While the Standard Model of Particle Physics has remained mostly unchanged since its initialconception, experimental observations for neutrinos have forced the neutrino part of the theory to be reconsidered in its entirety. Neutrino oscillation was the first observation inconsistent with the predictions and proves thatneutrinos have non-zero masses, a property that contradicts the Standard Model. In 2015, this discovery was rewarded with the Nobel Prize.

Additionally, there is the longstanding conjecture that neutrinos are so-called Majorana particles: Unlike all other constituents of matter, neutrinos might be their own antiparticles. This would also help explain why there is so much more matter than antimatter in the Universe.

The GERDA experiment is designed to scrutinize the Majorana hypothesis by searching for the neutrinoless double beta decay of the germanium isotope 76Ge: Two neutrons inside a 76Ge nucleus simultaneously transform into two protons with the emission of two electrons.This decay is forbidden in the Standard Model because the two antineutrinos – the balancing antimatter – are missing.
 

GERDA is the first experiment to reach exceptionally low levels of background noise and has now surpassed the half-life sensitivity for decay of 10²⁶ years. In other words: GERDA proves that the process has a half-life of at least 1026 years, or 10,000,000,000,000,000 times the age of the Universe.

Physicists know that neutrinos are at least 100,000 times lighter than electrons, the next heaviest particles. What mass they have exactly, however, is still unknown and another important research topic. In the standard interpretation, the half-life of the neutrinoless double beta decay is related to a special variant of the neutrino mass called the Majorana mass. Based on the new GERDA limit and those from other experiments, this mass must be at least a million times smaller than that of an electron, or in the terms of physicists, less than 0.07 to 0.16 eV/c² _(1).

Also other experiments limit the neutrino mass: the Planck mission provides a limit on
another variant of the neutrino mass: The sum of the masses of all known neutrino types is less than 0.12 to 0.66 eV/c². The tritium decay experiment KATRIN at the Karlsruhe Institute of Technology (KIT) is set-up to measure the neutrino mass with a sensitivity of about 0.2 eV/c² in the coming years. These masses are not directly comparable, but they provide a cross check on the paradigm that neutrinos are Majorana particles. So far, no discrepancy has been observed.

During the reported data collection period, GERDA operated detectors with a total mass of 35.6 kg of 76Ge. Now, a newly formed international collaboration, LEGEND, will increase this mass to 200 kg of 76Ge until 2021 and further reduce the background noise. The aim is to achieve a sensitivity of 10²⁷ years within the next five years.

Publication:
The GERDA collaboration: “Probing Majorana neutrinos with double beta decay”
Science, published online on Thursday 5 September, 2019 DOI: 10.1126/science/

More information:
GERDA is an international European collaboration of more than 100 physicists from Belgium, Germany, Italy, Russia, Poland and Switzerland. 

(1) In particle physics masses are specified not in kilograms, but rather in accordance with Einstein’s equation E=mc²: electron volts [eV] divided by the speed of light squared. Electron volts are a measure of energy. This convention is used to circumvent unfathomably small units of mass: 1 eV/c² corresponds to 1.8 × 10^-37 kilograms.

Contact:
Technical University of Dresden
Prof. Dr. Kai Zuber
Tel.: +49 351 463 42250
E-Mail: zuber@physik.tu-dresden.de

Max Planck Institute for Nuclear Physics, Heidelberg
Prof. Dr. Werner Hoffmann
Tel.: +49 6221 516 330
E-Mail: Werner.Hofmann@mpi-hd.mpg.de
Prof. Dr. Manfred Lindner
Tel.: +49 6221 516 800
E-Mail: lindner@mpi-hd.mpg.de

Max Planck Institute for Physics, Munich
Prof. Dr. Allen Caldwell
Tel.: +49 89 323 54207
E-Mail: caldwell@mpp.mpg.de

Technical University of Munich
Prof. Dr. Stefan Schönert
Tel.: +49 89 289 12511
E-Mail: schoenert@ph.tum.de

University of Tübingen
Prof. Dr. Josef Jochum
Tel.: +49 7071 297 4453
E-Mail: Josef.Jochum@uni-tuebingen.de

University of Zürich
Prof. Dr. Laura Baudis
Tel.: +41 44 635 5777
E-Mail: lbaudis@physik.uzh.ch