Experiments Designed to Measure Neutrino Mass with Increased Sensitivity

In a pursuit to show that it is possible to produce matter without antimatter, the GERDA experiment carried out at the Gran Sasso Underground Laboratory is searching for signs of neutrinoless double beta decay.

The sensitivity of the experiment is the highest in the world for detecting the concerned decay. In order to further enhance the chances of success, a follow-up project, known as LEGEND, employs a more advanced decay experiment.

Although the Standard Model of Particle Physics has mostly been unchanged from the time it was initially proposed, experimental observations of neutrinos have compelled the theory’s neutrino part to be revisited on the whole.

Neutrino oscillation was the first observation incongruent with the hypotheses and shows that neutrinos have non-zero masses—a property that is in contrast to the Standard Model. This discovery was awarded the Nobel Prize in 2015.

Are Neutrinos Their Own Antiparticles?

In addition, for a long time, there has been speculation that neutrinos are so-called Majorana particles: in contrast to all other constituents of matter, neutrinos could be their own antiparticles. This would also enable researchers to elucidate why there is so much more matter than antimatter in the Universe.

The GERDA experiment has been developed to investigate the Majorana hypothesis by looking for the neutrinoless double beta decay of the germanium isotope ⁷⁶Ge: Two neutrons within a ⁷⁶Ge nucleus transform into two protons at the same time, emitting two electrons. According to the Standard Model, this decay is prohibited since the two antineutrinos—the balancing antimatter—do not exist.

For several years, the Technical University of Munich (TUM) has been the main collaborator of the GERDA project (GERmanium Detector Array). Prof. Stefan Schönert, who leads the TUM research group, is the speaker of the new LEGEND project.

The GERDA Experiment Achieves Extreme Levels of Sensitivity

GERDA is the first experiment to achieve extremely low background noise levels and has now outperformed the half-life sensitivity for decay of 10²⁶ years. Put differently, GERDA shows that the half-life of the process is at least 10²⁶ years, or 10,000,000,000,000,000 times the age of the Universe.

Physicists are aware that neutrinos are at least 100,000 times lighter compared to electrons, the next heaviest particles. However, their precise mass is not yet known, which is another crucial research topic.

As per standard interpretation, the half-life of the neutrinoless double beta decay depends on a unique variant of the neutrino mass known as the Majorana mass. According to the new GERDA limit and those from other experiments, this mass could be at least a million times smaller when compared to that of an electron or, as per the physicists, <0.07–0.16 eV/c².

Consistent with Other Experiments

Moreover, the mass of the neutrino is limited by other experiments: the Planck mission offers a limit on another variant of the neutrino mass: The sum of the masses of all familiar neutrino types is <0.12–0.66 eV/c².

At the Karlsruhe Institute of Technology (KIT), the tritium decay experiment KATRIN has been designed to measure the mass of the neutrino with a sensitivity of around 0.2 eV/c² in future. Although these masses are not directly analogous, they offer a cross verification of the concept that neutrinos are Majorana particles. To date, there has been no observed discrepancy.

From GERDA to LEGEND

During the period when the reported data was gathered, GERDA ran detectors with a total mass of 35.6 kg of ⁷⁶Ge. Currently, this mass will be increased by a newly formed international collaboration—LEGEND—to 200 kg of ⁷⁶Ge until 2021 and the background noise will be reduced further. The goal is to realize a sensitivity of 10²⁷ years within the next five years.