Coupling between quadrupole, octupole and hexadecapole degrees of freedom in nuclei

Coupling among quadrupole, octupole and hexadecapole degrees of freedom in nuclei
The nucleus is a quantum mechanical system where the strong, electromagnetic and weak forces are in action
and nucleons occupy orbitals forming structured shells. Nuclei with filled shells, so-called “closed shells” take
spherical shapes, while non-spherical “deformed” shapes are known for open-shell nuclei. The most common
deformed shapes observed for nuclei across the Segre chart are "rugby ball" or prolate and "disc" or oblate
shapes that are characterised by a non-zero quadrupole deformation (|beta2|>0). Some nuclei take "spinning
top" shapes with hexadecapole deformations (|beta4|>0). Both beta2 and beta4 deformations give rise to
axially symmetric reflection-symmetry shapes, while octupole deformations (|beta3|>0) break reflection
symmetry leading to axially asymmetric “pear” shapes for nuclei. It is an open problem – how deformations
and properties of nuclei transpire near open shells as a result of competition among quadrupole, octupole, and
hexadecapole degrees of freedom. In addition, the matter-antimatter asymmetry puzzle may have a solution in
“pear-shaped” nuclei that are good candidates for searching electron dipole moment.

In general, all three quadrupole, octupole, and hexadecapole deformations may be present in a deformed
nucleus. The magnitudes of these deformations may also change with the excitation energy within the
nucleus. This complexity is rooted in the effective interaction between nucleons, which is composed of
couplings between quadrupole and octupole, and/or octupole and hexadecapole correlations. Our
collaboration proposes to study nuclei with such complicated shapes to provide an insight into the
aforementioned couplings for the first time by invoking beta3 and beta4 that are usually not considered.