|
One of the most fundamental open questions in physics is the puzzle of the observed matter-antimatter asymmetry in the Universe. This imbalance remains unexplained by the Standard Model of particle physics. The electrically neutral neutrinos may play a key role in the origin of this asymmetry in the early stages of the universe, if they prove to be their own antiparticles, so-called Majorana fermions. In such scenarios, the lepton number conservation, a feature that has been observed in all particle reactions investigated until now, would be violated.
The observation of the theoretically well motivated neutrinoless double-beta decay (0νββ) is a proposed experimental pathway to prove this concept. Here, two neutrons in an atomic nucleus decay into two protons and two electrons simultaneously, but without accompanying antineutrinos (see Fig 2.) Therefore, matter is created without the corresponding number of antiparticles, which violates our current understanding of lepton number conservation and inherently proves the Majorana nature of neutrinos, marking a revolutionary step in our understanding of the universe. Additionally, it would offer insights into a new theory of fermion masses and the absolute neutrino mass scale, bridging the gap between cosmology and particle physics.
Since 2023, the LEGEND-200 experiment is searching for signs of this neutrinoless double beta decay of the germanium isotope 76Ge. LEGEND is an upgrade of the former GERDA and MAJORANA DEMONSTRATOR experiments. Due to the very rare nature of this process, care has to be taken to minimise background signals in the experiments as much as possible. The Germanium used in the detectors are extremely purified. In order to suppress any incoming cosmic radiation, it is located under 1400 meters of solid rock at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy.
Scientists from the collaboration have now reported results of the first search for neutrinoless double beta decay performed with LEGEND-200, in combination with data from its predecessor experiments. The analysis revealed no signature of this decay and the researchers could therefore obtain a new lower limit on the half-lifetime of the (0νββ) decay of 76Ge of 1,9 ×1026 years- for reference the age of the universe of 1.4 x1010 years. Assuming the process is mediated by Majorana neutrinos, their corresponding effective mass would be less than 75–200 meV/c2. With the deployment of additional detectors and further background reduction, the sensitivity of LEGEND-200 is expected to reach 1027 yr in the coming years. The future LEGEND-1000 infrastructure with 1000 kg of germanium detectors is designed to achieve sensitivities beyond 1028 years. LEGEND-1000 was recognised by the German Federal Ministry of Research, Technology and Space (BMFTR) as one of the nine most promising projects for large-scale research infrastructures in the future.
Original publication:
First Results on the Search for Lepton Number Violating Neutrinoless Double-Beta Decay with the LEGEND-200 Experiment
LEGEND collaboration
Phys. Rev. Lett. 136, 022701, DOI: https://doi.org/10.1103/25tk-nctn
Weblinks:
