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LEGEND: the Majorana nature of the neutrino

Are neutrinos their own antiparticles? The future LEGEND experiment will enable scientists to learn more about this propertie of neutrino, also called "Majorana nature". LEGEND was formed as a follow-up to the previous GERDA and Majorana experiments.

Current insights suggest that for every charged spin-1/2 particle there exists an antiparticle with an opposite electric charge: anti-quarks and anti-leptons (positron, anti-muon, and anti-tau). As the corresponding neutrinos are electrically neutral, they may differ from this scheme and be their own antiparticle. If this was confirmed experimentally, it could open up new ways to improve our understanding of the physics of the universe.

How, then, can scientists discover if neutrinos are their own antiparticles?

They can search for a very special type of decay that only occurs if this is the case: neutrinoless double beta decay, in which two neutrons are converted into two protons and two electrons. In standard double beta decay, two neutrinos are also released. In neutrinoless decay, however, these are exchanged within the nucleus. But this can only occur if

•    neutrinos and their antineutrinos are identical, and
•    they have the same mass.

The role of germanium-76

In order to detect neutrinoless double beta decay, the LEGEND experiment will use detectors made of germanium-76, an unusual element that is, at least theoretically, capable of producing this decay. If the experiment can detect such decays, the presumed “dual nature” of the neutrino will be confirmed. Such detection relies on the fact that germanium-76 is not only the source of the decay, but also the material used to detect it.

In the first phase, the participating research groups will conduct the LEGEND 200 experiment at the LNGS using 200 kilograms of detector material. The experiment is scheduled to begin collecting data in 2022. In the second phase, an experiment employing 1000 kilograms of germanium detector material will be constructed. Both the location and time-line of the experiment are still under discussion.

Shielding from environmental radiation

Even if neutrinoless double beta decay does exist, it is an extremely rare phenomenon. It could never be detected in the presence of normal natural radiation, as this produces background events similar to signal events.

For this reason, experiments searching for neutrinoless double beta decay need to be extremely well shielded from environmental radiation. However, background events also occur due to the radioactivity of the materials used in the experiment itself and due to cosmic rays able to pass through solid objects.

To minimize the influence of cosmic rays, scientists conduct the experiments very deep underground. LEGEND 200 will therefore be assembled at the Gran Sasso underground laboratory in Italy, shielded by 1.4 kilometers of mountain rock against cosmic rays from space.

LEGEND 200 will largely use the infrastructure of the successful GERDA experiment, which achieved the lowest ever background radiation rate. The germanium detectors are placed in a container made of specialized steel that emits very low levels of radiation, and it is filled with liquid argon. This container is embedded in a ten-meter diameter tank filled with high-purity water, providing additional shielding. Moreover, the experiment will also detect muons with specialized detectors to rule out signals that might be caused by these particles.

Research for LEGEND

LEGEND involves more than 30 research institutions from nearly 10 countries. The group at the Max Planck Institute for Physics is working on optimizing the germanium detectors (GeDET). The group is also involved in further subprojects, such as MINIDEX, which studies neutrons as a potential source of disturbances, and PEN, which is developing and implementing new types of optically active components made of plastic, which are already being installed in LEGEND 200.


GeDet is committed to to further develop and optimize germanium detector technology. The goal is to design improved detectors in which background can be better identified. (Photo: I. Abt/MPP)



Neutrons are a source for background events difficult to detect. The goal of the MINIDEX project is to study cosmic-muon-induced neutrons which cause an especially dangerous background.  (Photo: I. Abt/MPP)


The goal of the PEN project is to provide an active veto against radioactive background and at the same time wrap and protect the germanium detectors. (Photo: F. Fischer/MPP)


Further information on the LEGEND group


Germanium detector mounted on a PEN holder: The transparent plate also carries the electronic components for signal detection. (Photo: M. Willers/MPP)

The LEGEND 200 experiment is set to search for extremely rare neutrinoless double beta decay. Finding it could answer a central physics question: is the neutrino its own antiparticle? To detect the decay, scientists use highly sensitive Germanium detectors. However, in order to identify “real”…

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The search for neutrinoless double-beta decay continues with new LEGEND experiment

Final results and completion of the GERDA experiment

The last measurements of the GERDA experiment were analyzed and published in 2020. The phote shows the experiment's opening in 2010. (Photo: MPP)

The GERDA experiment designed to prove the existence of neutrinoless double beta decay is coming to an end. The research collaboration has now evaluated the full data set produced by the experiment and published it in Physical Review Letters. Even though no signal was found, all goals set for the…

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Group members

name function extension office www

Abt, Iris, Dr.

Scientist 295 117

Brunst, Tim

Scientist 401 030C

Caldwell, Allen, Prof. Dr.

Director 529 212

Fischer, Felix

PhD-Student 280 135

Glas, Cynthia

Student 373 28C

Gooch, Christopher

Engineering 242 115

Hagemann, Felix

PhD-Student 327 112

Hauertmann, Lukas

PhD-Student 764 112

Henkes, Florian

Student 373 028C

Hervas Aguilar, David

PhD-Student 764 112

Koehler, Christoph

PhD-Student 361 025C

Liu, Xiang, Ph.D.

Scientist 415 114

Majorovits, Béla, PD Dr.

Scientist 262 118

Manzanillas, Luis

Postdoc 280 135

Meinrad Ettengruber, Manuel

PhD-Student 561 203

Schlueter, Lisa

PhD-Student 419 021C

Schulz, Oliver, Dr.

Scientist 521 113

Schuster, Martin

PhD-Student 327 112

Urban, Korbinian

PhD-Student 361 025C

Wacker, Ina

Secretary 207 213

Weidenthaler, Matthias

Student 373 028C

Wunderl, Lena

Student 373 28C

Zsigmond, Anna Julia, Dr.

Postdoc 337 116

Key publications

Alpha-event and surface characterisation in segmented true-coaxial HPGe detectors
Nucl. Instrum. Meth. A 858 (2017) 80-89

The GALATEA test-facility for high purity germanium detectors
Nucl.Instrum.Meth. A 782 (2015) 56

Measurement of the temperature dependence of pulse lengths in an n-type germanium detector
Eur. Phys. J. Appl. Phys. 56 (2011) 10104

Pulse shape simulation for segmented true-coaxial HPGe detectors
Eur. Phys. J. C 68, 609-618 (2010)

Neutron Interactions as Seen by A Segmented Germanium Detector
Eur. Phys. J. A 36, 139-149 (2008)

Characterization of the first true coaxial 18-fold segmented n-type prototype detector for the GERDA project Nucl.Instrum.Meth. A 577 (2007) 574