An elongation versus departure from axial symmetry (triaxiality) plot showing distinctive differences in the shapes of the parent (germanium-76, “rigid”) and daughter (selenium-76, “soft”) nuclei for neutrinoless double beta decay.
An elongation versus departure from axial symmetry (triaxiality) plot showing distinctive differences in the shapes of the parent (germanium-76, “rigid”) and daughter (selenium-76, “soft”) nuclei for neutrinoless double beta decay.
Image courtesy of Jack Henderson, University of Surrey

The Science

Understanding the elusive nature of neutrinos, the ghostly particles that permeate our universe, has captivated scientists for decades. Two of the many mysteries surrounding neutrinos are at the forefront of science: their actual mass, and whether they exist as both matter and antimatter. Studies of a phenomenon called neutrinoless double beta decay (0νββ) could shed light on these mysteries. One isotope, germanium-76 (76Ge), has emerged as a key player in this research. Determining the shape and properties of 76Ge contributes to understanding the process by which neutrinos acquire mass, and if neutrinos exist as both matter and antimatter.

The Impact

The discovery that neutrinos have mass was groundbreaking. However, their absolute mass remains unknown. Neutrinoless double beta decay experiments aim to determine whether neutrinos are their own antiparticles and, if so, provide a means to determine the mass of the neutrino species involved. Determining the mass through neutrinoless double beta decay experiments using 76Ge is only possible if scientists understand the properties of the decay of 76Ge into selenium-76 (76Se). The present study provides key input for these kinds of experiments.

Summary

Germanium-based neutrinoless double beta decay (0νββ) experiments hold great promise for unraveling the mysteries surrounding neutrinos. The observation of this rare decay process not only offers the prospect of determining the nature of these enigmatic particles, but also the determination of their mass, provided the probability governing the decay is reliably known. This probability is not a direct experimental observable and thus can only be determined theoretically. Although significant discrepancies between probability values calculated by different theoretical methods remain, efforts to understand and minimize such differences have progressed remarkably. Among the structure effects studied, research has shown that deformation (deviation from sphericity) and hence the nuclear shape have a significant effect on these decay probability values. Specifically, scientists expect a low probability when the parent and daughter nuclei assume different shapes but higher probability for nuclei with similar deformations. In addition, scientists find a maximum value when they assume spherical symmetry in both parent and daughter nuclei.

Research on the structure of 76Ge, led by physicists at the Triangle Universities Nuclear Laboratory (TUNL), has found that the 76Ge (parent) and 76Se (daughter) have different shapes. In particular, the experiment showed that, while the ground state of 76Ge exhibits rigid triaxial deformation, that of 76Se is characterized by a soft triaxial potential. These conclusions are important for calculations aiming to determine the probability relevant for 76Ge 0νββ decay.

Contact

Akaa Daniel Ayangeakaa
University of North Carolina at Chapel Hill and Triangle Universities Nuclear Laboratory
ayangeak@unc.edu  

Robert V. F. Janssens
University of North Carolina at Chapel Hill and Triangle Universities Nuclear Laboratory
rvfj@email.unc.edu 

Funding

This work is supported by the Department of Energy Office of Science, Office of Nuclear Physics and by the National Science Foundation.

Publications

Ayangeakaa, D., et al., Triaxiality and the nature of low-energy excitations in 76Ge. Physical Review C 107, 044314 (2023). [DOI: 10.1103/PhysRevC.107.044314]