Major steps forward in understanding neutrino properties

Closing in on elusive particles

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.

The Technical University of Munich (TUM) has been a key partner of the GERDA project (GERmanium Detector Array) for many years. Prof. Stefan Schönert, who heads the TUM research group, is the speaker of the new LEGEND project.

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 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.