2=M.Agostini |display-authors=0 |periodical=Nature |title=Background-free search for neutrinoless double-β decay of <sup>76</sup>Ge with GERDA |volume=544 |issue=7648 |pages=47–52 |date=2017-04-05 |doi=10.1038/nature21717 |pmid=28382980 |arxiv=1703.00570 |bibcode=2017Natur.544...47A |s2cid=4456764}}</ref> The background index for BEGe detectors was 0.7·10<sup>−3</sup> counts/(keV·kg·yr), which translated to less than one count in the signal region after an exposure of 100&nbsp;kg·yr. Again no neutrinoless decays were observed, bringing the present limit on the half life to T<sub>1/2</sub>{{nbsp}}>{{nbsp}}5.3·10<sup>25</sup> yr (90% C.L.).

As of 2018, the Phase II data-taking continued.

In December 2020, the final results of GERDA were reported. There was no detection of 0νββ, and the experiment reported lower limit for the 0νββ half-life in Ge-76 of <math>T_{0 \nu \beta \beta} > 1.8 \cdot 10^{26} yr </math>. The reported final lower limit agreed with the expected value for the sensitivity of the experiment, and was the most stringent value for the decay of any 0νββ isotope ever measured. Also the background event rate of GERDA was cutting-edge level in the field. In its final phase GERDA deployed 41 germanium detectors with a total mass of 44.2&nbsp;kg, with very high germanium-76 enrichment percent.<ref name="gerdafinal"/>

== References ==
{{reflist}}

== Publications ==
* {{cite journal |arxiv=1307.4720 |bibcode=2013PhRvL.111l2503A |doi=10.1103/PhysRevLett.111.122503 |pmid=24093254 |title=Results on Neutrinoless Double-β Decay of <sup>76</sup>Ge from Phase I of the GERDA Experiment |journal=[[Physics:Physical Review Letters|Physical Review Letters]] |volume=111 |issue=12 |article-number=122503 |date=19 September 2013 |author=GERDA collaboration, Agostini M. |display-authors=0|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:1849853/component/escidoc:1849852/1307.4720.pdf }}
* {{cite journal |arxiv=1212.3210 |bibcode=2013JPhG...40c5110T |doi=10.1088/0954-3899/40/3/035110 |title=Measurement of the half-life of the two-neutrino double beta decay of <sup>76</sup>Ge with the GERDA experiment |journal=[[Physics:Journal of Physics G|Journal of Physics G]] |volume=40 |issue=3 |article-number=035110 |date=12 February 2013 |author=GERDA collaboration, Agostini M. |s2cid=119118050 |display-authors=0}}
* {{cite journal |arxiv=1212.4067 |bibcode=2013EPJC...73.2330A |doi=10.1140/epjc/s10052-013-2330-0 |doi-access=free |title=The GERDA experiment for the search of 0νββ decay in <sup>76</sup>Ge |journal=[[Physics:European Physical Journal C|European Physical Journal C]] |volume=73 |issue=3 |page=2330 |date=March 2013 |author=GERDA collaboration, Ackermann K.-H. |display-authors=0}}
* {{cite journal |arxiv=1703.00570 |doi=10.1038/nature21717 |pmid=28382980 |title=Background-free search for neutrinoless double-β decay of <sup>76</sup>Ge with GERDA|journal=Nature |volume=544 |issue=7648|pages=47–52 |date=5 April 2017 |author=GERDA collaboration, Agostini M. |display-authors=0|bibcode=2017Natur.544...47A|s2cid=4456764 }}

== External links ==
* [http://www.mpi-hd.mpg.de/gerda/public/index.html GERDA Collaboration]
* [https://inspirehep.net/experiments/1108265 GERDA experiment] record on [[Physics:INSPIRE-HEP|INSPIRE-HEP]]
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{{Sourceattribution|Germanium Detector Array}}