Nuclear structure relevant to neutrinoless double beta decay: 76Ge and 76Se

Area of Science:

• Nuclear Physics
• Particle Physics
• Astroparticle Physics

Background:

• Neutrinoless double beta decay (0νββ) is a hypothetical radioactive decay.
• Observing 0νββ would determine the effective neutrino mass, a fundamental property of neutrinos.
• Accurate nuclear matrix elements are essential for interpreting 0νββ experiments, but theoretical calculations have uncertainties.

Purpose of the Study:

• To experimentally determine the occupation of valence neutron orbits in the ground states of 76Ge and 76Se.
• To provide crucial data for refining theoretical calculations of nuclear matrix elements relevant to 0νββ decay.

Main Methods:

• Precise measurement of cross sections for neutron-adding and neutron-removing transfer reactions.
• Utilizing 76Ge as a candidate for 0νββ decay and 76Se as its daughter nucleus.
• Analyzing reaction data to deduce the populations of specific nucleon orbits.

Main Results:

• The Fermi surface in 76Ge and 76Se is found to be significantly more diffuse than predicted by theoretical models.
• Populations of at least three valence neutron orbits show substantial changes between the ground states of 76Ge and 76Se.
• Theoretical calculations predominantly predict changes in only one orbit, highlighting a discrepancy with experimental findings.

Conclusions:

• Experimental data on valence orbit occupations provide critical insights into nuclear structure relevant to 0νββ decay.
• The observed diffuse Fermi surface and multi-orbit changes necessitate revisions in theoretical models for nuclear matrix elements.
• Improved theoretical calculations based on these findings will enhance the interpretation of future 0νββ experiments and neutrino mass determination.